Nursing mixtures can enhance long-term productivity of Sitka spruce (Picea sitchensis (Bong.) Carr.) stands on nutrient-poor soils

Nursing mixtures can enhance long-term productivity of Sitka spruce (Picea sitchensis (Bong.)... Abstract The growth of Sitka spruce (Picea sitchensis (Bong.) Carr.) on nutrient-poor sites in the British Isles is often improved during the late establishment phase when grown in intimate (‘nursing’) mixture with pioneer species such as pines and larches. However, there is very little information on the longer term effects of such mixtures on stand development and productivity. Therefore, we analysed results from four experiments established in northern Scotland in the 1960s which included nursing mixtures of Sitka spruce with Scots pine (Pinus sylvestris L.), lodgepole pine (Pinus contorta Dougl.) or larches (Larix spp.), as well as plots of pure Sitka spruce given different establishment regimes, sometimes including nitrogen fertilization. At three nutrient-poor sites, spruce in nursing mixture was significantly taller after 15 years than pure spruce without nitrogen. Analysis of foliar nitrogen status showed that pure spruce without nitrogen became deficient by ~8–10 years with no recovery for at least 20 years. Spruce grown in mixture also showed an initial nitrogen deficiency, but with recovery by 10–15 years to optimal or marginal levels. After more than 40 years growth, on the nutrient-poor sites the basal areas in the nursing mixtures were significantly higher than pure spruce without remedial nitrogen and comparable to those of pure spruce given several applications of nitrogen, However, these differences were not evident at the more fertile site. Nearly all mixed plots self-thinned towards dominance by Sitka spruce. The magnitude of this nursing effect is striking and harnessing the mechanisms underpinning this effect could be very important for sustaining productivity of forests on nutrient-poor soils in upland Britain. Introduction Recently, there has been increasing advocacy for growing tree species in mixture as part of a strategy of adapting British forests to projected climate change (Read et al., 2009, pp. 174–175). For example, the UK Forestry Standard, which sets out the national basis for sustainable forest management, encourages forestry practices which promote species diversity, such as mixed stands (UKFS 2011, p. 96). Separate policy documents in Wales and Scotland seek to increase the diversity of planted forests by wider use of a range of species mixtures (Anonymous, 2010; Grant et al., 2012). Nevertheless, planted forests in Britain, not to mention Ireland and other adjoining regions of Atlantic Europe, are mostly characterized by single species plantations of fast growing non-native conifers grown on relatively short rotations (Mason, 2007; Mason & Perks, 2011). Successful establishment and management of mixed-species forests depends on an understanding of the characteristics of the component species (e.g. growth rate, shade tolerance) and the way in which their mutual interactions change over time (Pretzsch, 2009; Chapter 9). Everything else being equal, one would expect higher production from a mixed-species stand where the niches occupied by the component species are different so that the species can be said to have complementary characteristics (Kelty, 2006). Positive mixing effects can occur where the growth of a valuable species is favoured by mixture with another, either because of a reduction in competition between the species growing in mixture (termed ‘competitive reduction’) or because one component of the mixture increases the availability of a limiting resource (e.g. nutrients, water) to the benefit of the other components, a process termed ‘facilitation’ (Kelty, 2006). Paquette and Messier (2011) suggested that beneficial interactions between tree species may be more important in stressful environments such as the boreal forests while reviews of facilitation in wider plant communities have also highlighted the need for taking environmental gradients into account (Brooker et al., 2008). The complexity of these interactions suggests that, despite increasing reports of the value of mixed stands for the provision of a range of ecosystem services including productivity (Felton et al., 2010, 2016; Zhang et al., 2012; Gamfeldt et al., 2013), the potential benefits of mixtures need to be carefully tested in individual climatic regions or site types. A further complication is that many studies of mixing effects in forests have used data derived from specific forest plots or from national or regional forest inventories (Paquette and Messier, 2011; Gamfeldt et al., 2013; Toigo et al., 2015) where variations due to site, silvicultural practices and disturbance history can be confounding factors (Forrester and Pretzsch, 2015). In planted conifer forests in Britain and Ireland, the main use of mixtures was in the afforestation of nutrient-poor soils (Carey et al., 1988). Studies in the early decades of the last century had shown the importance of site cultivation and drainage combined with remedial phosphate fertilization for the establishment of tree species on these difficult sites (Zehetmayr, 1960). While this proved sufficient to establish less demanding species such as Scots pine (Pinus sylvestris L.), lodgepole pine (Pinus contorta Dougl.) and Japanese larch (Larix kaempferi (Lamb.) Carr.), growth of more productive species such as Sitka spruce (Picea sitchensis (Bong.) Carr.) rapidly stagnated (‘checked’) particularly in the presence of ericaceous vegetation such as heather (Calluna vulgaris (L.) Hull) (Morgan et al., 1992). This checked growth was caused by an antagonistic effect of heather upon spruce mycorrhiza resulting in nitrogen deficiency in the spruce (Robinson, 1972). As a result, depending upon the lithology, a combination of herbicide control of the heather and one or more applications of nitrogen could be required to achieve canopy closure in pure spruce stands (McIntosh, 1983; Taylor, 1991). This comparatively expensive establishment regime meant that managers of such sites were often forced to plant less productive species such as pines and larches. However, from the 1930s onwards, researchers had observed that where Sitka spruce was growing in close proximity to either Scots pine or Japanese larch, the growth of the spruce improved after an initial period of check and that the trees eventually closed canopy (Macdonald, 1936; Macdonald & Macdonald, 1952). This ‘nursing’ effect (Weatherell, 1957) was associated with improved nitrogen status of the spruce (O’Carroll, 1978) which reflected higher nitrogen concentrations in soils in the mixed stands (Carlyle & Malcolm, 1986) and this effect occurred irrespective of the nurse species. Provided that the nurse species was not too vigorous, which was a problem with faster growing provenances of lodgepole pine (Garforth, 1979), once canopy closure had occurred the mixed stands were expected to progressively self-thin towards a spruce dominated stand (Carey et al., 1988). As a result, the use of ‘nursing’ mixtures where Sitka spruce was planted in combination with either pine or larch became a recommended practice for afforestation and reforestation regimes on the most nutrient-poor soils in upland regions of the British Isles (Carey et al., 1988; Taylor, 1991; Smith & McKay, 2002). The nursing benefit provided by pines and larches was also reported to occur when these species were grown with a number of broadleaved species on nutrient-poor soils (Gabriel et al., 2005) and recently has been found on more fertile sites where nitrogen deficiency would not have been anticipated (Mason & Connolly, 2014). Despite this history, there is little information about the long-term effects of these mixtures upon stand growth and productivity, with most studies being concentrated in stands 15–25 years of age (Carey et al., 1988; Morgan et al., 1992). A pair of studies on the impacts of nursing mixtures upon the growth and wood properties of 25–30-years-old Sitka spruce found that Japanese larch had the greatest positive influence upon spruce diameter and volume increment (Watson & Cameron, 1995; Cameron & Watson, 1999), but resulted in spruce with wider annual rings, larger branch diameters and more detrimental knot characteristics. They concluded that pines would be preferable species for nursing Sitka spruce because of fewer negative impacts on timber quality, particularly if sawlog production was envisaged (Cameron & Watson, 1999). However, a recent study in northern Scotland found no difference in harvested volume or sawlog outturn from 41-year-old self-thinning mixtures of Sitka spruce and larch when compared with pure Sitka spruce stands (Mason, 2014). The objective of this paper is to examine the growth and productivity of nursing mixtures with Sitka spruce from early establishment until close to rotation age using results from four experiments on soil types of differing fertility. Our working hypothesis involved the following: (1) there would be better growth and productivity at time of canopy closure of nursing mixtures over pure Sitka spruce without nitrogen, (2) this would be reflected in improved foliage nutrient status of the Sitka spruce growing in mixture, (3) any growth improvements found at canopy closure would be sustained over the remainder of the rotation and (4) that these effects would be mediated by site fertility. Materials and methods General The four experiments examined in this paper were all established in the north of Scotland (Figure 1) between 1965 and 1969 as part of a wider research programme that explored different silvicultural practices relevant to the establishment of forests on nutrient-poor acid soils. Three of the experiments (Strathy, Drumtochty and Inchnacardoch) were designed mainly to investigate the effects of different levels of nitrogen input on the growth and development of Sitka spruce. The fourth (Culloden) was primarily intended to study the effects of varying intensities of cultivation and drainage upon the growth of conifer stands. As a result, there is a range of treatments represented in these experiments (Table 1) and only two experiments (Strathy and Drumtochty) share a common design. However, the important feature for this paper is that all four experiments contain one or more mixture treatment and a pure Sitka spruce control, fully randomized into the design. Table 1 Details of the subsidiary experimental treatments (i.e. not including mixtures) used in the four experiments Experiment name and number1  Fertilizer (code)  Herbicide (code)  Assessment history  Strathy 6  ANNUAL (NA) applications (13 in total) of nitrogen to pure Sitka spruce until canopy closure – rate 168 kg N ha−1: applications in 1966–1971, 1973, 1975, 1977, 1980, 1983, 1986, 1989. PERIODIC(NP – rate as NA) applications (5 in total) of nitrogen until canopy closure: applications in 1966, 1972, 1977, 1982, 1987 NONE (0) – no nitrogen application  Control of competing heather in pure Sitka spruce plots (W); no herbicide treatment (V). Herbicide applied in 1969 and 1971  Height at 3, 6, 17, 20, 25, 47 years; dbh and basal area at 25, 30, 32 and 47 years  Drumtochty 28  ANNUAL (NA) applications (11 in total) of nitrogen to pure Sitka spruce until canopy closure – rate 168 kg N ha−1: applications in 1968–1971, 1973, 1975, 1977, 1979, 1981, 1983, and 1986. PERIODIC (NP – rate as NA) applications (4 in total) of nitrogen until canopy closure: applications in 1968, 1973, 1977, and 1984 NONE (0) – no nitrogen application  Control of competing heather in pure Sitka spruce plots (W); no herbicide treatment (V) Treatment applied in 1971  Height at 3, 6, 10, 16, 30, 35, 44 years; dbh and basal area at 16–25, 29, 35 and 44 years  Inchnacardoch 164  PERIODIC (NP) applications (5 in total) of nitrogen to pure and mixed plots – rate 168 kg N ha−1 in 1973, 1976, 1979, 1982 and 1986; NONE (0) – no application of nitrogen  N/A  Height at 9, 12, 15, 18, 20 and 45 years; dbh and basal area at 20–23, 25, 30, 35 and 45 years  Experiment name and number  Cultivation treatment (code)  Drainage treatment (code)  Assessment history  Culloden 2  Shallow spaced furrow ploughing (ST); Complete shallow ploughing (CST); Deep spaced furrow ploughing (DT); Complete deep ploughing (DT)  Drains (75 cm deep) at 40 m spacing (D40); drains as above at 20 m spacing (D20)  Height at 3, 6, 10, 12, 18, 20, 34 and 42 years; dbh and basal area at 12, 20, 34 and 42 years  Experiment name and number1  Fertilizer (code)  Herbicide (code)  Assessment history  Strathy 6  ANNUAL (NA) applications (13 in total) of nitrogen to pure Sitka spruce until canopy closure – rate 168 kg N ha−1: applications in 1966–1971, 1973, 1975, 1977, 1980, 1983, 1986, 1989. PERIODIC(NP – rate as NA) applications (5 in total) of nitrogen until canopy closure: applications in 1966, 1972, 1977, 1982, 1987 NONE (0) – no nitrogen application  Control of competing heather in pure Sitka spruce plots (W); no herbicide treatment (V). Herbicide applied in 1969 and 1971  Height at 3, 6, 17, 20, 25, 47 years; dbh and basal area at 25, 30, 32 and 47 years  Drumtochty 28  ANNUAL (NA) applications (11 in total) of nitrogen to pure Sitka spruce until canopy closure – rate 168 kg N ha−1: applications in 1968–1971, 1973, 1975, 1977, 1979, 1981, 1983, and 1986. PERIODIC (NP – rate as NA) applications (4 in total) of nitrogen until canopy closure: applications in 1968, 1973, 1977, and 1984 NONE (0) – no nitrogen application  Control of competing heather in pure Sitka spruce plots (W); no herbicide treatment (V) Treatment applied in 1971  Height at 3, 6, 10, 16, 30, 35, 44 years; dbh and basal area at 16–25, 29, 35 and 44 years  Inchnacardoch 164  PERIODIC (NP) applications (5 in total) of nitrogen to pure and mixed plots – rate 168 kg N ha−1 in 1973, 1976, 1979, 1982 and 1986; NONE (0) – no application of nitrogen  N/A  Height at 9, 12, 15, 18, 20 and 45 years; dbh and basal area at 20–23, 25, 30, 35 and 45 years  Experiment name and number  Cultivation treatment (code)  Drainage treatment (code)  Assessment history  Culloden 2  Shallow spaced furrow ploughing (ST); Complete shallow ploughing (CST); Deep spaced furrow ploughing (DT); Complete deep ploughing (DT)  Drains (75 cm deep) at 40 m spacing (D40); drains as above at 20 m spacing (D20)  Height at 3, 6, 10, 12, 18, 20, 34 and 42 years; dbh and basal area at 12, 20, 34 and 42 years  Notes: 1. The number following the experiment (forest) name is a unique identifier. Thus, ‘Strathy 6’ indicates the sixth experiment established in Strathy forest. Figure 1 View largeDownload slide A map of Great Britain showing the approximate location of the four experiments described in this paper. Figure 1 View largeDownload slide A map of Great Britain showing the approximate location of the four experiments described in this paper. Sites The experiments were all located on afforestation sites that were typical of many areas planted in upland Britain in the second half of the last century, being in cool, exposed locations with relatively high levels of rainfall (Supplementary data, Table 1). Three sites (Strathy, Inchnacardoch and Culloden) were located on moist soils of very low nutrient availability, while the remaining site (Drumtochty) was on a slightly drier and more fertile site (Supplementary data, Table 1). Apart from the experimental treatments, all other aspects of experiment establishment and management followed normal procedures characteristic of upland afforestation (Hibberd, 1991) with the site being cultivated before planting and trees planted either on the upturned ridge (Strathy, Inchnacardoch and Culloden) or in the side of the furrow (Drumtochty) as appropriate for the soil type. Planting stock was either 2 or 3-years-old bare-root transplants depending on species. Any failures were replaced with plants of the same species in the years following planting to ensure full stocking in the plots. There was serious frost damage to Sitka spruce and larch 8 years after planting at Strathy (Supplementary data Table 2). No thinning has taken place in any of the experiments. Experimental treatments and design (see also Table 1) At Strathy and Drumtochty the design contrasted pure Sitka spruce given all combinations of three intensities of nitrogen fertilizer application (annual (NA), periodic (NP) or none (0)) and two herbicide regimes (treatment of heather or none) with Sitka spruce planted in two different nursing mixtures. However the combination of Sitka spruce given no nitrogen fertilizer and no herbicide input was considered ‘impractical’ and not implemented: therefore these experiments contained seven treatments replicated three times in a randomized block design (i.e. 21 plots) with a plot size of 0.27–0.1 ha and an internal assessment plot of 0.032 ha (Strathy) or 0.04 ha (Drumtochty). The initial design at Inchnacardoch involved pure Sitka spruce and two nursing mixtures replicated five times in 0.12 ha plots. From 1973, all three treatments were randomly split with one split plot being given periodic nitrogen application (NP) and the other receiving no nitrogen (0) (i.e. 30 plots). All split plots had an internal assessment plot of 0.02 ha. At Culloden, the design compared two species (i.e. pure Sitka spruce or a nursing mixture) by four cultivation treatments (different intensities and depths of ploughing) with the eight combinations split for two different drain