TY - JOUR
AU - Vogel, Jason G.
AB - Two randomized, complete block design experiments were established in north Florida to examine the inter-rotational effects of fertilization and herbicide applications on understory community responses in 2-year-old Pinus taeda L. stands. One experiment was left untreated (carryover [C]; CC, CF, CW, and CFW) and the second received the same first-rotation treatments—control (C), fertilizer (F), herbicide (W), and fertilizer and weed control (FW)—in the second rotation. In both experiments, herbicide applications alone and when combined with fertilization suppressed woody species such as Ilex glabra (L.) A. Gray and Serenoa repens (W. Bartram) Small (CFW: 0.26; FW: 0.01 Mg ha−1 biomass) but favored graminoids such as Andropogon virginicus L. var. glaucus Hack. and Dichanthelium acuminatum (Sw.) Gould & C.A. Clark (CFW: 3.1; FW: 1.1 Mg ha−1 biomass). Although the CW (0.99) and W (0.99) treatments did not affect understory diversity (H′), the CFW (0.45) and FW (0.5) treatments exhibited reductions compared with the CF (1.24) and F (1.33) treatments, respectively. In both experiments, fertilizer alone did not affect understory composition and diversity compared with the control. These results suggest that intensive additions of either herbicide alone or in combination with fertilization affected understory composition and diversity in the subsequent rotation. Management and Policy Implications The maintenance of long-term site productivity and associated ecological functions are central tenets for developing sustainable forest management systems. In addition to the overstory, the composition and diversity of understory vegetation represents an important component of forest ecosystems. Understanding how forest practices affect the inter-rotational development of understory vegetation is critical for advancing future management systems used in southern pine plantation ecosystems because of possible impacts, for example, on species diversity and nutrient cycling processes. This study examined how fertilizer additions and herbicide treatments affected understory redevelopment in second-rotation juvenile loblolly pine stands growing in north Florida. One experiment remained untreated to examine carryover effects of previous treatments whereas the second was actively managed using the same treatments as the prior rotation. The results for both experiments showed that intensive weed control treatments applied alone or in combination with fertilizer additions suppressed woody understory species development and favored graminoid vegetation (grasslike morphology) early in the next rotation. However, fertilizer additions alone did not affect understory composition and diversity when compared with the untreated control. When making forest management decisions in loblolly pine plantations, land managers should recognize that historical and intensive weed control treatments can have inter-rotational effects on understory species richness, diversity, and composition. sustainability, species richness, diversity, biomass, nutrients In nutrient-limited terrestrial ecosystems, site amelioration treatments that include fertilizer additions can increase aboveground net primary productivity (Gough et al. 2000, LeBauer and Treseder 2008). Herbicide applications increase overstory productivity mainly by reducing the competitive influence of understory vegetation on site resources (Martin and Jokela 2004). These changes in productivity are often accompanied by changes in abundance, richness, and diversity of understory plant communities (Tilman 1987, Swindel et al. 1989, Vitousek and Farrington 1997, Miller et al. 2003, Wassen et al. 2005, LeBauer and Treseder 2008, Bobbink et al. 2010). Such changes in measures of understory community biodiversity affect resource use efficiency, overall biomass productivity, nutrient cycling, and stability of these ecosystem processes (Cardinale et al. 2007, Hector et al. 2010, Tilman et al. 2014). Understanding whether such changes in understory community responses are transient or permanent is critical to the recovery or reorganization of ecosystem functions after perturbations induced by long-term site management practices (Folke et al. 2004). The effects of nitrogen (N) and phosphorus (P) fertilizer additions on vegetation responses have been well studied in agricultural and grassland ecosystems, where consistent declines in plant diversity after N and P enrichment have been observed (DiTommaso and Aarssen 1989, Hautier et al. 2009, Dickson and Foster, 2011). For example, in grassland ecosystems, P enrichment has been an important driver of species loss (Ceulemans et al. 2011). In addition to changes in diversity, shifts in understory community composition have been observed with long-term N and P enrichments (Avolio et al. 2014). Such shifts in plant communities are often attributed to changes in competitive interactions among species, via shifts in competition from belowground to aboveground, or creation of less heterogeneous sites that affect species coexistence (Tilman and Pacala 1993, Rajaniemi 2003, Hautier et al. 2009, Dickson and Foster 2011, Dickson and Gross 2013). In comparison, changes in understory species abundance and diversity associated with nutrient enrichment in forested ecosystems have been less studied, which likely reflects a historical focus of treatment effects on overstory productivity. In general, research findings are less conclusive, with reports suggesting patterns of increase (Kellner 1993, Prescott et al. 1993), decrease (Thomas et al. 1999, Lu et al. 2010), or no net change (VanderSchaff et al. 2000, Ostertag and Verville 2002). For example, nutrient additions have reportedly altered functional groups in the understory community (Sword et al. 1998, Thomas et al. 1999, Miller et al. 2003). Thomas et al. (1999) documented a reduction in understory vegetation cover, especially herbaceous species, after urea fertilization in managed Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) plantations. Sword et al. (1998) reported that fertilization significantly increased the density of competing woody plants in a 3-year