spacings (40 m or 20 m). There were four replicate blocks but the shallow ploughing was confined to two of the blocks and the deep ploughing was located in the other two (i.e. a total of 32 plots). Plot size was 0.12 ha with an internal assessment plot of 0.04 ha. The nurse species represented in the four experiments were: hybrid larch and lodgepole pine (Alaskan provenance) at Strathy, Japanese larch and lodgepole pine (Alaskan provenance) at Drumtochty and Inchnacardoch, and Scots pine at Culloden. Two patterns of mixture were used: at Strathy, Drumtochty and Inchnacardoch a 3:1 nurse–Sitka spruce ratio was achieved by alternating pure rows of the nurse species with rows where the nurse and Sitka spruce were planted in successive groups of three plants. At Culloden, Scots pine and Sitka spruce were planted in a 1:1 ratio in a three row: three row mixture of each species. Growth measurements The assessment history varied between these experiments reflecting their different objectives and history (Table 1). In general, the early assessments up to about year 10 were only for height and were carried out on a whole plot basis. From about year 15, a central assessment plot was located in each treatment where measurements of top height, mean dbh and basal area were made. Early growth data from two experiments (Inchnacardoch, Culloden) were published over two decades ago (Carey et al.,1988; Morgan et al., 1992). Foliage analysis Because of interest in the nutritional response of the Sitka spruce to the various treatments, foliage analysis of the spruce trees was carried out on a regular basis in all experiments from ~3 to 5 years after planting until the early 1990s. One final analysis was carried out in 2010 at Strathy, Inchnacardoch and Culloden. Foliage samples were collected in October–November each year from 15 dominant or co-dominant trees per plot up to canopy closure and this sample was reduced to five trees for later assessments. Although the nutrient levels of all macronutrients in the Sitka spruce foliage were analysed, in this paper we concentrate upon the nitrogen status since this is the best indicator of satisfactory tree growth (Taylor, 1991). Interpretation of the results for foliar nitrogen concentrations used standard levels (per cent dry weight) as defined by Taylor (1991) as follows: >1.5 – optimal levels; 1.2–1.5 – marginal; <1.2 – deficient; <1.0 – severely checked growth. In addition to experimental fertilizer treatments (see above and Table 1) all treatments in these experiments received a standard fertilizer regime recommended for upland afforestation (Taylor, 1991) involving up to three phosphorus applications during the establishment phase supplemented by potassium on the poorest sites (Supplementary data, Table 2). Data analysis Analysis of the data differed between the experiments. At Strathy and Drumtochty, which had similar designs, we used analysis of variance procedures for a randomized block design, separating overall treatment effects into: those with no nitrogen application (i.e. control and the two mixture treatments); the two different intensities of nitrogen application; the two different rates of herbicide application; and any interaction between the intensities of nitrogen application and the herbicide treatment. This analysis was carried out twice, once on a whole plot basis including the growth of the nurses in the mixed plots, and the other time only examining the response of the Sitka spruce. The procedure at Inchnacardoch was similar except that the analysis reflected that the design was a split-plot with the species/mixtures as the main plots and the presence or absence of nitrogen application being the secondary factor. At Culloden, the responses observed in blocks one and two were examined separately from those found in blocks three and four because the ploughing treatments were confounded with the blocks (see above and Supplementary data, Table 2). Inspection of the data showed a consistent response between the two parts of the experiment and therefore for ease of presentation only the results from blocks one and two are presented here. Here, we again carried out the analysis twice, namely with and without the nurse species. In the experiments at Strathy, Drumtochty and Inchnacardoch with a more complicated experimental structure we used Fisher’s Unprotected LSD to test for significant differences between treatments. This method sets the significance level for each pairwise comparison at 5 per cent, but is reported only for those instances where the overall analysis of variance had proved significant. The data were analysed independently on a date by date basis and no time by treatment interaction was investigated. This was because trends over time were readily apparent in the results. All data analyses were conducted using the statistical software Genstat (VSN International, 2013) Results Growth and basal area Drumtochty There were significant differences in height of Sitka spruce between treatments from the ages of 6 to 29 years, reflecting better growth where nitrogen had been applied, and lesser growth of spruce in mixture, especially in the mixtures with lodgepole pine (Table 2a). A significant difference in height at 44 years was due to the spruce in mixture with larch being taller while the spruce mixed with pine was smaller. There were no differences in response to the two periodicities of nitrogen fertilization, but there was a significant increase in height at 10 years following herbicide application (P < 0.01). At 16 years after planting, the difference in height between the spruce and each of the nurse species in the mixed plots was less than 1 m (data not shown). Table 2 Growth in height and diameter at breast height (dbh) for 44 years at Drumtochty (a) or for 47 years at Strathy (b) of Sitka spruce grown either in mixture or in pure plots with various combinations of nitrogen fertilizer and herbicide application (see footnote for treatement codes) (a) Drumtochty    Height (m)  Dbh (cm)  Age (years)  3  6  10  16  29  35  44  16  20  25  29  35  44  Treatment   With larch nurse  0.7  1.4ab  2.8b  7.1bc  16.0b  19.3  26.1b  9.1c  12.8d  16.5c  19.5c  21.7b  29.1b   With pine nurse  0.7  1.4a  2.1a  5.2a  13.8a  18.0  22.8a  6.2a  9.4a  13.2b  17.1b  21.2b  29.4b   NAV  0.7  1.6bc  3.5cde  7.5c  15.7b  19.1  22.6a  9.3c  11.4c  13.2b  14.4a  15.2a  20.3a   NAW  0.8  1.7c  3.8e  7.4c  16.2b  19.2  24.4ab  9.8c  11.8cd  13.4b  14.6a  15.3a  20.3a   NPV  0.7  1.7c  3.2bc  6.7bc  15.4b  18.4  24.0ab  8.8bc  11.0bc  13.1ab  14.5a  15.4a  20.8a   NPW  0.7  1.7c  3.7de  7.3bc  15.9b  19.3  22.9a  9.6c  11.5c  13.4b  14.6a  15.5a  20.2a   OW- control  0.8  1.6c  3.3cd  6.5b  14.9ab  18.4  24.0ab  7.6b  9.8ab  12.0a  13.5a  14.5a  19.8a   Significance  ns  **  ***  **  *  ns  *  ***  ***  ***  ***  ***  ***   SED  0.05  0.07  0.20  0.40  0.65  0.77  0.98  0.62  0.57  0.54  0.78  0.74  0.67   5% LSD  0.11  0.16  0.42  0.86  1.41  1.68  2.12  1.35  1.25  1.18  1.70  1.61  1.50  (b) Strathy    Height (m)  Dbh (cm)  Age (years)  3  6  17  20  25  47  25  30  47          Treatment   With larch nurse  0.6cd  1.7c  4.9d  6.5c  10.5  18.3bc  13.9b  15.4c  20.9c           With pine nurse  0.6bcd  1.6bc  4.7cd  6.4c  10.7  20.3cd  9.9a  12.1b  18.6c           NAV  0.5a  1.4a  4.1bc  5.1b  9.9  21.1d  11.5a  13.6bc  20.2c           NAW  0.5ab  1.4a  3.7 b  4.7b  9.7  19.8bcd  11.1a  13.5 bc  19.5c           NPV  0.5ab  1.5ab  4.3 bcd  5.3b  9.8  17.7b  10.3a  11.8b  15.1b           NPW  0.6abc  1.5abc  4.7cd  6.4c  10.9  20.3cd  11.9ab  13.4bc  18.0bc           OW – control  0.7d  1.7c  2.5a  2.6a  Nd  9.0a  Nd  5.5a  8.2a           Significance  **  **  ***  ***  ns  ***  *  ***  ***           SED  0.04  0.08  0.30  0.47  0.66  1.04  1.02  1.13  1.46           5% LSD  0.08  0.18  0.65  1.02  1.47  2.26  2.26  2.47  3.17          (a) Drumtochty    Height (m)  Dbh (cm)  Age (years)  3  6  10  16  29  35  44  16  20  25  29  35  44  Treatment   With larch nurse  0.7  1.4ab  2.8b  7.1bc  16.0b  19.3  26.1b  9.1c  12.8d  16.5c  19.5c  21.7b  29.1b   With pine nurse  0.7  1.4a  2.1a  5.2a  13.8a  18.0  22.8a  6.2a  9.4a  13.2b  17.1b  21.2b  29.4b   NAV  0.7  1.6bc  3.5cde  7.5c  15.7b  19.1  22.6a  9.3c  11.4c  13.2b  14.4a  15.2a  20.3a   NAW  0.8  1.7c  3.8e  7.4c  16.2b  19.2  24.4ab  9.8c  11.8cd  13.4b  14.6a  15.3a  20.3a   NPV  0.7  1.7c  3.2bc  6.7bc  15.4b  18.4  24.0ab  8.8bc  11.0bc  13.1ab  14.5a  15.4a  20.8a   NPW  0.7  1.7c  3.7de  7.3bc  15.9b  19.3  22.9a  9.6c  11.5c  13.4b  14.6a  15.5a  20.2a   OW- control  0.8  1.6c  3.3cd  6.5b  14.9ab  18.4  24.0ab  7.6b  9.8ab  12.0a  13.5a  14.5a  19.8a   Significance  ns  **  ***  **  *  ns  *  ***  ***  ***  ***  ***  ***   SED  0.05  0.07  0.20  0.40  0.65  0.77  0.98  0.62  0.57  0.54  0.78  0.74  0.67   5% LSD  0.11  0.16  0.42  0.86  1.41  1.68  2.12  1.35  1.25  1.18  1.70  1.61  1.50  (b) Strathy    Height (m)  Dbh (cm)  Age (years)  3  6  17  20  25  47  25  30  47          Treatment   With larch nurse  0.6cd  1.7c  4.9d  6.5c  10.5  18.3bc  13.9b  15.4c  20.9c           With pine nurse  0.6bcd  1.6bc  4.7cd  6.4c  10.7  20.3cd  9.9a  12.1b  18.6c           NAV  0.5a  1.4a  4.1bc  5.1b  9.9  21.1d  11.5a  13.6bc  20.2c           NAW  0.5ab  1.4a  3.7 b  4.7b  9.7  19.8bcd  11.1a  13.5 bc  19.5c           NPV  0.5ab  1.5ab  4.3 bcd  5.3b  9.8  17.7b  10.3a  11.8b  15.1b           NPW  0.6abc  1.5abc  4.7cd  6.4c  10.9  20.3cd  11.9ab  13.4bc  18.0bc           OW – control  0.7d  1.7c  2.5a  2.6a  Nd  9.0a  Nd  5.5a  8.2a           Significance  **  **  ***  ***  ns  ***  *  ***  ***           SED  0.04  0.08  0.30  0.47  0.66  1.04  1.02  1.13  1.46           5% LSD  0.08  0.18  0.65  1.02  1.47  2.26  2.26  2.47  3.17          At any given age, treatments with different letter suffices are significantly different from one another. Treatment codes are: NAV, annual applications of nitrogen and no vegetation control; NAW, annual application of nitrogen plus weed control; NPV, periodic application of nitrogen and no vegetation control; NPW, periodic application of nitrogen plus weed control; OW, no application of nitrogen plus weed control. For details of timings and rates of applications of fertilizer and/or herbicides, see Table 1. Nd indicates ‘no data’ where a particular treatment was not measured. Significance values are presented as: ns = non-significant; *P < 0.05; **P < 0.01; ***P < 0.001. Diameter growth followed a similar pattern with highly significant differences in the early years due to larger values in nitrogen treatments or in trees in mixture with larch. However, by year 44 the major difference was the greater diameter of spruce in both mixtures compared with all the pure spruce plots. There were no effects due to timing of nitrogen input or herbicide application. There were highly significant differences in basal area between treatments at all ages (Table 3). For up to 35 years, this reflected better growth in the nitrogen treatments and lower productivity in both mixtures. At the last assessment, the difference was due to continued better growth in some of the nitrogen treatments plus the poorer performance of the mixture with lodgepole pine, which may reflect the loss of one plot of this mixture before the last assessment (Supplementary Table 2). Over the period of assessment, Sitka spruce progressed from being a 25 per cent component to becoming the dominant species in both mixtures. Table 3 Basal area production (m2 ha−1) of plots of nursing mixtures with Sitka spruce and of pure spruce plots given various combinations of nitrogen fertilizer and herbicide application at Drumtochty and Strathy   Drumtochty  Strathy  Age  16  20  25  29  35  44  25  30  47  Treatment   With larch nurse  20.4b (0.37)  32.7ab (0.45)  48.2ab (0.51)  59.9a (0.59)  71.3a (0.62)  77.1bc (0.78)  29.3 (0.73)  34.5b (0.77)  36.3b (1.0)   With pine nurse  13.0a (0.24)  24.9a (0.29)  40.1a (0.35)  54.5a (0.47)  66.5a (0.55)  53.0a (0.93)  49.3 (0.19)  59.9c (0.22)  70.8c (0.3)   NAV  28.3c  42.6c  58.2c  69.9bc  79.5bc  68.9b  39.9  50.0c  58.0c   NAW  31.6c  46.1c  60.9c  73.5c  82.6c  81.2c  43.0  54.9c  67.8c   NPV  25.3bc  39.8bc  56.8bc  70.4bc  80.9bc  80.6c  39.9  50.5c  62.5c   NPW  29.5c  42.6c  58.0c  70.7bc  80.8bc  74.8bc  43.3  54.0c  69.9c   OW – control  19.8ab  32.7ab  48.6ab  62.4ab  73.5ab  74.3bc  Nd  10.4a  17.8a   Significance  **  **  **  **  **  **  ns  ***  ***   SED  3.25  3.94  4.06  3.88  3.73  4.78  5.32  5.71  6.26   5% LSD  7.08  8.59  8.86  8.45  8.12  10.65  11.87  12.44  13.63    Drumtochty  Strathy  Age  16  20  25  29  35  44  25  30  47  Treatment   With larch nurse  20.4b (0.37)  32.7ab (0.45)  48.2ab (0.51)  59.9a (0.59)  71.3a (0.62)  77.1bc (0.78)  29.3 (0.73)  34.5b (0.77)  36.3b (1.0)   With pine nurse  13.0a (0.24)  24.9a (0.29)  40.1a (0.35)  54.5a (0.47)  66.5a (0.55)  53.0a (0.93)  49.3 (0.19)  59.9c (0.22)  70.8c (0.3)   NAV  28.3c  42.6c  58.2c  69.9bc  79.5bc  68.9b  39.9  50.0c  58.0c   NAW  31.6c  46.1c  60.9c  73.5c  82.6c  81.2c  43.0  54.9c  67.8c   NPV  25.3bc  39.8bc  56.8bc  70.4bc  80.9bc  80.6c  39.9  50.5c  62.5c   NPW  29.5c  42.6c  58.0c  70.7bc  80.8bc  74.8bc  43.3  54.0c  69.9c   OW – control  19.8ab  32.7ab  48.6ab  62.4ab  73.5ab  74.3bc  Nd  10.4a  17.8a   Significance  **  **  **  **  **  **  ns  ***  ***   SED  3.25  3.94  4.06  3.88  3.73  4.78  5.32  5.71  6.26   5% LSD  7.08  8.59  8.86  8.45  8.12  10.65  11.87  12.44  13.63  Values for the nursing mixtures are the combined values for both nurse and Sitka spruce: the values in parentheses show the proportion of Sitka spruce in the mixture. In any given column, treatments with different letter suffices are significantly different from one another. Treatment codes are: NAV, annual applications of nitrogen and no vegetation control; NAW, annual application of nitrogen plus weed control; NPV, periodic application of nitrogen and no vegetation control; NPW, periodic application of nitrogen plus weed control; OW, no application of nitrogen plus weed control. For details of timings and rates of applications of fertilizer and/or herbicides, see Table 1. Nd indicates ‘no data’ where a particular treatment was not measured. Significance values are presented as: ns = non-significant; *P < 0.05; **P < 0.01; ***P < 0.001. Strathy On this northerly site, there were significant differences in height at most ages of assessment, which mainly reflected the much poorer growth of Sitka spruce in the control plots where no nitrogen was applied, especially from age 17 (Table 2b). At years 17 and 20, there was a significant difference between the two timings of nitrogen input (P < 0.05) with spruce trees given periodic inputs being taller than those with annual applications. At time of canopy closure (17 years), the differences in height between the spruce and each of the nurse species in the mixed plots were less than 1 m (data not shown). Patterns of diameter growth were similar to those for height, with the smaller values in the control being the most significant difference. In addition, spruce trees in the NPV treatment also tended to be smaller in diameter than those in the remainder of the nitrogen treatments and in the mixtures. Basal area values were consistently much lower in the control than in the other treatments (Table 3). It was also lower in the mixture with larch, with no differences between the other treatments. Sitka spruce became the dominant component of the mixture with larch, but formed only 30 per cent of the mixture with lodgepole pine. Inchnacardoch In this split-plot experiment, the only effect of species/mixture upon height growth was at 20 years when the Sitka spruce in the pure plots was significantly smaller (8.0 m vs 9.5 m and 9.4 m for the spruce in the larch and pine mixtures, respectively: P < 0.05) (Table 4). There was a positive height growth response to nitrogen at years 15 and 20 (P < 0.001). At 18 years after planting, the difference in height between the spruce and each of the nurse species in the mixed plots was less than 2 m (data not shown). By contrast, there was a significant effect of species/mixture upon Sitka spruce diameter at all years of assessment (P < 0.01) with the trees in the pure plots always having a lower diameter than those in mixture (thus at 45 years diameter of pure spruce was 14.1 cm vs 24.5 cm and 22.8 cm, respectively, for spruce in mixture with larch and pine). Diameter of spruce was always greater in the presence of nitrogen (P < 0.01 or 0.001). In the early years of this experiment, these effects were generally due to the poorer performance of the pure spruce plots where no nitrogen had been applied (Table 4). Table 4 45 year growth in height and diameter at breast height (dbh) at Inchnacardoch of Sitka spruce grown either in mixture or in pure plots with (+) or without (–) nitrogen (N) fertilizer     Height (m)  Dbh (cm)    Age (years)  9  15  20  45  20  25  30  35  45  Treatment   With larch nurse  +N  1.9a  5.7c  10.2b  23.9  13.6d  17.5e  19.8e  21.4  25.6  –N  1.7b  4.4b  8.9b  21.8  10.6bc  14.4cd  17.0cd  19.2  23.5   With pine nurse  +N  1.6b  5.2bc  9.5b  23.7  11.1c  15.0de  17.6de  19.4  23.7  –N  1.8ab  4.8bc  9.4b  24.3  9.0b  12.2bc  14.9bc  17.2  22.0   Pure  +N  1.8ab  5.1bc  9.8b  20.9  9.8bc  11.5b  12.5b  13.2  16.3  –N  1.7b  2.9a  6.3a  19.2  5.5a  6.4a  7.5a  8.7  11.9   Significance  *  **  **  ns  *  *  *  ns  ns   SED  0.09  0.32  0.63  1.16  0.40  0.54  0.63  0.69  1.24   5% LSD  0.19  0.69  1.38  2.54  0.91  1.20  1.40  1.54  2.72      Height (m)  Dbh (cm)    Age (years)  9  15  20  45  20  25  30  35  45  Treatment   With larch nurse  +N  1.9a  5.7c  10.2b  23.9  13.6d  17.5e  19.8e  21.4  25.6  –N  1.7b  4.4b  8.9b  21.8  10.6bc  14.4cd  17.0cd  19.2  23.5   With pine nurse  +N  1.6b  5.2bc  9.5b  23.7  11.1c  15.0de  17.6de  19.4  23.7  –N  1.8ab  4.8bc  9.4b  24.3  9.0b  12.2bc  14.9bc  17.2  22.0   Pure  +N  1.8ab  5.1bc  9.8b  20.9  9.8bc  11.5b  12.5b  13.2  16.3  –N  1.7b  2.9a  6.3a  19.2  5.5a  6.4a  7.5a  8.7  11.9   Significance  *  **  **  ns  *  *  *  ns  ns   SED  0.09  0.32  0.63  1.16  0.40  0.54  0.63  0.69  1.24   5% LSD  0.19  0.69  1.38  2.54  0.91  1.20  1.40  1.54  2.72  Note that the significance values presented are for the split-plot analysis and not for the main treatment effects. At any given age, treatments with different letter suffices are significantly different from one another. Significance values are presented as: ns, non-significant; *P < 0.05; **P < 0.01; ***P < 0.001. Analysis of whole treatment basal area showed a significant effect of species/mixture at years 25–35 (P < 0.05) due to lower values in the pure spruce plots compared with the mixtures with lodgepole pine. There was a very highly beneficial effect of nitrogen in all years (P < 0.001). There were interactions evident (Table 5) since the production of pure spruce without nitrogen was consistently low. The values in the mixture with Japanese larch without nitrogen were lower than in the same mixture when nitrogen was applied, while no such response was evident in the mixture with lodgepole pine. Both mixed treatments self-thinned towards dominance by Sitka spruce. However, this rate was faster in the mixture with Japanese larch and in the presence of nitrogen (Table 5). Table 5 Basal area production (m2 ha−1) of plots of nursing mixtures with Sitka spruce and of pure spruce plots at Inchnacardoch with (+) and without (–) nitrogen (N) fertilizer application     Basal area    Age (years)  20  25  30  35  45  Treatment   With larch nurse  +N  29.9 (0.66)c  45.0 (0.73)c  55.2 (0.76)c  62.6 (0.79)c  62.8 (0.98)c  –N  18.2 (0.54)b  29.9 (0.61)b  38.9 (0.67)b  47.0 (0.71)b  50.9 (0.92)ab   With pine nurse  +N  30.6 (0.37)c  45.1 (0.46)c  55.3 (0.52)c  63.6 (0.56)c  62.1 (0.75)bc  –N  28.8 (0.31)c  42.7 (0.36)c  53.8 (0.43)c  63.8 (0.49)c  61.0 (0.76)bc   Pure  +N  35.4c  49.7c  59.5c  66.4c  70.8c  –N  7.3a  12.4a  17.3a  23.9a  37.0a   Significance  *  ***  ***  ***  **   SED  2.56  3.50  4.32  4.82  4.83   5% LSD  5.58  7.63  9.43  10.50  10.52      Basal area    Age (years)  20  25  30  35  45  Treatment   With larch nurse  +N  29.9 (0.66)c  45.0 (0.73)c  55.2 (0.76)c  62.6 (0.79)c  62.8 (0.98)c  –N  18.2 (0.54)b  29.9 (0.61)b  38.9 (0.67)b  47.0 (0.71)b  50.9 (0.92)ab   With pine nurse  +N  30.6 (0.37)c  45.1 (0.46)c  55.3 (0.52)c  63.6 (0.56)c  62.1 (0.75)bc  –N  28.8 (0.31)c  42.7 (0.36)c  53.8 (0.43)c  63.8 (0.49)c  61.0 (0.76)bc   Pure  +N  35.4c  49.7c  59.5c  66.4c  70.8c  –N  7.3a  12.4a  17.3a  23.9a  37.0a   Significance  *  ***  ***  ***  **   SED  2.56  3.50  4.32  4.82  4.83   5% LSD  5.58  7.63  9.43  10.50  10.52  Values for the nursing mixtures are the combined values for both nurse and Sitka spruce: the values in parentheses show the proportion of Sitka spruce in the mixture. Note that the significance values presented are for the split-plot analysis and not for the main treatment effects. At any given age, treatments with different letter suffices are significantly different from one another. Significance values are presented as: ns, non-significant; *P < 0.05; **P < 0.01; ***P < 0.001. Culloden From 12 years after planting, there were differences in height growth between the pure spruce plots and the spruce grown in mixture with Scots pine with the latter being the taller: these differences were highly significant in the years up to canopy closure (Table 6). At time of canopy closure (18 years), the differences in height between the spruce and the nurse species in the mixed plots were less than 1 m (data not shown). Table 6 Growth of Sitka spruce when planted pure and in mixture with Scots pine at Culloden Parameter  Height (m)  Dbh (cm)  Basal area (m2 ha−1)  Age (years)  3  6  12  20  34  42  12  34  42  34  42  Treatment   Pure spruce  0.4  1.3  3.3  7.0  15.9  21.8  5.0  10.5  13.3  33.5  43.7   In mixture with Scots pine  0.5  1.3  5.2  10.8  18.7  24.0  5.9  24.1  28.1  60.9 (0.57)  64.6 (0.67)   Significance  **  ns  ***  **  ns  ns  ns  **  **  **  *   SED  0.02  0.05  0.1  0.4  1.4  0.7  0.4  1.4  1.4  3.5  4.9   5% LSD  0.07  0.07  0.4  1.4  4.4  2.2  1.3  4.4  4.5  11.2  15.6  Parameter  Height (m)  Dbh (cm)  Basal area (m2 ha−1)  Age (years)  3  6  12  20  34  42  12  34  42  34  42  Treatment   Pure spruce  0.4  1.3  3.3  7.0  15.9  21.8  5.0  10.5  13.3  33.5  43.7   In mixture with Scots pine  0.5  1.3  5.2  10.8  18.7  24.0  5.9  24.1  28.1  60.9 (0.57)  64.6 (0.67)   Significance  **  ns  ***  **  ns  ns  ns  **  **  **  *   SED  0.02  0.05  0.1  0.4  1.4  0.7  0.4  1.4  1.4  3.5  4.9   5% LSD  0.07  0.07  0.4  1.4  4.4  2.2  1.3  4.4  4.5  11.2  15.6  Basal area figures for the mixture are the total value for both species with the Sitka spruce proportion in parentheses. Height and diameter values for the mixed plot are for the Sitka spruce component only. There were significant differences (P < 0.01) in Sitka spruce diameter at years 34 and 42 due to the much larger size of the trees growing in mixture. At both these times, larger diameter of spruce was found when the trees were growing in the closer drainage treatment (P < 0.01). There was also a significant mixture/cultivation interaction (P < 0.01) due to the spruce in mixture having a larger diameter when growing on the spaced furrow ploughing (data not shown). The mixed plots had a much greater basal area than the pure plots at both years 34 and 42 (P < 0.01 and P < 0.05 respectively), indeed the spruce production in mixture was almost identical with that of the pure plots even though the latter contained twice as many spruce trees. By year 42 Sitka spruce represented an increasing proportion of the mixture. At year 34 basal area was higher on the complete ploughing (P < 0.05), while there was a significant interaction (P < 0.05) between mixture and drainage due to poorer growth of the pure spruce planted on the wider drain spacing (data not shown). Foliage analysis In Figure 2, the values for up to three representative treatments are presented over time at each experiment. At three of the sites (Inchnacardoch, Strathy, Culloden) the foliar nitrogen concentrations observed during the late establishment and early stem exclusion phase are significantly lower in the controls than in the mixture with pine, or where nitrogen was applied. The nitrogen concentrations observed in the controls were indicative of ‘deficient’ or ‘severely checked growth’ whereas those in the mixtures indicated a ‘marginal’ or ‘optimal’ status. However, at Drumtochty the differences between the control and the other treatments were less substantial. Figure 2 View largeDownload slide Comparison of Sitka spruce foliar nitrogen concentration (per cent dry weight) over time in selected treatments at the four experimental sites. Treatments are: Sitka spruce mixed with lodgepole or Scots pine, pure spruce with periodic nitrogen application, control (i.e. pure spruce with no nitrogen application). Open triangles indicate that there was a significant difference (P < 0.05) between the values in the control and those of the spruce in mixture; closed triangles indicate no significant difference. The upper dotted line shows the boundary between ‘optimal’ and ‘marginal’ nitrogen status; the lower dotted line gives that between ‘marginal’ and deficient’. Figure 2 View largeDownload slide Comparison of Sitka spruce foliar nitrogen concentration (per cent dry weight) over time in selected treatments at the four experimental sites. Treatments are: Sitka spruce mixed with lodgepole or Scots pine, pure spruce with periodic nitrogen application, control (i.e. pure spruce with no nitrogen application). Open triangles indicate that there was a significant difference (P < 0.05) between the values in the control and those of the spruce in mixture; closed triangles indicate no significant difference. The upper dotted line shows the boundary between ‘optimal’ and ‘marginal’ nitrogen status; the lower dotted line gives that between ‘marginal’ and deficient’. At Drumtochty, there were significant differences between treatments from age 6 until 21, but subsequent effects were non-significant (Supplementary data, Table 3a). Initially these differences reflected lower nitrogen status in spruce growing in mixture compared with all pure plots. Thereafter, lowest nitrogen levels were found in the control treatment while highest values occurred in plots where spruce was either mixed with Japanese larch or given regular nitrogen applications. At the last date of assessment (year 30), all treatments had optimal nitrogen foliar levels. By contrast, at Strathy there were significant differences between treatments at all ages. The highest values were generally found where nitrogen was applied annually, but from age 15 onwards the lowest values were consistently found in the controls and in the mixture with hybrid larch while higher values occurred in the mixture with lodgepole pine. At the last assessment (year 46), the control value would have been classed as ‘severely checked’ while all the other treatments were on the borderline between ‘deficient’ and ‘marginal’ (Supplementary data, Table 3b). In the Inchnacardoch experiment, there were significant differences in nitrogen status between the main ‘mixture’ treatments from ages 11 until 36 years reflecting consistently lower values in the pure spruce plots. There were also significant differences from 11 to 27 years between treatments given nitrogen as opposed to those with no nitrogen application. Interaction between the mixture and the nitrogen treatments occurred at intervals until year 30, reflecting lower nutrient levels in the pure spruce plots grown without nitrogen (Supplementary data, Table 4). At the last assessment, all treatments would have been classed as being ‘deficient’ in nitrogen. At Culloden, there were also significant differences from age 11 years until 22 years, but no difference at age 42 years. This reflected a consistently higher nitrogen status in the spruce grown in mixture with Scots pine. At the last assessment both treatments would have been classed as having ‘marginal’ nitrogen foliar concentrations. Discussion Results from these four experiments have indicated that, on nutrient-poor sites, the better growth of Sitka spruce in mixture compared with that of pure spruce without nitrogen was evident at ~10 and 20 years after planting (Tables 2, 4 and 6). This improvement resulted in longer-term increases in basal area in accord with our working hypothesis. These increases are also likely to have resulted in a greater volume outturn, but the lack of any measurements of stem taper in the different treatments means that a linear relationship between basal area and volume should not be assumed (Pretzsch, 2009, Chapter 9). The magnitude and duration of these basal area gains is mediated by site quality since on the more nutrient-poor sites at Strathy, Inchnacardoch and Culloden, basal area after more than 40 years was higher in the mixed plots than in the pure spruce without nitrogen (Tables 3, 5 and 6). In contrast, on a more fertile site at Drumtochty, over time there was no difference between the nursing mixture with Japanese larch and the pure spruce without nitrogen (Table 3), while the nursing mixture with lodgepole pine had a lower basal area. In another experiment in the same region where the soil nutrient regime was on the border between ‘poor’ and ‘very poor’, basal area was higher in a larch–Sitka spruce mixture than pure spruce at 20 and 25 years of age but there were no differences evident at 41 years (Mason, 2014). In terms of soil fertility, that site would have been intermediate between Drumtochty and the three other experiments presented in the present paper which were all located in the most nutrient-poor category of the ESC system (see Supplementary data Table 1). In general, all mixtures have self-thinned towards domination by Sitka spruce, although the proportion of the basal area occupied by the spruce has varied between sites and with species used as a nurse. Except on the most fertile site at Drumtochty, there was a tendency for the larch species used as a nurse to die out sooner than the pines (Tables 3 and 5), possibly reflecting a lower tolerance of competition and of the wetter peat soils at Strathy and Inchnacardoch. One consequence of this self-thinning process was that at the last assessment the average diameter of the spruce grown in nursing mixture was nearly always larger than that of the spruce grown in pure plots (Tables 2, 4 and 6). This increase could be an indicator of improved wind stability in mixture since, everything else being equal, tree resistance to overturning by wind is proportionate to the dbh of a tree squared (Gardiner et al., 1997). The temporal pattern of foliar nitrogen values whereby a decline is followed by a slow recovery after canopy closure mediated by site fertility agrees with results from a series of other fertilizer experiments on second rotation sites across northern Britain (Smith and McKay, 2002). The period of deficiency can be offset by remedial fertilizer application as shown by the higher foliar concentrations in spruce trees from treatments where nitrogen was applied at Drumtochty and Inchnacardoch (supplementary data, Tables 3 and 4). However, this may not be a cost-effective option, given that the cost of a single aerial fertilizer application can exceed £200 ha−1 (pers. comm. Bill Rayner, Forest Research) and that several such inputs might be needed to ensure canopy closure of a pure spruce stand on a nutrient-poor site (Taylor, 1991). In addition, certification protocols favour a reduction in the use of synthetic chemicals such as nitrogen fertilizer in British forests (Smith and McKay, 2002). By contrast, the spruce trees in the nursing mixtures experienced an initial period where foliar nitrogen levels were at ‘deficient’ or ‘severely checked’ levels, but by 10–15 years after planting the values rose to ‘marginal’ or ‘optimal’ status (Figure 2) and the tree growth rates were comparable to those found in the pure plots where nitrogen had been applied. The end-result in the mixed plots was the establishment of a spruce dominated stand of equivalent productivity to pure spruce where nitrogen was applied but without the need for costly remedial inputs. In recent decades, a common approach used to explore species interactions in mixed stands has been to compare the productivity of a mixture against that of the component species in pure plots on the same site (Kelty, 1992; Pretzsch, 2009; Forrester and Pretzsch, 2015). If the mixture is more productive than the average production of the component species in pure stands, ‘overyielding’ is said to occur, whereas if it less productive it indicates ‘underyielding’. Where a mixture is higher yielding than the most productive of the pure stands, this is termed ‘transgressive overyielding’. The feasibility of carrying out a similar analysis in these experiments was constrained by the lack of pure plots of the nurse species. We attempted to compensate for this by estimating the likely productivity of the pine nurses in pure plots through a combination of site details and yield tables (see Supplementary Paper Two). This exercise suggested possible transgressive overyielding at the three most nutrient-poor sites, but no evidence of overyielding at the more fertile site (Drumtochty). Other results indicate that overyielding in mixtures declines with improving site fertility (Bielak et al., 2014; Toigo et al., 2015), in line with the stress-gradient hypothesis (Bertness and Callaway, 1994) proposing that interactions between species change from competition in favourable conditions to facilitation in harsher situations. Other possible explanations for this nursing effect could include competitive reduction through canopy stratification (Forrester and Pretzsch, 2015), but this seems unlikely given the lack of any differentiation in height between the components of the mixture at time of canopy closure when the mixture effect was most pronounced. There are variations in crown architecture between the nurse species and the spruce which might make for more efficient use of growing space (Forrester and Bauhus, 2016), but this seems unlikely to explain the major temporal changes in spruce foliage nitrogen status found in these experiments. Therefore, we consider that facilitation is the driving process in these experiments. In the long-term the spruce component of the nursing mixture benefits at the expense of the nurse which is gradually suppressed through competitive self-thinning, an example of asymmetric facilitation (Lin et al., 2012). Our understanding of the factors driving the facilitation effect in nursing mixtures has hardly advanced since the comprehensive summary of detailed studies at several sites including Culloden and Inchnacardoch provided by Morgan et al. (1992). In brief, these showed that there were enhanced mineralization rates in the soil in mixed stands which increased the amount of nitrogen available to the spruce. There were differential rooting patterns between the nurse species and the spruce, with the nurse being found to root to greater depths leading to greater aeration and enhanced microbial activity within the soil. There were also differences in mycorrhizal associations present in pure spruce stands compared with mixtures. For instance, Heslin et al. (1992) found a more diverse mycorrhizal flora in nursing mixtures with Japanese larch or lodgepole pine in Ireland compared with pure spruce. Studies at the Culloden experiment showed that at least one ectomycorrhizal species associated with the roots of Scots pine in the mixed stands was capable of degrading proteins and so enhancing the pool of organic nitrogen in the soil which would then be accessible for uptake by the spruce. By contrast, the generalist mycorrhizal fungi associated with spruce roots were not capable of degrading proteins, but were capable of utilizing the products of protein breakdown (Ryan & Alexander, 1992). Further studies at Strathy, Inchnacardoch and Culloden showed that the foliage of spruce in pure stands had significantly depleted levels of 15N compared with spruce in mixtures and that in the mixtures there were no significant differences between the levels found in the spruce and in the nurse species (Horsburgh, 1997). This confirmed that the spruce in nursing mixtures had access to a pool of organic nitrogen that was not present in pure stands. There was no effect of different nurse species upon the levels of 15N, but an investigation in another experiment in northern Scotland suggested that these levels increased with an increasing proportion (from 25 to 75 per cent of stems) of the nurse in the mixture (Horsburgh, 1997). Although this nursing effect is mentioned in reviews of nutritional interactions in mixtures (Rothe and Binkley, 2001; Richards et al., 2010), it has been overlooked in meta-analyses of mixture performance (Piotto, 2008; Hulvey et al., 2013) where only the contribution of nitrogen fixing species such as alders or acacias has been considered. Furthermore, given that mycorrhizal associations are critical to the functioning of ecosystem processes on nutrient-poor soils in the boreal forests and other nutrient-poor sites (Read et al., 2003; Collier and Bidartondo, 2009), we suggest that the beneficial mycorrhizal interaction believed to drive the nursing mixtures effect in northern Britain may be more widespread than currently recognized. Thus a number of recent papers have reported positive growth responses where Scots pine is grown in mixture with other European species, for example with Norway spruce (Linden and Agestam, 2003; Bielak et al., 2014; Mason and Connolly, 2014), with sessile or pedunculate oaks (Gabriel et al., 2005; Lu et al., 2016), or with beech (Gabriel et al., 2005; Pretzsch et al., 2015a,b). Often such responses are primarily attributed to aspects of tree architecture (e.g. deep or shallow rooting) or to functional traits such as shade tolerance, while seemingly neglecting any possible mycorrhizal contribution, especially in the establishment phase. It remains a major research challenge to identify those regions where this nursing effect is likely to occur and, within those regions, to achieve a better understanding of the precise mechanisms of nutrient enhancement and transfer. Other aspects which merit investigation are determining if there is a minimum proportion of the nurse species required to initiate the nursing effect, and whether the mycorrhizal associates of the nurse species can persist as the latter is outcompeted in the mixture. The practical significance of these experiments is considerable. First, they indicate that the use of nursing mixtures can allow a substantial increase in stand productivity on nutrient-poor soils in Britain, and possibly elsewhere in northern Europe, without requiring intensive and expensive fertilizer inputs. However, achieving these gains depends upon choosing a nurse species with early growth rates that are compatible with the admixed species. For example, a survey of 20 000 ha of mixtures of lodgepole pine and Sitka spruce in southern Scotland found that a mixed stand would self-thin towards spruce dominance provided that the pine was no more than 2 m taller than the spruce at the beginning of canopy closure (Garforth, 1979). Where this did not apply (e.g. with more vigorous provenances of lodgepole pine), the pine largely suppressed the spruce. Second, wider and continued use of these mixtures is probably essential for sustaining productivity of planted conifer forests on soils overlying nutrient-poor lithologies in Britain (e.g. the Moine schists and Torridonian sandstones of northern Scotland), not least because remedial fertilizer inputs are rarely used because of their cost. Third, users of existing growth models and of other decision support tools which predict species response in relation to site or climatic features need to be aware that such aids currently make little allowance for the possibility of species interaction in mixture (Pretzsch et al., 2015a,b). Taken overall, the findings of these experiments are a salutary reminder that much silvicultural practice and many management prescriptions are based on an understanding gleaned from experiments and experience with single species stands. The performance and dynamics of species in mixture is complex, cannot necessarily be predicted from that of the individual components in isolation, and requires a good appreciation of the mechanisms that sustain the functioning of the forest ecosystem. Supplementary data Supplementary data are available at Forestry online. Conflict of interest statement None declared. Acknowledgements This is a revised version of a paper originally presented at the IUFRO conference held in August 2015 in Edmonton, Canada on the topic of the ‘Ecology, silviculture and management of spruce species in mixed forests’. WLM acknowledges support from the EU Cost Action FP1206 ‘EuMixFor’ that enabled him to attend the meeting. We are grateful to past members of Forest Research’s Silviculture (North) branch for the design and oversight of these experiments and for staff of the Technical Support Unit for their management and assessment. We thank Dr Victoria Stokes for helpful comments on an earlier version of this manuscript and to Stephen Bathgate for assistance in identifying plots to validate the overyielding analysis. Further suggestions provided by Dr Gary Kerr and two anonymous referees considerably improved the final version of the paper. References Anonymous 2010 A Guide for Increasing Tree Species Diversity in Wales . Forestry Commission Wales, p. 41. Bertness, M.D. and Callaway, R. 1994 Positive interactions in communities. Trends Ecol. Evol.  9, 191– 193. Google Scholar CrossRef Search ADS PubMed  Bielak, K., Dudzinska, M. and Pretzsch, H. 2014 Mixed stands of Scots pine (Pinus sylvestris L.) and Norway spruce (Picea abies (L.) Karst.) can be more productive than monocultures. Evidence from over 100 years of observation of long-term experiments. Forest Syst.  23 ( 3), 573– 589. Google Scholar CrossRef Search ADS   Brooker, R.W., Maestre, F.T., Callaway, R.M., Lortie, C.L., Cavieres, L.A., Kunstler, G., et al.  . 2008 Facilitation in plant communities: the past, the present, and the future. J. Ecol.  96, 18– 34. Google Scholar CrossRef Search ADS   Cameron, A.D. and Watson, B.A. 1999 Effect of nursing mixtures on stem form, crown size, branching habit and wood properties of Sitka spruce (Picea sitchensis (Bong.) Carr. For. Ecol. Manage.  122, 113– 124. Google Scholar CrossRef Search ADS   Carey, M.L., McCarthy, R.G. and Miller, H.G. 1988 More on nursing mixtures. Ir. Forestry  45, 7– 20. Carlyle, J.C. and Malcolm, D.C. 1986 Nitrogen availability beneath pure spruce and mixed larch and spruce stands growing on a deep peat. I. Net N mineralization measured by field and laboratory incubations. Plant. Soil.  93, 95– 113. Google Scholar CrossRef Search ADS   Collier, F.A. and Bidartondo, M. 2009 Waiting for fungi: the ecotmycorrhizal invasion of lowland heathlands. J. Ecol.  97, 950– 963. Google Scholar CrossRef Search ADS   Felton, A., Lindbladh, M., Brunet, J. and Fritz, O. 2010 Replacing coniferous monocultures with mixed-species production stands: an assessment of the potential benefits for forest biodiversity in northern Europe. For. Ecol. Manage.  260, 939– 947. Google Scholar CrossRef Search ADS   Felton, A., Nilsson, U., Sonesson, J., Felton, A.M., Roberge, J.-M., Ranius, T., et al.  . 2016 Replacing monocultures with mixed-species stands: ecosystem service implications of two production forest alternatives in Sweden. Ambio  45, S124– S139. Google Scholar CrossRef Search ADS   Forrester, D.I. and Bauhus, J. 2016 A review of processes behind diversity-productivity relationships in forests. Curr. Forestry Rep.  2, 45– 61. Google Scholar CrossRef Search ADS   Forrester, D.I. and Pretzsch, H. 2015 Tamm review: on the strength of evidence when comparing ecosystem functions of mixtures with monocultures. For. Ecol. Manage.  356, 41– 53. Google Scholar CrossRef Search ADS   Gabriel, K., Blair, I. and Mason, W.L. 2005 Growing broadleaved trees on the North York Moors: results after nearly 50 years. Q. J. For.  99, 21– 30. Gamfeldt, L., Snall, T., Bagchi, R., Jonsson, M., Gustaffson, L., Kjellander, P., et al.  . 2013 Higher levels of multiple ecosystem services are found in forests with more tree species. Nat. Commun . doi:10.1038/ncomms2328. Gardiner, B.A., Stacey, G.R., Belcher, R.E. and Wood, C.J. 1997 Field and wind tunnel assessments of the effects of respacing on tree stability. Forestry  70, 233– 252. Google Scholar CrossRef Search ADS   Garforth, M.F. 1979 Mixtures of Sitka spruce and lodgepole pine in South Scotland: history and future management. Scot. For.  33, 15– 28. Grant, A., Worrell, R., Wilson, S., Ray, D. and Mason, W.L. 2012 Achieving diversity in Scotland’s forest landscapes. Forestry Commission Scotland Practice Guide . Forestry Commission, p. 30. Heslin, M.C., Blasius, D., McElhinney, C. and Mitchell, D.T. 1992 Mycorrhizal and associated fungi of Sitka spruce in Irish forest mixed stands. Eur. J. For. Path  22, 46– 57. Google Scholar CrossRef Search ADS   Hibberd, B.G. 1991 Forestry practice. Forestry Commission Handbook 6 . HMSO. Horsburgh, A.M. 1997 Patterns of N concentration and 15N natural abundance in pure and mixed stands of Sitka spruce. Unpublished Ph.D. thesis, University of Aberdeen. Hulvey, K.B., Hobbs, R.J., Standish, R.J., Lindenmayer, D.B., Lach, L. and Perring, M.P. 2013 Benefits of tree mixes in carbon plantings. Nat. Clim. Change  3, 869– 874. Google Scholar CrossRef Search ADS   Kelty, M.J. 1992 Comparative productivity of monocultures and mixed species stands. In The Ecology and Silviculture of Mixed Species Forests . Kelty M.J., Larson B.C. and Oliver C.D. (eds). Kluwer, pp. 125– 141. Google Scholar CrossRef Search ADS   Kelty, M.J. 2006 The role of species mixtures in plantation forestry. For. Ecol. Manage.  233, 195– 204. Google Scholar CrossRef Search ADS   Lin, Y., Berger, U., Grimm, V. and Ji, Q.-R. 2012 Differences between symmetric and asymmetric facilitation matter: exploring the interplay between modes of positive and negative plant interactions. J. Ecol.  100, 1482– 1491. Google Scholar CrossRef Search ADS   Linden, M. and Agestam, E. 2003 Increment and yield in mixed and monoculture stands of Pinus sylvestris and Picea abies based on an experiment in southern Sweden. Scan. J. For. Res.  18, 155– 162. Google Scholar CrossRef Search ADS   Lu, H., Mohren, G.M.J., den Ouden, J., Goudiaby, V. and Sterck, F.J. 2016 Overyielding of temperate mixed forests occurs in evergreen-deciduous but not in deciduous–deciduous species mixtures over time in the Netherlands. For. Ecol. Man.  376, 321– 332. Google Scholar CrossRef Search ADS   Macdonald, J.A.B. 1936 The effect of introducing pine species among checked Sitka spruce on a dry, Calluna-clad slope. Trans. R. Soc. Arboricultural Soc.  50, 83– 86. Macdonald, J.A.B. and Macdonald, A. 1952 The effect of interplanting with pine on the emergence of Sitka spruce from check on heather land. Scot. For.  6, 77– 79. Mason, W.L. 2007 Changes in the management of British forests between 1945 and 2000 and possible future trends. Ibis  149, 41– 52. Google Scholar CrossRef Search ADS   Mason, W.L. 2014 Long-term development of nursing mixtures of Sitka spruce and larch species in an experiment in northern Scotland. Forest. Syst.  23 ( 3), 590– 597. Google Scholar CrossRef Search ADS   Mason, W.L. and Connolly, T. 2014 Mixtures with spruce species can be more productive than monocultures: evidence from the Gisburn experiment in Britain. Forestry . doi:10.1093/forestry/cpt042. Mason, W.L. and Perks, M.P. 2011 Sitka spruce (Picea sitchensis) forests in Atlantic Europe: changes in forest management and possible consequences for carbon sequestration. Scan. J. For. Res. Suppl.  11, 72– 81. Google Scholar CrossRef Search ADS   McIntosh, R. 1983 Nitrogen deficiency in established phase Sitka spruce in upland Britain. Scot. For.  35, 185– 193. Morgan, J.L., Campbell, J.M. and Malcolm, D.C. 1992 Nitrogen relations of mixed-species stands on oligotrophic soils. In The Ecology of Mixed-species Stands of Trees . Cannell M.G.R., Malcolm D.C. and Robertson P.A. (eds). Blackwell, Oxford, pp. 65– 85. O’Carroll, N. 1978 The nursing of Sitka spruce I. Japanese larch. Ir. Forestry  35, 60– 65. Paquette, A. and Messier, C. 2011 The effect of biodiversity on tree productivity: from temperate to boreal forests. Glob. Ecol. Biogeogr.  20, 170– 180. Google Scholar CrossRef Search ADS   Piotto, D. 2008 A meta-analysis comparing tree growth in monocultures and mixed plantations. For. Ecol. Manage.  255, 781– 786. Google Scholar CrossRef Search ADS   Pretzsch, H. 2009 Forest Dynamics, Growth and Yield . Springer-Verlag, p. 664. Pretzsch, H., del Rio, M., Ammer, C., Avdagic, A., Barbeito, I., Bielak, K., et al.  . 2015a Growth and yield of mixed versus pure stands of Scots pine (Pinus sylvestris L.) and European beech (Fagus sylvatica L.) analysed along a productivity gradient through Europe. Euro. J. For. Res . doi:10.1007/s10342-015-0900-4. Pretzsch, H., Forrester, D.I. and Rotzer, T. 2015b Representation of species mixing in forest growth models; a review and perspective. Ecol. Model.  313, 276– 292. Google Scholar CrossRef Search ADS   Read, D.J., Leake, J.R. and Perez-Moreno, J. 2003 Mycorrhizal fungi as drivers of processes in heathland and boreal forest biomes. Can. J. Bot.  82, 1243– 1263. Google Scholar CrossRef Search ADS   Read, D.J., Freer-Smith, P.H., Morison, J.I.L., Hanley, N., West, C.C. and Snowdon, P. (eds). 2009 Combating climate change – a role for UK forests. An Assessment of the Potential of the UK’s Trees and Woodlands to Mitigate and Adapt to Climate Change . The Stationery Office. Richards, A.E., Forrester, D.I., Bauhus, J. and Scherer-Lorenzen, M. 2010 The influence of mixed tree plantations on the nutrition of individual species: a review. Tree Physiol.  30, 1192– 1208. Google Scholar CrossRef Search ADS PubMed  Robinson, R.K. 1972 The production by roots of Calluna vulgaris of a factor inhibitory to the growth of some mycorrhizal fungi. J. Ecol.  60, 219– 224. Google Scholar CrossRef Search ADS   Rothe, A. and Binkley, D. 2001 Nutritional interactions in mixed species forests: a synthesis. Canadian J. For. Res.  31, 1855– 1870. Google Scholar CrossRef Search ADS   Ryan, E.A. and Alexander, I.J. 1992 Mycorrhizal aspects of improved growth of spruce when grown in mixed stands on heathland soils. In Mycorrhizas in Ecosystems . Read D.J., Lewis D.H., Fitter A.H. and Alexander I.J. (eds). CAB International, pp. 237– 245. Smith, S.A. and McKay, H.M. 2002 Nutrition of Sitka Spruce on Upland Restock Sites. Forestry Commission Information Note 47 . Forestry Commission. Taylor, C.M.A. 1991 Forest fertilisation in Great Britain. Forestry Commission Bulletin 95 . HMSO. Toigo, M., Vallet, P., Perot, T., Bontemps, J.-D., Piedallu, C. and Courbaud, B. 2015 Overyielding in mixed forests decreases with site productivity. J. Ecol.  103, 505– 512. Google Scholar CrossRef Search ADS   UKFS. 2011 The UK Forestry Standard, pp. 116. http://www.forestry.gov.uk/pdf/FCFC001.pdf/$FILE/FCFC001.pdf (accessed on 15 March, 2014). VSN International, 2013. GenStat for Windows 16th Edition. VSN International, Hemel Hempstead, UK. Web page: GenStat.co.uk Watson, B.A. and Cameron, A.D. 1995 Some effects of nursing species on stem form, branching habit and compression wood content of Sitka spruce. Scot. For.  49, 146– 154. Weatherell, J. 1957 The use of nurse species in the afforestation of upland heaths. Q. J. For.  51, 298– 304. Zehetmayr, J.W.L. 1960 Afforestation of upland heaths. Forestry Commission Bulletin No. 32 . HMSO. Zhang, Y., Chen, H.Y.H. and Reich, P.B. 2012 Forest productivity increases with evenness, species richness and trait variation: a global meta-analysis. J. Ecol. , 8. doi:10.1111/j.1365-2745.2011.01944.x. © Crown copyright 2018. This article contains public sector information licensed under the Open Government Licence v3.0 (http://www.nationalarchives.gov.uk/doc/open-government-licence/version/3/). http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Forestry: An International Journal Of Forest Research Oxford University Press

Nursing mixtures can enhance long-term productivity of Sitka spruce (Picea sitchensis (Bong.) Carr.) stands on nutrient-poor soils

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Abstract

Abstract The growth of Sitka spruce (Picea sitchensis (Bong.) Carr.) on nutrient-poor sites in the British Isles is often improved during the late establishment phase when grown in intimate (‘nursing’) mixture with pioneer species such as pines and larches. However, there is very little information on the longer term effects of such mixtures on stand development and productivity. Therefore, we analysed results from four experiments established in northern Scotland in the 1960s which included nursing mixtures of Sitka spruce with Scots pine (Pinus sylvestris L.), lodgepole pine (Pinus contorta Dougl.) or larches (Larix spp.), as well as plots of pure Sitka spruce given different establishment regimes, sometimes including nitrogen fertilization. At three nutrient-poor sites, spruce in nursing mixture was significantly taller after 15 years than pure spruce without nitrogen. Analysis of foliar nitrogen status showed that pure spruce without nitrogen became deficient by ~8–10 years with no recovery for at least 20 years. Spruce grown in mixture also showed an initial nitrogen deficiency, but with recovery by 10–15 years to optimal or marginal levels. After more than 40 years growth, on the nutrient-poor sites the basal areas in the nursing mixtures were significantly higher than pure spruce without remedial nitrogen and comparable to those of pure spruce given several applications of nitrogen, However, these differences were not evident at the more fertile site. Nearly all mixed plots self-thinned towards dominance by Sitka spruce. The magnitude of this nursing effect is striking and harnessing the mechanisms underpinning this effect could be very important for sustaining productivity of forests on nutrient-poor soils in upland Britain. Introduction Recently, there has been increasing advocacy for growing tree species in mixture as part of a strategy of adapting British forests to projected climate change (Read et al., 2009, pp. 174–175). For example, the UK Forestry Standard, which sets out the national basis for sustainable forest management, encourages forestry practices which promote species diversity, such as mixed stands (UKFS 2011, p. 96). Separate policy documents in Wales and Scotland seek to increase the diversity of planted forests by wider use of a range of species mixtures (Anonymous, 2010; Grant et al., 2012). Nevertheless, planted forests in Britain, not to mention Ireland and other adjoining regions of Atlantic Europe, are mostly characterized by single species plantations of fast growing non-native conifers grown on relatively short rotations (Mason, 2007; Mason & Perks, 2011). Successful establishment and management of mixed-species forests depends on an understanding of the characteristics of the component species (e.g. growth rate, shade tolerance) and the way in which their mutual interactions change over time (Pretzsch, 2009; Chapter 9). Everything else being equal, one would expect higher production from a mixed-species stand where the niches occupied by the component species are different so that the species can be said to have complementary characteristics (Kelty, 2006). Positive mixing effects can occur where the growth of a valuable species is favoured by mixture with another, either because of a reduction in competition between the species growing in mixture (termed ‘competitive reduction’) or because one component of the mixture increases the availability of a limiting resource (e.g. nutrients, water) to the benefit of the other components, a process termed ‘facilitation’ (Kelty, 2006). Paquette and Messier (2011) suggested that beneficial interactions between tree species may be more important in stressful environments such as the boreal forests while reviews of facilitation in wider plant communities have also highlighted the need for taking environmental gradients into account (Brooker et al., 2008). The complexity of these interactions suggests that, despite increasing reports of the value of mixed stands for the provision of a range of ecosystem services including productivity (Felton et al., 2010, 2016; Zhang et al., 2012; Gamfeldt et al., 2013), the potential benefits of mixtures need to be carefully tested in individual climatic regions or site types. A further complication is that many studies of mixing effects in forests have used data derived from specific forest plots or from national or regional forest inventories (Paquette and Messier, 2011; Gamfeldt et al., 2013; Toigo et al., 2015) where variations due to site, silvicultural practices and disturbance history can be confounding factors (Forrester and Pretzsch, 2015). In planted conifer forests in Britain and Ireland, the main use of mixtures was in the afforestation of nutrient-poor soils (Carey et al., 1988). Studies in the early decades of the last century had shown the importance of site cultivation and drainage combined with remedial phosphate fertilization for the establishment of tree species on these difficult sites (Zehetmayr, 1960). While this proved sufficient to establish less demanding species such as Scots pine (Pinus sylvestris L.), lodgepole pine (Pinus contorta Dougl.) and Japanese larch (Larix kaempferi (Lamb.) Carr.), growth of more productive species such as Sitka spruce (Picea sitchensis (Bong.) Carr.) rapidly stagnated (‘checked’) particularly in the presence of ericaceous vegetation such as heather (Calluna vulgaris (L.) Hull) (Morgan et al., 1992). This checked growth was caused by an antagonistic effect of heather upon spruce mycorrhiza resulting in nitrogen deficiency in the spruce (Robinson, 1972). As a result, depending upon the lithology, a combination of herbicide control of the heather and one or more applications of nitrogen could be required to achieve canopy closure in pure spruce stands (McIntosh, 1983; Taylor, 1991). This comparatively expensive establishment regime meant that managers of such sites were often forced to plant less productive species such as pines and larches. However, from the 1930s onwards, researchers had observed that where Sitka spruce was growing in close proximity to either Scots pine or Japanese larch, the growth of the spruce improved after an initial period of check and that the trees eventually closed canopy (Macdonald, 1936; Macdonald & Macdonald, 1952). This ‘nursing’ effect (Weatherell, 1957) was associated with improved nitrogen status of the spruce (O’Carroll, 1978) which reflected higher nitrogen concentrations in soils in the mixed stands (Carlyle & Malcolm, 1986) and this effect occurred irrespective of the nurse species. Provided that the nurse species was not too vigorous, which was a problem with faster growing provenances of lodgepole pine (Garforth, 1979), once canopy closure had occurred the mixed stands were expected to progressively self-thin towards a spruce dominated stand (Carey et al., 1988). As a result, the use of ‘nursing’ mixtures where Sitka spruce was planted in combination with either pine or larch became a recommended practice for afforestation and reforestation regimes on the most nutrient-poor soils in upland regions of the British Isles (Carey et al., 1988; Taylor, 1991; Smith & McKay, 2002). The nursing benefit provided by pines and larches was also reported to occur when these species were grown with a number of broadleaved species on nutrient-poor soils (Gabriel et al., 2005) and recently has been found on more fertile sites where nitrogen deficiency would not have been anticipated (Mason & Connolly, 2014). Despite this history, there is little information about the long-term effects of these mixtures upon stand growth and productivity, with most studies being concentrated in stands 15–25 years of age (Carey et al., 1988; Morgan et al., 1992). A pair of studies on the impacts of nursing mixtures upon the growth and wood properties of 25–30-years-old Sitka spruce found that Japanese larch had the greatest positive influence upon spruce diameter and volume increment (Watson & Cameron, 1995; Cameron & Watson, 1999), but resulted in spruce with wider annual rings, larger branch diameters and more detrimental knot characteristics. They concluded that pines would be preferable species for nursing Sitka spruce because of fewer negative impacts on timber quality, particularly if sawlog production was envisaged (Cameron & Watson, 1999). However, a recent study in northern Scotland found no difference in harvested volume or sawlog outturn from 41-year-old self-thinning mixtures of Sitka spruce and larch when compared with pure Sitka spruce stands (Mason, 2014). The objective of this paper is to examine the growth and productivity of nursing mixtures with Sitka spruce from early establishment until close to rotation age using results from four experiments on soil types of differing fertility. Our working hypothesis involved the following: (1) there would be better growth and productivity at time of canopy closure of nursing mixtures over pure Sitka spruce without nitrogen, (2) this would be reflected in improved foliage nutrient status of the Sitka spruce growing in mixture, (3) any growth improvements found at canopy closure would be sustained over the remainder of the rotation and (4) that these effects would be mediated by site fertility. Materials and methods General The four experiments examined in this paper were all established in the north of Scotland (Figure 1) between 1965 and 1969 as part of a wider research programme that explored different silvicultural practices relevant to the establishment of forests on nutrient-poor acid soils. Three of the experiments (Strathy, Drumtochty and Inchnacardoch) were designed mainly to investigate the effects of different levels of nitrogen input on the growth and development of Sitka spruce. The fourth (Culloden) was primarily intended to study the effects of varying intensities of cultivation and drainage upon the growth of conifer stands. As a result, there is a range of treatments represented in these experiments (Table 1) and only two experiments (Strathy and Drumtochty) share a common design. However, the important feature for this paper is that all four experiments contain one or more mixture treatment and a pure Sitka spruce control, fully randomized into the design. Table 1 Details of the subsidiary experimental treatments (i.e. not including mixtures) used in the four experiments Experiment name and number1  Fertilizer (code)  Herbicide (code)  Assessment history  Strathy 6  ANNUAL (NA) applications (13 in total) of nitrogen to pure Sitka spruce until canopy closure – rate 168 kg N ha−1: applications in 1966–1971, 1973, 1975, 1977, 1980, 1983, 1986, 1989. PERIODIC(NP – rate as NA) applications (5 in total) of nitrogen until canopy closure: applications in 1966, 1972, 1977, 1982, 1987 NONE (0) – no nitrogen application  Control of competing heather in pure Sitka spruce plots (W); no herbicide treatment (V). Herbicide applied in 1969 and 1971  Height at 3, 6, 17, 20, 25, 47 years; dbh and basal area at 25, 30, 32 and 47 years  Drumtochty 28  ANNUAL (NA) applications (11 in total) of nitrogen to pure Sitka spruce until canopy closure – rate 168 kg N ha−1: applications in 1968–1971, 1973, 1975, 1977, 1979, 1981, 1983, and 1986. PERIODIC (NP – rate as NA) applications (4 in total) of nitrogen until canopy closure: applications in 1968, 1973, 1977, and 1984 NONE (0) – no nitrogen application  Control of competing heather in pure Sitka spruce plots (W); no herbicide treatment (V) Treatment applied in 1971  Height at 3, 6, 10, 16, 30, 35, 44 years; dbh and basal area at 16–25, 29, 35 and 44 years  Inchnacardoch 164  PERIODIC (NP) applications (5 in total) of nitrogen to pure and mixed plots – rate 168 kg N ha−1 in 1973, 1976, 1979, 1982 and 1986; NONE (0) – no application of nitrogen  N/A  Height at 9, 12, 15, 18, 20 and 45 years; dbh and basal area at 20–23, 25, 30, 35 and 45 years  Experiment name and number  Cultivation treatment (code)  Drainage treatment (code)  Assessment history  Culloden 2  Shallow spaced furrow ploughing (ST); Complete shallow ploughing (CST); Deep spaced furrow ploughing (DT); Complete deep ploughing (DT)  Drains (75 cm deep) at 40 m spacing (D40); drains as above at 20 m spacing (D20)  Height at 3, 6, 10, 12, 18, 20, 34 and 42 years; dbh and basal area at 12, 20, 34 and 42 years  Experiment name and number1  Fertilizer (code)  Herbicide (code)  Assessment history  Strathy 6  ANNUAL (NA) applications (13 in total) of nitrogen to pure Sitka spruce until canopy closure – rate 168 kg N ha−1: applications in 1966–1971, 1973, 1975, 1977, 1980, 1983, 1986, 1989. PERIODIC(NP – rate as NA) applications (5 in total) of nitrogen until canopy closure: applications in 1966, 1972, 1977, 1982, 1987 NONE (0) – no nitrogen application  Control of competing heather in pure Sitka spruce plots (W); no herbicide treatment (V). Herbicide applied in 1969 and 1971  Height at 3, 6, 17, 20, 25, 47 years; dbh and basal area at 25, 30, 32 and 47 years  Drumtochty 28  ANNUAL (NA) applications (11 in total) of nitrogen to pure Sitka spruce until canopy closure – rate 168 kg N ha−1: applications in 1968–1971, 1973, 1975, 1977, 1979, 1981, 1983, and 1986. PERIODIC (NP – rate as NA) applications (4 in total) of nitrogen until canopy closure: applications in 1968, 1973, 1977, and 1984 NONE (0) – no nitrogen application  Control of competing heather in pure Sitka spruce plots (W); no herbicide treatment (V) Treatment applied in 1971  Height at 3, 6, 10, 16, 30, 35, 44 years; dbh and basal area at 16–25, 29, 35 and 44 years  Inchnacardoch 164  PERIODIC (NP) applications (5 in total) of nitrogen to pure and mixed plots – rate 168 kg N ha−1 in 1973, 1976, 1979, 1982 and 1986; NONE (0) – no application of nitrogen  N/A  Height at 9, 12, 15, 18, 20 and 45 years; dbh and basal area at 20–23, 25, 30, 35 and 45 years  Experiment name and number  Cultivation treatment (code)  Drainage treatment (code)  Assessment history  Culloden 2  Shallow spaced furrow ploughing (ST); Complete shallow ploughing (CST); Deep spaced furrow ploughing (DT); Complete deep ploughing (DT)  Drains (75 cm deep) at 40 m spacing (D40); drains as above at 20 m spacing (D20)  Height at 3, 6, 10, 12, 18, 20, 34 and 42 years; dbh and basal area at 12, 20, 34 and 42 years  Notes: 1. The number following the experiment (forest) name is a unique identifier. Thus, ‘Strathy 6’ indicates the sixth experiment established in Strathy forest. Figure 1 View largeDownload slide A map of Great Britain showing the approximate location of the four experiments described in this paper. Figure 1 View largeDownload slide A map of Great Britain showing the approximate location of the four experiments described in this paper. Sites The experiments were all located on afforestation sites that were typical of many areas planted in upland Britain in the second half of the last century, being in cool, exposed locations with relatively high levels of rainfall (Supplementary data, Table 1). Three sites (Strathy, Inchnacardoch and Culloden) were located on moist soils of very low nutrient availability, while the remaining site (Drumtochty) was on a slightly drier and more fertile site (Supplementary data, Table 1). Apart from the experimental treatments, all other aspects of experiment establishment and management followed normal procedures characteristic of upland afforestation (Hibberd, 1991) with the site being cultivated before planting and trees planted either on the upturned ridge (Strathy, Inchnacardoch and Culloden) or in the side of the furrow (Drumtochty) as appropriate for the soil type. Planting stock was either 2 or 3-years-old bare-root transplants depending on species. Any failures were replaced with plants of the same species in the years following planting to ensure full stocking in the plots. There was serious frost damage to Sitka spruce and larch 8 years after planting at Strathy (Supplementary data Table 2). No thinning has taken place in any of the experiments. Experimental treatments and design (see also Table 1) At Strathy and Drumtochty the design contrasted pure Sitka spruce given all combinations of three intensities of nitrogen fertilizer application (annual (NA), periodic (NP) or none (0)) and two herbicide regimes (treatment of heather or none) with Sitka spruce planted in two different nursing mixtures. However the combination of Sitka spruce given no nitrogen fertilizer and no herbicide input was considered ‘impractical’ and not implemented: therefore these experiments contained seven treatments replicated three times in a randomized block design (i.e. 21 plots) with a plot size of 0.27–0.1 ha and an internal assessment plot of 0.032 ha (Strathy) or 0.04 ha (Drumtochty). The initial design at Inchnacardoch involved pure Sitka spruce and two nursing mixtures replicated five times in 0.12 ha plots. From 1973, all three treatments were randomly split with one split plot being given periodic nitrogen application (NP) and the other receiving no nitrogen (0) (i.e. 30 plots). All split plots had an internal assessment plot of 0.02 ha. At Culloden, the design compared two species (i.e. pure Sitka spruce or a nursing mixture) by four cultivation treatments (different intensities and depths of ploughing) with the eight combinations split for two different drain spacings (40 m or 20 m). There were four replicate blocks but the shallow ploughing was confined to two of the blocks and the deep ploughing was located in the other two (i.e. a total of 32 plots). Plot size was 0.12 ha with an internal assessment plot of 0.04 ha. The nurse species represented in the four experiments were: hybrid larch and lodgepole pine (Alaskan provenance) at Strathy, Japanese larch and lodgepole pine (Alaskan provenance) at Drumtochty and Inchnacardoch, and Scots pine at Culloden. Two patterns of mixture were used: at Strathy, Drumtochty and Inchnacardoch a 3:1 nurse–Sitka spruce ratio was achieved by alternating pure rows of the nurse species with rows where the nurse and Sitka spruce were planted in successive groups of three plants. At Culloden, Scots pine and Sitka spruce were planted in a 1:1 ratio in a three row: three row mixture of each species. Growth measurements The assessment history varied between these experiments reflecting their different objectives and history (Table 1). In general, the early assessments up to about year 10 were only for height and were carried out on a whole plot basis. From about year 15, a central assessment plot was located in each treatment where measurements of top height, mean dbh and basal area were made. Early growth data from two experiments (Inchnacardoch, Culloden) were published over two decades ago (Carey et al.,1988; Morgan et al., 1992). Foliage analysis Because of interest in the nutritional response of the Sitka spruce to the various treatments, foliage analysis of the spruce trees was carried out on a regular basis in all experiments from ~3 to 5 years after planting until the early 1990s. One final analysis was carried out in 2010 at Strathy, Inchnacardoch and Culloden. Foliage samples were collected in October–November each year from 15 dominant or co-dominant trees per plot up to canopy closure and this sample was reduced to five trees for later assessments. Although the nutrient levels of all macronutrients in the Sitka spruce foliage were analysed, in this paper we concentrate upon the nitrogen status since this is the best indicator of satisfactory tree growth (Taylor, 1991). Interpretation of the results for foliar nitrogen concentrations used standard levels (per cent dry weight) as defined by Taylor (1991) as follows: >1.5 – optimal levels; 1.2–1.5 – marginal; <1.2 – deficient; <1.0 – severely checked growth. In addition to experimental fertilizer treatments (see above and Table 1) all treatments in these experiments received a standard fertilizer regime recommended for upland afforestation (Taylor, 1991) involving up to three phosphorus applications during the establishment phase supplemented by potassium on the poorest sites (Supplementary data, Table 2). Data analysis Analysis of the data differed between the experiments. At Strathy and Drumtochty, which had similar designs, we used analysis of variance procedures for a randomized block design, separating overall treatment effects into: those with no nitrogen application (i.e. control and the two mixture treatments); the two different intensities of nitrogen application; the two different rates of herbicide application; and any interaction between the intensities of nitrogen application and the herbicide treatment. This analysis was carried out twice, once on a whole plot basis including the growth of the nurses in the mixed plots, and the other time only examining the response of the Sitka spruce. The procedure at Inchnacardoch was similar except that the analysis reflected that the design was a split-plot with the species/mixtures as the main plots and the presence or absence of nitrogen application being the secondary factor. At Culloden, the responses observed in blocks one and two were examined separately from those found in blocks three and four because the ploughing treatments were confounded with the blocks (see above and Supplementary data, Table 2). Inspection of the data showed a consistent response between the two parts of the experiment and therefore for ease of presentation only the results from blocks one and two are presented here. Here, we again carried out the analysis twice, namely with and without the nurse species. In the experiments at Strathy, Drumtochty and Inchnacardoch with a more complicated experimental structure we used Fisher’s Unprotected LSD to test for significant differences between treatments. This method sets the significance level for each pairwise comparison at 5 per cent, but is reported only for those instances where the overall analysis of variance had proved significant. The data were analysed independently on a date by date basis and no time by treatment interaction was investigated. This was because trends over time were readily apparent in the results. All data analyses were conducted using the statistical software Genstat (VSN International, 2013) Results Growth and basal area Drumtochty There were significant differences in height of Sitka spruce between treatments from the ages of 6 to 29 years, reflecting better growth where nitrogen had been applied, and lesser growth of spruce in mixture, especially in the mixtures with lodgepole pine (Table 2a). A significant difference in height at 44 years was due to the spruce in mixture with larch being taller while the spruce mixed with pine was smaller. There were no differences in response to the two periodicities of nitrogen fertilization, but there was a significant increase in height at 10 years following herbicide application (P < 0.01). At 16 years after planting, the difference in height between the spruce and each of the nurse species in the mixed plots was less than 1 m (data not shown). Table 2 Growth in height and diameter at breast height (dbh) for 44 years at Drumtochty (a) or for 47 years at Strathy (b) of Sitka spruce grown either in mixture or in pure plots with various combinations of nitrogen fertilizer and herbicide application (see footnote for treatement codes) (a) Drumtochty    Height (m)  Dbh (cm)  Age (years)  3  6  10  16  29  35  44  16  20  25  29  35  44  Treatment   With larch nurse  0.7  1.4ab  2.8b  7.1bc  16.0b  19.3  26.1b  9.1c  12.8d  16.5c  19.5c  21.7b  29.1b   With pine nurse  0.7  1.4a  2.1a  5.2a  13.8a  18.0  22.8a  6.2a  9.4a  13.2b  17.1b  21.2b  29.4b   NAV  0.7  1.6bc  3.5cde  7.5c  15.7b  19.1  22.6a  9.3c  11.4c  13.2b  14.4a  15.2a  20.3a   NAW  0.8  1.7c  3.8e  7.4c  16.2b  19.2  24.4ab  9.8c  11.8cd  13.4b  14.6a  15.3a  20.3a   NPV  0.7  1.7c  3.2bc  6.7bc  15.4b  18.4  24.0ab  8.8bc  11.0bc  13.1ab  14.5a  15.4a  20.8a   NPW  0.7  1.7c  3.7de  7.3bc  15.9b  19.3  22.9a  9.6c  11.5c  13.4b  14.6a  15.5a  20.2a   OW- control  0.8  1.6c  3.3cd  6.5b  14.9ab  18.4  24.0ab  7.6b  9.8ab  12.0a  13.5a  14.5a  19.8a   Significance  ns  **  ***  **  *  ns  *  ***  ***  ***  ***  ***  ***   SED  0.05  0.07  0.20  0.40  0.65  0.77  0.98  0.62  0.57  0.54  0.78  0.74  0.67   5% LSD  0.11  0.16  0.42  0.86  1.41  1.68  2.12  1.35  1.25  1.18  1.70  1.61  1.50  (b) Strathy    Height (m)  Dbh (cm)  Age (years)  3  6  17  20  25  47  25  30  47          Treatment   With larch nurse  0.6cd  1.7c  4.9d  6.5c  10.5  18.3bc  13.9b  15.4c  20.9c           With pine nurse  0.6bcd  1.6bc  4.7cd  6.4c  10.7  20.3cd  9.9a  12.1b  18.6c           NAV  0.5a  1.4a  4.1bc  5.1b  9.9  21.1d  11.5a  13.6bc  20.2c           NAW  0.5ab  1.4a  3.7 b  4.7b  9.7  19.8bcd  11.1a  13.5 bc  19.5c           NPV  0.5ab  1.5ab  4.3 bcd  5.3b  9.8  17.7b  10.3a  11.8b  15.1b           NPW  0.6abc  1.5abc  4.7cd  6.4c  10.9  20.3cd  11.9ab  13.4bc  18.0bc           OW – control  0.7d  1.7c  2.5a  2.6a  Nd  9.0a  Nd  5.5a  8.2a           Significance  **  **  ***  ***  ns  ***  *  ***  ***           SED  0.04  0.08  0.30  0.47  0.66  1.04  1.02  1.13  1.46           5% LSD  0.08  0.18  0.65  1.02  1.47  2.26  2.26  2.47  3.17          (a) Drumtochty    Height (m)  Dbh (cm)  Age (years)  3  6  10  16  29  35  44  16  20  25  29  35  44  Treatment   With larch nurse  0.7  1.4ab  2.8b  7.1bc  16.0b  19.3  26.1b  9.1c  12.8d  16.5c  19.5c  21.7b  29.1b   With pine nurse  0.7  1.4a  2.1a  5.2a  13.8a  18.0  22.8a  6.2a  9.4a  13.2b  17.1b  21.2b  29.4b   NAV  0.7  1.6bc  3.5cde  7.5c  15.7b  19.1  22.6a  9.3c  11.4c  13.2b  14.4a  15.2a  20.3a   NAW  0.8  1.7c  3.8e  7.4c  16.2b  19.2  24.4ab  9.8c  11.8cd  13.4b  14.6a  15.3a  20.3a   NPV  0.7  1.7c  3.2bc  6.7bc  15.4b  18.4  24.0ab  8.8bc  11.0bc  13.1ab  14.5a  15.4a  20.8a   NPW  0.7  1.7c  3.7de  7.3bc  15.9b  19.3  22.9a  9.6c  11.5c  13.4b  14.6a  15.5a  20.2a   OW- control  0.8  1.6c  3.3cd  6.5b  14.9ab  18.4  24.0ab  7.6b  9.8ab  12.0a  13.5a  14.5a  19.8a   Significance  ns  **  ***  **  *  ns  *  ***  ***  ***  ***  ***  ***   SED  0.05  0.07  0.20  0.40  0.65  0.77  0.98  0.62  0.57  0.54  0.78  0.74  0.67   5% LSD  0.11  0.16  0.42  0.86  1.41  1.68  2.12  1.35  1.25  1.18  1.70  1.61  1.50  (b) Strathy    Height (m)  Dbh (cm)  Age (years)  3  6  17  20  25  47  25  30  47          Treatment   With larch nurse  0.6cd  1.7c  4.9d  6.5c  10.5  18.3bc  13.9b  15.4c  20.9c           With pine nurse  0.6bcd  1.6bc  4.7cd  6.4c  10.7  20.3cd  9.9a  12.1b  18.6c           NAV  0.5a  1.4a  4.1bc  5.1b  9.9  21.1d  11.5a  13.6bc  20.2c           NAW  0.5ab  1.4a  3.7 b  4.7b  9.7  19.8bcd  11.1a  13.5 bc  19.5c           NPV  0.5ab  1.5ab  4.3 bcd  5.3b  9.8  17.7b  10.3a  11.8b  15.1b           NPW  0.6abc  1.5abc  4.7cd  6.4c  10.9  20.3cd  11.9ab  13.4bc  18.0bc           OW – control  0.7d  1.7c  2.5a  2.6a  Nd  9.0a  Nd  5.5a  8.2a           Significance  **  **  ***  ***  ns  ***  *  ***  ***           SED  0.04  0.08  0.30  0.47  0.66  1.04  1.02  1.13  1.46           5% LSD  0.08  0.18  0.65  1.02  1.47  2.26  2.26  2.47  3.17          At any given age, treatments with different letter suffices are significantly different from one another. Treatment codes are: NAV, annual applications of nitrogen and no vegetation control; NAW, annual application of nitrogen plus weed control; NPV, periodic application of nitrogen and no vegetation control; NPW, periodic application of nitrogen plus weed control; OW, no application of nitrogen plus weed control. For details of timings and rates of applications of fertilizer and/or herbicides, see Table 1. Nd indicates ‘no data’ where a particular treatment was not measured. Significance values are presented as: ns = non-significant; *P < 0.05; **P < 0.01; ***P < 0.001. Diameter growth followed a similar pattern with highly significant differences in the early years due to larger values in nitrogen treatments or in trees in mixture with larch. However, by year 44 the major difference was the greater diameter of spruce in both mixtures compared with all the pure spruce plots. There were no effects due to timing of nitrogen input or herbicide application. There were highly significant differences in basal area between treatments at all ages (Table 3). For up to 35 years, this reflected better growth in the nitrogen treatments and lower productivity in both mixtures. At the last assessment, the difference was due to continued better growth in some of the nitrogen treatments plus the poorer performance of the mixture with lodgepole pine, which may reflect the loss of one plot of this mixture before the last assessment (Supplementary Table 2). Over the period of assessment, Sitka spruce progressed from being a 25 per cent component to becoming the dominant species in both mixtures. Table 3 Basal area production (m2 ha−1) of plots of nursing mixtures with Sitka spruce and of pure spruce plots given various combinations of nitrogen fertilizer and herbicide application at Drumtochty and Strathy   Drumtochty  Strathy  Age  16  20  25  29  35  44  25  30  47  Treatment   With larch nurse  20.4b (0.37)  32.7ab (0.45)  48.2ab (0.51)  59.9a (0.59)  71.3a (0.62)  77.1bc (0.78)  29.3 (0.73)  34.5b (0.77)  36.3b (1.0)   With pine nurse  13.0a (0.24)  24.9a (0.29)  40.1a (0.35)  54.5a (0.47)  66.5a (0.55)  53.0a (0.93)  49.3 (0.19)  59.9c (0.22)  70.8c (0.3)   NAV  28.3c  42.6c  58.2c  69.9bc  79.5bc  68.9b  39.9  50.0c  58.0c   NAW  31.6c  46.1c  60.9c  73.5c  82.6c  81.2c  43.0  54.9c  67.8c   NPV  25.3bc  39.8bc  56.8bc  70.4bc  80.9bc  80.6c  39.9  50.5c  62.5c   NPW  29.5c  42.6c  58.0c  70.7bc  80.8bc  74.8bc  43.3  54.0c  69.9c   OW – control  19.8ab  32.7ab  48.6ab  62.4ab  73.5ab  74.3bc  Nd  10.4a  17.8a   Significance  **  **  **  **  **  **  ns  ***  ***   SED  3.25  3.94  4.06  3.88  3.73  4.78  5.32  5.71  6.26   5% LSD  7.08  8.59  8.86  8.45  8.12  10.65  11.87  12.44  13.63    Drumtochty  Strathy  Age  16  20  25  29  35  44  25  30  47  Treatment   With larch nurse  20.4b (0.37)  32.7ab (0.45)  48.2ab (0.51)  59.9a (0.59)  71.3a (0.62)  77.1bc (0.78)  29.3 (0.73)  34.5b (0.77)  36.3b (1.0)   With pine nurse  13.0a (0.24)  24.9a (0.29)  40.1a (0.35)  54.5a (0.47)  66.5a (0.55)  53.0a (0.93)  49.3 (0.19)  59.9c (0.22)  70.8c (0.3)   NAV  28.3c  42.6c  58.2c  69.9bc  79.5bc  68.9b  39.9  50.0c  58.0c   NAW  31.6c  46.1c  60.9c  73.5c  82.6c  81.2c  43.0  54.9c  67.8c   NPV  25.3bc  39.8bc  56.8bc  70.4bc  80.9bc  80.6c  39.9  50.5c  62.5c   NPW  29.5c  42.6c  58.0c  70.7bc  80.8bc  74.8bc  43.3  54.0c  69.9c   OW – control  19.8ab  32.7ab  48.6ab  62.4ab  73.5ab  74.3bc  Nd  10.4a  17.8a   Significance  **  **  **  **  **  **  ns  ***  ***   SED  3.25  3.94  4.06  3.88  3.73  4.78  5.32  5.71  6.26   5% LSD  7.08  8.59  8.86  8.45  8.12  10.65  11.87  12.44  13.63  Values for the nursing mixtures are the combined values for both nurse and Sitka spruce: the values in parentheses show the proportion of Sitka spruce in the mixture. In any given column, treatments with different letter suffices are significantly different from one another. Treatment codes are: NAV, annual applications of nitrogen and no vegetation control; NAW, annual application of nitrogen plus weed control; NPV, periodic application of nitrogen and no vegetation control; NPW, periodic application of nitrogen plus weed control; OW, no application of nitrogen plus weed control. For details of timings and rates of applications of fertilizer and/or herbicides, see Table 1. Nd indicates ‘no data’ where a particular treatment was not measured. Significance values are presented as: ns = non-significant; *P < 0.05; **P < 0.01; ***P < 0.001. Strathy On this northerly site, there were significant differences in height at most ages of assessment, which mainly reflected the much poorer growth of Sitka spruce in the control plots where no nitrogen was applied, especially from age 17 (Table 2b). At years 17 and 20, there was a significant difference between the two timings of nitrogen input (P < 0.05) with spruce trees given periodic inputs being taller than those with annual applications. At time of canopy closure (17 years), the differences in height between the spruce and each of the nurse species in the mixed plots were less than 1 m (data not shown). Patterns of diameter growth were similar to those for height, with the smaller values in the control being the most significant difference. In addition, spruce trees in the NPV treatment also tended to be smaller in diameter than those in the remainder of the nitrogen treatments and in the mixtures. Basal area values were consistently much lower in the control than in the other treatments (Table 3). It was also lower in the mixture with larch, with no differences between the other treatments. Sitka spruce became the dominant component of the mixture with larch, but formed only 30 per cent of the mixture with lodgepole pine. Inchnacardoch In this split-plot experiment, the only effect of species/mixture upon height growth was at 20 years when the Sitka spruce in the pure plots was significantly smaller (8.0 m vs 9.5 m and 9.4 m for the spruce in the larch and pine mixtures, respectively: P < 0.05) (Table 4). There was a positive height growth response to nitrogen at years 15 and 20 (P < 0.001). At 18 years after planting, the difference in height between the spruce and each of the nurse species in the mixed plots was less than 2 m (data not shown). By contrast, there was a significant effect of species/mixture upon Sitka spruce diameter at all years of assessment (P < 0.01) with the trees in the pure plots always having a lower diameter than those in mixture (thus at 45 years diameter of pure spruce was 14.1 cm vs 24.5 cm and 22.8 cm, respectively, for spruce in mixture with larch and pine). Diameter of spruce was always greater in the presence of nitrogen (P < 0.01 or 0.001). In the early years of this experiment, these effects were generally due to the poorer performance of the pure spruce plots where no nitrogen had been applied (Table 4). Table 4 45 year growth in height and diameter at breast height (dbh) at Inchnacardoch of Sitka spruce grown either in mixture or in pure plots with (+) or without (–) nitrogen (N) fertilizer     Height (m)  Dbh (cm)    Age (years)  9  15  20  45  20  25  30  35  45  Treatment   With larch nurse  +N  1.9a  5.7c  10.2b  23.9  13.6d  17.5e  19.8e  21.4  25.6  –N  1.7b  4.4b  8.9b  21.8  10.6bc  14.4cd  17.0cd  19.2  23.5   With pine nurse  +N  1.6b  5.2bc  9.5b  23.7  11.1c  15.0de  17.6de  19.4  23.7  –N  1.8ab  4.8bc  9.4b  24.3  9.0b  12.2bc  14.9bc  17.2  22.0   Pure  +N  1.8ab  5.1bc  9.8b  20.9  9.8bc  11.5b  12.5b  13.2  16.3  –N  1.7b  2.9a  6.3a  19.2  5.5a  6.4a  7.5a  8.7  11.9   Significance  *  **  **  ns  *  *  *  ns  ns   SED  0.09  0.32  0.63  1.16  0.40  0.54  0.63  0.69  1.24   5% LSD  0.19  0.69  1.38  2.54  0.91  1.20  1.40  1.54  2.72      Height (m)  Dbh (cm)    Age (years)  9  15  20  45  20  25  30  35  45  Treatment   With larch nurse  +N  1.9a  5.7c  10.2b  23.9  13.6d  17.5e  19.8e  21.4  25.6  –N  1.7b  4.4b  8.9b  21.8  10.6bc  14.4cd  17.0cd  19.2  23.5   With pine nurse  +N  1.6b  5.2bc  9.5b  23.7  11.1c  15.0de  17.6de  19.4  23.7  –N  1.8ab  4.8bc  9.4b  24.3  9.0b  12.2bc  14.9bc  17.2  22.0   Pure  +N  1.8ab  5.1bc  9.8b  20.9  9.8bc  11.5b  12.5b  13.2  16.3  –N  1.7b  2.9a  6.3a  19.2  5.5a  6.4a  7.5a  8.7  11.9   Significance  *  **  **  ns  *  *  *  ns  ns   SED  0.09  0.32  0.63  1.16  0.40  0.54  0.63  0.69  1.24   5% LSD  0.19  0.69  1.38  2.54  0.91  1.20  1.40  1.54  2.72  Note that the significance values presented are for the split-plot analysis and not for the main treatment effects. At any given age, treatments with different letter suffices are significantly different from one another. Significance values are presented as: ns, non-significant; *P < 0.05; **P < 0.01; ***P < 0.001. Analysis of whole treatment basal area showed a significant effect of species/mixture at years 25–35 (P < 0.05) due to lower values in the pure spruce plots compared with the mixtures with lodgepole pine. There was a very highly beneficial effect of nitrogen in all years (P < 0.001). There were interactions evident (Table 5) since the production of pure spruce without nitrogen was consistently low. The values in the mixture with Japanese larch without nitrogen were lower than in the same mixture when nitrogen was applied, while no such response was evident in the mixture with lodgepole pine. Both mixed treatments self-thinned towards dominance by Sitka spruce. However, this rate was faster in the mixture with Japanese larch and in the presence of nitrogen (Table 5). Table 5 Basal area production (m2 ha−1) of plots of nursing mixtures with Sitka spruce and of pure spruce plots at Inchnacardoch with (+) and without (–) nitrogen (N) fertilizer application     Basal area    Age (years)  20  25  30  35  45  Treatment   With larch nurse  +N  29.9 (0.66)c  45.0 (0.73)c  55.2 (0.76)c  62.6 (0.79)c  62.8 (0.98)c  –N  18.2 (0.54)b  29.9 (0.61)b  38.9 (0.67)b  47.0 (0.71)b  50.9 (0.92)ab   With pine nurse  +N  30.6 (0.37)c  45.1 (0.46)c  55.3 (0.52)c  63.6 (0.56)c  62.1 (0.75)bc  –N  28.8 (0.31)c  42.7 (0.36)c  53.8 (0.43)c  63.8 (0.49)c  61.0 (0.76)bc   Pure  +N  35.4c  49.7c  59.5c  66.4c  70.8c  –N  7.3a  12.4a  17.3a  23.9a  37.0a   Significance  *  ***  ***  ***  **   SED  2.56  3.50  4.32  4.82  4.83   5% LSD  5.58  7.63  9.43  10.50  10.52      Basal area    Age (years)  20  25  30  35  45  Treatment   With larch nurse  +N  29.9 (0.66)c  45.0 (0.73)c  55.2 (0.76)c  62.6 (0.79)c  62.8 (0.98)c  –N  18.2 (0.54)b  29.9 (0.61)b  38.9 (0.67)b  47.0 (0.71)b  50.9 (0.92)ab   With pine nurse  +N  30.6 (0.37)c  45.1 (0.46)c  55.3 (0.52)c  63.6 (0.56)c  62.1 (0.75)bc  –N  28.8 (0.31)c  42.7 (0.36)c  53.8 (0.43)c  63.8 (0.49)c  61.0 (0.76)bc   Pure  +N  35.4c  49.7c  59.5c  66.4c  70.8c  –N  7.3a  12.4a  17.3a  23.9a  37.0a   Significance  *  ***  ***  ***  **   SED  2.56  3.50  4.32  4.82  4.83   5% LSD  5.58  7.63  9.43  10.50  10.52  Values for the nursing mixtures are the combined values for both nurse and Sitka spruce: the values in parentheses show the proportion of Sitka spruce in the mixture. Note that the significance values presented are for the split-plot analysis and not for the main treatment effects. At any given age, treatments with different letter suffices are significantly different from one another. Significance values are presented as: ns, non-significant; *P < 0.05; **P < 0.01; ***P < 0.001. Culloden From 12 years after planting, there were differences in height growth between the pure spruce plots and the spruce grown in mixture with Scots pine with the latter being the taller: these differences were highly significant in the years up to canopy closure (Table 6). At time of canopy closure (18 years), the differences in height between the spruce and the nurse species in the mixed plots were less than 1 m (data not shown). Table 6 Growth of Sitka spruce when planted pure and in mixture with Scots pine at Culloden Parameter  Height (m)  Dbh (cm)  Basal area (m2 ha−1)  Age (years)  3  6  12  20  34  42  12  34  42  34  42  Treatment   Pure spruce  0.4  1.3  3.3  7.0  15.9  21.8  5.0  10.5  13.3  33.5  43.7   In mixture with Scots pine  0.5  1.3  5.2  10.8  18.7  24.0  5.9  24.1  28.1  60.9 (0.57)  64.6 (0.67)   Significance  **  ns  ***  **  ns  ns  ns  **  **  **  *   SED  0.02  0.05  0.1  0.4  1.4  0.7  0.4  1.4  1.4  3.5  4.9   5% LSD  0.07  0.07  0.4  1.4  4.4  2.2  1.3  4.4  4.5  11.2  15.6  Parameter  Height (m)  Dbh (cm)  Basal area (m2 ha−1)  Age (years)  3  6  12  20  34  42  12  34  42  34  42  Treatment   Pure spruce  0.4  1.3  3.3  7.0  15.9  21.8  5.0  10.5  13.3  33.5  43.7   In mixture with Scots pine  0.5  1.3  5.2  10.8  18.7  24.0  5.9  24.1  28.1  60.9 (0.57)  64.6 (0.67)   Significance  **  ns  ***  **  ns  ns  ns  **  **  **  *   SED  0.02  0.05  0.1  0.4  1.4  0.7  0.4  1.4  1.4  3.5  4.9   5% LSD  0.07  0.07  0.4  1.4  4.4  2.2  1.3  4.4  4.5  11.2  15.6  Basal area figures for the mixture are the total value for both species with the Sitka spruce proportion in parentheses. Height and diameter values for the mixed plot are for the Sitka spruce component only. There were significant differences (P < 0.01) in Sitka spruce diameter at years 34 and 42 due to the much larger size of the trees growing in mixture. At both these times, larger diameter of spruce was found when the trees were growing in the closer drainage treatment (P < 0.01). There was also a significant mixture/cultivation interaction (P < 0.01) due to the spruce in mixture having a larger diameter when growing on the spaced furrow ploughing (data not shown). The mixed plots had a much greater basal area than the pure plots at both years 34 and 42 (P < 0.01 and P < 0.05 respectively), indeed the spruce production in mixture was almost identical with that of the pure plots even though the latter contained twice as many spruce trees. By year 42 Sitka spruce represented an increasing proportion of the mixture. At year 34 basal area was higher on the complete ploughing (P < 0.05), while there was a significant interaction (P < 0.05) between mixture and drainage due to poorer growth of the pure spruce planted on the wider drain spacing (data not shown). Foliage analysis In Figure 2, the values for up to three representative treatments are presented over time at each experiment. At three of the sites (Inchnacardoch, Strathy, Culloden) the foliar nitrogen concentrations observed during the late establishment and early stem exclusion phase are significantly lower in the controls than in the mixture with pine, or where nitrogen was applied. The nitrogen concentrations observed in the controls were indicative of ‘deficient’ or ‘severely checked growth’ whereas those in the mixtures indicated a ‘marginal’ or ‘optimal’ status. However, at Drumtochty the differences between the control and the other treatments were less substantial. Figure 2 View largeDownload slide Comparison of Sitka spruce foliar nitrogen concentration (per cent dry weight) over time in selected treatments at the four experimental sites. Treatments are: Sitka spruce mixed with lodgepole or Scots pine, pure spruce with periodic nitrogen application, control (i.e. pure spruce with no nitrogen application). Open triangles indicate that there was a significant difference (P < 0.05) between the values in the control and those of the spruce in mixture; closed triangles indicate no significant difference. The upper dotted line shows the boundary between ‘optimal’ and ‘marginal’ nitrogen status; the lower dotted line gives that between ‘marginal’ and deficient’. Figure 2 View largeDownload slide Comparison of Sitka spruce foliar nitrogen concentration (per cent dry weight) over time in selected treatments at the four experimental sites. Treatments are: Sitka spruce mixed with lodgepole or Scots pine, pure spruce with periodic nitrogen application, control (i.e. pure spruce with no nitrogen application). Open triangles indicate that there was a significant difference (P < 0.05) between the values in the control and those of the spruce in mixture; closed triangles indicate no significant difference. The upper dotted line shows the boundary between ‘optimal’ and ‘marginal’ nitrogen status; the lower dotted line gives that between ‘marginal’ and deficient’. At Drumtochty, there were significant differences between treatments from age 6 until 21, but subsequent effects were non-significant (Supplementary data, Table 3a). Initially these differences reflected lower nitrogen status in spruce growing in mixture compared with all pure plots. Thereafter, lowest nitrogen levels were found in the control treatment while highest values occurred in plots where spruce was either mixed with Japanese larch or given regular nitrogen applications. At the last date of assessment (year 30), all treatments had optimal nitrogen foliar levels. By contrast, at Strathy there were significant differences between treatments at all ages. The highest values were generally found where nitrogen was applied annually, but from age 15 onwards the lowest values were consistently found in the controls and in the mixture with hybrid larch while higher values occurred in the mixture with lodgepole pine. At the last assessment (year 46), the control value would have been classed as ‘severely checked’ while all the other treatments were on the borderline between ‘deficient’ and ‘marginal’ (Supplementary data, Table 3b). In the Inchnacardoch experiment, there were significant differences in nitrogen status between the main ‘mixture’ treatments from ages 11 until 36 years reflecting consistently lower values in the pure spruce plots. There were also significant differences from 11 to 27 years between treatments given nitrogen as opposed to those with no nitrogen application. Interaction between the mixture and the nitrogen treatments occurred at intervals until year 30, reflecting lower nutrient levels in the pure spruce plots grown without nitrogen (Supplementary data, Table 4). At the last assessment, all treatments would have been classed as being ‘deficient’ in nitrogen. At Culloden, there were also significant differences from age 11 years until 22 years, but no difference at age 42 years. This reflected a consistently higher nitrogen status in the spruce grown in mixture with Scots pine. At the last assessment both treatments would have been classed as having ‘marginal’ nitrogen foliar concentrations. Discussion Results from these four experiments have indicated that, on nutrient-poor sites, the better growth of Sitka spruce in mixture compared with that of pure spruce without nitrogen was evident at ~10 and 20 years after planting (Tables 2, 4 and 6). This improvement resulted in longer-term increases in basal area in accord with our working hypothesis. These increases are also likely to have resulted in a greater volume outturn, but the lack of any measurements of stem taper in the different treatments means that a linear relationship between basal area and volume should not be assumed (Pretzsch, 2009, Chapter 9). The magnitude and duration of these basal area gains is mediated by site quality since on the more nutrient-poor sites at Strathy, Inchnacardoch and Culloden, basal area after more than 40 years was higher in the mixed plots than in the pure spruce without nitrogen (Tables 3, 5 and 6). In contrast, on a more fertile site at Drumtochty, over time there was no difference between the nursing mixture with Japanese larch and the pure spruce without nitrogen (Table 3), while the nursing mixture with lodgepole pine had a lower basal area. In another experiment in the same region where the soil nutrient regime was on the border between ‘poor’ and ‘very poor’, basal area was higher in a larch–Sitka spruce mixture than pure spruce at 20 and 25 years of age but there were no differences evident at 41 years (Mason, 2014). In terms of soil fertility, that site would have been intermediate between Drumtochty and the three other experiments presented in the present paper which were all located in the most nutrient-poor category of the ESC system (see Supplementary data Table 1). In general, all mixtures have self-thinned towards domination by Sitka spruce, although the proportion of the basal area occupied by the spruce has varied between sites and with species used as a nurse. Except on the most fertile site at Drumtochty, there was a tendency for the larch species used as a nurse to die out sooner than the pines (Tables 3 and 5), possibly reflecting a lower tolerance of competition and of the wetter peat soils at Strathy and Inchnacardoch. One consequence of this self-thinning process was that at the last assessment the average diameter of the spruce grown in nursing mixture was nearly always larger than that of the spruce grown in pure plots (Tables 2, 4 and 6). This increase could be an indicator of improved wind stability in mixture since, everything else being equal, tree resistance to overturning by wind is proportionate to the dbh of a tree squared (Gardiner et al., 1997). The temporal pattern of foliar nitrogen values whereby a decline is followed by a slow recovery after canopy closure mediated by site fertility agrees with results from a series of other fertilizer experiments on second rotation sites across northern Britain (Smith and McKay, 2002). The period of deficiency can be offset by remedial fertilizer application as shown by the higher foliar concentrations in spruce trees from treatments where nitrogen was applied at Drumtochty and Inchnacardoch (supplementary data, Tables 3 and 4). However, this may not be a cost-effective option, given that the cost of a single aerial fertilizer application can exceed £200 ha−1 (pers. comm. Bill Rayner, Forest Research) and that several such inputs might be needed to ensure canopy closure of a pure spruce stand on a nutrient-poor site (Taylor, 1991). In addition, certification protocols favour a reduction in the use of synthetic chemicals such as nitrogen fertilizer in British forests (Smith and McKay, 2002). By contrast, the spruce trees in the nursing mixtures experienced an initial period where foliar nitrogen levels were at ‘deficient’ or ‘severely checked’ levels, but by 10–15 years after planting the values rose to ‘marginal’ or ‘optimal’ status (Figure 2) and the tree growth rates were comparable to those found in the pure plots where nitrogen had been applied. The end-result in the mixed plots was the establishment of a spruce dominated stand of equivalent productivity to pure spruce where nitrogen was applied but without the need for costly remedial inputs. In recent decades, a common approach used to explore species interactions in mixed stands has been to compare the productivity of a mixture against that of the component species in pure plots on the same site (Kelty, 1992; Pretzsch, 2009; Forrester and Pretzsch, 2015). If the mixture is more productive than the average production of the component species in pure stands, ‘overyielding’ is said to occur, whereas if it less productive it indicates ‘underyielding’. Where a mixture is higher yielding than the most productive of the pure stands, this is termed ‘transgressive overyielding’. The feasibility of carrying out a similar analysis in these experiments was constrained by the lack of pure plots of the nurse species. We attempted to compensate for this by estimating the likely productivity of the pine nurses in pure plots through a combination of site details and yield tables (see Supplementary Paper Two). This exercise suggested possible transgressive overyielding at the three most nutrient-poor sites, but no evidence of overyielding at the more fertile site (Drumtochty). Other results indicate that overyielding in mixtures declines with improving site fertility (Bielak et al., 2014; Toigo et al., 2015), in line with the stress-gradient hypothesis (Bertness and Callaway, 1994) proposing that interactions between species change from competition in favourable conditions to facilitation in harsher situations. Other possible explanations for this nursing effect could include competitive reduction through canopy stratification (Forrester and Pretzsch, 2015), but this seems unlikely given the lack of any differentiation in height between the components of the mixture at time of canopy closure when the mixture effect was most pronounced. There are variations in crown architecture between the nurse species and the spruce which might make for more efficient use of growing space (Forrester and Bauhus, 2016), but this seems unlikely to explain the major temporal changes in spruce foliage nitrogen status found in these experiments. Therefore, we consider that facilitation is the driving process in these experiments. In the long-term the spruce component of the nursing mixture benefits at the expense of the nurse which is gradually suppressed through competitive self-thinning, an example of asymmetric facilitation (Lin et al., 2012). Our understanding of the factors driving the facilitation effect in nursing mixtures has hardly advanced since the comprehensive summary of detailed studies at several sites including Culloden and Inchnacardoch provided by Morgan et al. (1992). In brief, these showed that there were enhanced mineralization rates in the soil in mixed stands which increased the amount of nitrogen available to the spruce. There were differential rooting patterns between the nurse species and the spruce, with the nurse being found to root to greater depths leading to greater aeration and enhanced microbial activity within the soil. There were also differences in mycorrhizal associations present in pure spruce stands compared with mixtures. For instance, Heslin et al. (1992) found a more diverse mycorrhizal flora in nursing mixtures with Japanese larch or lodgepole pine in Ireland compared with pure spruce. Studies at the Culloden experiment showed that at least one ectomycorrhizal species associated with the roots of Scots pine in the mixed stands was capable of degrading proteins and so enhancing the pool of organic nitrogen in the soil which would then be accessible for uptake by the spruce. By contrast, the generalist mycorrhizal fungi associated with spruce roots were not capable of degrading proteins, but were capable of utilizing the products of protein breakdown (Ryan & Alexander, 1992). Further studies at Strathy, Inchnacardoch and Culloden showed that the foliage of spruce in pure stands had significantly depleted levels of 15N compared with spruce in mixtures and that in the mixtures there were no significant differences between the levels found in the spruce and in the nurse species (Horsburgh, 1997). This confirmed that the spruce in nursing mixtures had access to a pool of organic nitrogen that was not present in pure stands. There was no effect of different nurse species upon the levels of 15N, but an investigation in another experiment in northern Scotland suggested that these levels increased with an increasing proportion (from 25 to 75 per cent of stems) of the nurse in the mixture (Horsburgh, 1997). Although this nursing effect is mentioned in reviews of nutritional interactions in mixtures (Rothe and Binkley, 2001; Richards et al., 2010), it has been overlooked in meta-analyses of mixture performance (Piotto, 2008; Hulvey et al., 2013) where only the contribution of nitrogen fixing species such as alders or acacias has been considered. Furthermore, given that mycorrhizal associations are critical to the functioning of ecosystem processes on nutrient-poor soils in the boreal forests and other nutrient-poor sites (Read et al., 2003; Collier and Bidartondo, 2009), we suggest that the beneficial mycorrhizal interaction believed to drive the nursing mixtures effect in northern Britain may be more widespread than currently recognized. Thus a number of recent papers have reported positive growth responses where Scots pine is grown in mixture with other European species, for example with Norway spruce (Linden and Agestam, 2003; Bielak et al., 2014; Mason and Connolly, 2014), with sessile or pedunculate oaks (Gabriel et al., 2005; Lu et al., 2016), or with beech (Gabriel et al., 2005; Pretzsch et al., 2015a,b). Often such responses are primarily attributed to aspects of tree architecture (e.g. deep or shallow rooting) or to functional traits such as shade tolerance, while seemingly neglecting any possible mycorrhizal contribution, especially in the establishment phase. It remains a major research challenge to identify those regions where this nursing effect is likely to occur and, within those regions, to achieve a better understanding of the precise mechanisms of nutrient enhancement and transfer. Other aspects which merit investigation are determining if there is a minimum proportion of the nurse species required to initiate the nursing effect, and whether the mycorrhizal associates of the nurse species can persist as the latter is outcompeted in the mixture. The practical significance of these experiments is considerable. First, they indicate that the use of nursing mixtures can allow a substantial increase in stand productivity on nutrient-poor soils in Britain, and possibly elsewhere in northern Europe, without requiring intensive and expensive fertilizer inputs. However, achieving these gains depends upon choosing a nurse species with early growth rates that are compatible with the admixed species. For example, a survey of 20 000 ha of mixtures of lodgepole pine and Sitka spruce in southern Scotland found that a mixed stand would self-thin towards spruce dominance provided that the pine was no more than 2 m taller than the spruce at the beginning of canopy closure (Garforth, 1979). Where this did not apply (e.g. with more vigorous provenances of lodgepole pine), the pine largely suppressed the spruce. Second, wider and continued use of these mixtures is probably essential for sustaining productivity of planted conifer forests on soils overlying nutrient-poor lithologies in Britain (e.g. the Moine schists and Torridonian sandstones of northern Scotland), not least because remedial fertilizer inputs are rarely used because of their cost. Third, users of existing growth models and of other decision support tools which predict species response in relation to site or climatic features need to be aware that such aids currently make little allowance for the possibility of species interaction in mixture (Pretzsch et al., 2015a,b). Taken overall, the findings of these experiments are a salutary reminder that much silvicultural practice and many management prescriptions are based on an understanding gleaned from experiments and experience with single species stands. The performance and dynamics of species in mixture is complex, cannot necessarily be predicted from that of the individual components in isolation, and requires a good appreciation of the mechanisms that sustain the functioning of the forest ecosystem. Supplementary data Supplementary data are available at Forestry online. Conflict of interest statement None declared. Acknowledgements This is a revised version of a paper originally presented at the IUFRO conference held in August 2015 in Edmonton, Canada on the topic of the ‘Ecology, silviculture and management of spruce species in mixed forests’. WLM acknowledges support from the EU Cost Action FP1206 ‘EuMixFor’ that enabled him to attend the meeting. We are grateful to past members of Forest Research’s Silviculture (North) branch for the design and oversight of these experiments and for staff of the Technical Support Unit for their management and assessment. We thank Dr Victoria Stokes for helpful comments on an earlier version of this manuscript and to Stephen Bathgate for assistance in identifying plots to validate the overyielding analysis. Further suggestions provided by Dr Gary Kerr and two anonymous referees considerably improved the final version of the paper. References Anonymous 2010 A Guide for Increasing Tree Species Diversity in Wales . Forestry Commission Wales, p. 41. Bertness, M.D. and Callaway, R. 1994 Positive interactions in communities. Trends Ecol. Evol.  9, 191– 193. Google Scholar CrossRef Search ADS PubMed  Bielak, K., Dudzinska, M. and Pretzsch, H. 2014 Mixed stands of Scots pine (Pinus sylvestris L.) and Norway spruce (Picea abies (L.) Karst.) can be more productive than monocultures. Evidence from over 100 years of observation of long-term experiments. Forest Syst.  23 ( 3), 573– 589. Google Scholar CrossRef Search ADS   Brooker, R.W., Maestre, F.T., Callaway, R.M., Lortie, C.L., Cavieres, L.A., Kunstler, G., et al.  . 2008 Facilitation in plant communities: the past, the present, and the future. J. Ecol.  96, 18– 34. Google Scholar CrossRef Search ADS   Cameron, A.D. and Watson, B.A. 1999 Effect of nursing mixtures on stem form, crown size, branching habit and wood properties of Sitka spruce (Picea sitchensis (Bong.) Carr. For. Ecol. Manage.  122, 113– 124. Google Scholar CrossRef Search ADS   Carey, M.L., McCarthy, R.G. and Miller, H.G. 1988 More on nursing mixtures. Ir. Forestry  45, 7– 20. Carlyle, J.C. and Malcolm, D.C. 1986 Nitrogen availability beneath pure spruce and mixed larch and spruce stands growing on a deep peat. I. Net N mineralization measured by field and laboratory incubations. Plant. Soil.  93, 95– 113. Google Scholar CrossRef Search ADS   Collier, F.A. and Bidartondo, M. 2009 Waiting for fungi: the ecotmycorrhizal invasion of lowland heathlands. J. Ecol.  97, 950– 963. Google Scholar CrossRef Search ADS   Felton, A., Lindbladh, M., Brunet, J. and Fritz, O. 2010 Replacing coniferous monocultures with mixed-species production stands: an assessment of the potential benefits for forest biodiversity in northern Europe. For. Ecol. Manage.  260, 939– 947. Google Scholar CrossRef Search ADS   Felton, A., Nilsson, U., Sonesson, J., Felton, A.M., Roberge, J.-M., Ranius, T., et al.  . 2016 Replacing monocultures with mixed-species stands: ecosystem service implications of two production forest alternatives in Sweden. Ambio  45, S124– S139. Google Scholar CrossRef Search ADS   Forrester, D.I. and Bauhus, J. 2016 A review of processes behind diversity-productivity relationships in forests. Curr. Forestry Rep.  2, 45– 61. Google Scholar CrossRef Search ADS   Forrester, D.I. and Pretzsch, H. 2015 Tamm review: on the strength of evidence when comparing ecosystem functions of mixtures with monocultures. For. Ecol. Manage.  356, 41– 53. Google Scholar CrossRef Search ADS   Gabriel, K., Blair, I. and Mason, W.L. 2005 Growing broadleaved trees on the North York Moors: results after nearly 50 years. Q. J. For.  99, 21– 30. Gamfeldt, L., Snall, T., Bagchi, R., Jonsson, M., Gustaffson, L., Kjellander, P., et al.  . 2013 Higher levels of multiple ecosystem services are found in forests with more tree species. Nat. Commun . doi:10.1038/ncomms2328. Gardiner, B.A., Stacey, G.R., Belcher, R.E. and Wood, C.J. 1997 Field and wind tunnel assessments of the effects of respacing on tree stability. Forestry  70, 233– 252. Google Scholar CrossRef Search ADS   Garforth, M.F. 1979 Mixtures of Sitka spruce and lodgepole pine in South Scotland: history and future management. Scot. For.  33, 15– 28. Grant, A., Worrell, R., Wilson, S., Ray, D. and Mason, W.L. 2012 Achieving diversity in Scotland’s forest landscapes. Forestry Commission Scotland Practice Guide . Forestry Commission, p. 30. Heslin, M.C., Blasius, D., McElhinney, C. and Mitchell, D.T. 1992 Mycorrhizal and associated fungi of Sitka spruce in Irish forest mixed stands. Eur. J. For. Path  22, 46– 57. Google Scholar CrossRef Search ADS   Hibberd, B.G. 1991 Forestry practice. Forestry Commission Handbook 6 . HMSO. Horsburgh, A.M. 1997 Patterns of N concentration and 15N natural abundance in pure and mixed stands of Sitka spruce. Unpublished Ph.D. thesis, University of Aberdeen. Hulvey, K.B., Hobbs, R.J., Standish, R.J., Lindenmayer, D.B., Lach, L. and Perring, M.P. 2013 Benefits of tree mixes in carbon plantings. Nat. Clim. Change  3, 869– 874. Google Scholar CrossRef Search ADS   Kelty, M.J. 1992 Comparative productivity of monocultures and mixed species stands. In The Ecology and Silviculture of Mixed Species Forests . Kelty M.J., Larson B.C. and Oliver C.D. (eds). Kluwer, pp. 125– 141. Google Scholar CrossRef Search ADS   Kelty, M.J. 2006 The role of species mixtures in plantation forestry. For. Ecol. Manage.  233, 195– 204. Google Scholar CrossRef Search ADS   Lin, Y., Berger, U., Grimm, V. and Ji, Q.-R. 2012 Differences between symmetric and asymmetric facilitation matter: exploring the interplay between modes of positive and negative plant interactions. J. Ecol.  100, 1482– 1491. Google Scholar CrossRef Search ADS   Linden, M. and Agestam, E. 2003 Increment and yield in mixed and monoculture stands of Pinus sylvestris and Picea abies based on an experiment in southern Sweden. Scan. J. For. Res.  18, 155– 162. Google Scholar CrossRef Search ADS   Lu, H., Mohren, G.M.J., den Ouden, J., Goudiaby, V. and Sterck, F.J. 2016 Overyielding of temperate mixed forests occurs in evergreen-deciduous but not in deciduous–deciduous species mixtures over time in the Netherlands. For. Ecol. Man.  376, 321– 332. Google Scholar CrossRef Search ADS   Macdonald, J.A.B. 1936 The effect of introducing pine species among checked Sitka spruce on a dry, Calluna-clad slope. Trans. R. Soc. Arboricultural Soc.  50, 83– 86. Macdonald, J.A.B. and Macdonald, A. 1952 The effect of interplanting with pine on the emergence of Sitka spruce from check on heather land. Scot. For.  6, 77– 79. Mason, W.L. 2007 Changes in the management of British forests between 1945 and 2000 and possible future trends. Ibis  149, 41– 52. Google Scholar CrossRef Search ADS   Mason, W.L. 2014 Long-term development of nursing mixtures of Sitka spruce and larch species in an experiment in northern Scotland. Forest. Syst.  23 ( 3), 590– 597. Google Scholar CrossRef Search ADS   Mason, W.L. and Connolly, T. 2014 Mixtures with spruce species can be more productive than monocultures: evidence from the Gisburn experiment in Britain. Forestry . doi:10.1093/forestry/cpt042. Mason, W.L. and Perks, M.P. 2011 Sitka spruce (Picea sitchensis) forests in Atlantic Europe: changes in forest management and possible consequences for carbon sequestration. Scan. J. For. Res. Suppl.  11, 72– 81. Google Scholar CrossRef Search ADS   McIntosh, R. 1983 Nitrogen deficiency in established phase Sitka spruce in upland Britain. Scot. For.  35, 185– 193. Morgan, J.L., Campbell, J.M. and Malcolm, D.C. 1992 Nitrogen relations of mixed-species stands on oligotrophic soils. In The Ecology of Mixed-species Stands of Trees . Cannell M.G.R., Malcolm D.C. and Robertson P.A. (eds). Blackwell, Oxford, pp. 65– 85. O’Carroll, N. 1978 The nursing of Sitka spruce I. Japanese larch. Ir. Forestry  35, 60– 65. Paquette, A. and Messier, C. 2011 The effect of biodiversity on tree productivity: from temperate to boreal forests. Glob. Ecol. Biogeogr.  20, 170– 180. Google Scholar CrossRef Search ADS   Piotto, D. 2008 A meta-analysis comparing tree growth in monocultures and mixed plantations. For. Ecol. Manage.  255, 781– 786. Google Scholar CrossRef Search ADS   Pretzsch, H. 2009 Forest Dynamics, Growth and Yield . Springer-Verlag, p. 664. Pretzsch, H., del Rio, M., Ammer, C., Avdagic, A., Barbeito, I., Bielak, K., et al.  . 2015a Growth and yield of mixed versus pure stands of Scots pine (Pinus sylvestris L.) and European beech (Fagus sylvatica L.) analysed along a productivity gradient through Europe. Euro. J. For. Res . doi:10.1007/s10342-015-0900-4. Pretzsch, H., Forrester, D.I. and Rotzer, T. 2015b Representation of species mixing in forest growth models; a review and perspective. Ecol. Model.  313, 276– 292. Google Scholar CrossRef Search ADS   Read, D.J., Leake, J.R. and Perez-Moreno, J. 2003 Mycorrhizal fungi as drivers of processes in heathland and boreal forest biomes. Can. J. Bot.  82, 1243– 1263. Google Scholar CrossRef Search ADS   Read, D.J., Freer-Smith, P.H., Morison, J.I.L., Hanley, N., West, C.C. and Snowdon, P. (eds). 2009 Combating climate change – a role for UK forests. An Assessment of the Potential of the UK’s Trees and Woodlands to Mitigate and Adapt to Climate Change . The Stationery Office. Richards, A.E., Forrester, D.I., Bauhus, J. and Scherer-Lorenzen, M. 2010 The influence of mixed tree plantations on the nutrition of individual species: a review. Tree Physiol.  30, 1192– 1208. Google Scholar CrossRef Search ADS PubMed  Robinson, R.K. 1972 The production by roots of Calluna vulgaris of a factor inhibitory to the growth of some mycorrhizal fungi. J. Ecol.  60, 219– 224. Google Scholar CrossRef Search ADS   Rothe, A. and Binkley, D. 2001 Nutritional interactions in mixed species forests: a synthesis. Canadian J. For. Res.  31, 1855– 1870. Google Scholar CrossRef Search ADS   Ryan, E.A. and Alexander, I.J. 1992 Mycorrhizal aspects of improved growth of spruce when grown in mixed stands on heathland soils. In Mycorrhizas in Ecosystems . Read D.J., Lewis D.H., Fitter A.H. and Alexander I.J. (eds). CAB International, pp. 237– 245. Smith, S.A. and McKay, H.M. 2002 Nutrition of Sitka Spruce on Upland Restock Sites. Forestry Commission Information Note 47 . Forestry Commission. Taylor, C.M.A. 1991 Forest fertilisation in Great Britain. Forestry Commission Bulletin 95 . HMSO. Toigo, M., Vallet, P., Perot, T., Bontemps, J.-D., Piedallu, C. and Courbaud, B. 2015 Overyielding in mixed forests decreases with site productivity. J. Ecol.  103, 505– 512. Google Scholar CrossRef Search ADS   UKFS. 2011 The UK Forestry Standard, pp. 116. http://www.forestry.gov.uk/pdf/FCFC001.pdf/$FILE/FCFC001.pdf (accessed on 15 March, 2014). VSN International, 2013. GenStat for Windows 16th Edition. VSN International, Hemel Hempstead, UK. Web page: GenStat.co.uk Watson, B.A. and Cameron, A.D. 1995 Some effects of nursing species on stem form, branching habit and compression wood content of Sitka spruce. Scot. For.  49, 146– 154. Weatherell, J. 1957 The use of nurse species in the afforestation of upland heaths. Q. J. For.  51, 298– 304. Zehetmayr, J.W.L. 1960 Afforestation of upland heaths. Forestry Commission Bulletin No. 32 . HMSO. Zhang, Y., Chen, H.Y.H. and Reich, P.B. 2012 Forest productivity increases with evenness, species richness and trait variation: a global meta-analysis. J. Ecol. , 8. doi:10.1111/j.1365-2745.2011.01944.x. © Crown copyright 2018. This article contains public sector information licensed under the Open Government Licence v3.0 (http://www.nationalarchives.gov.uk/doc/open-government-licence/version/3/).

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Forestry: An International Journal Of Forest ResearchOxford University Press

Published: Apr 1, 2018

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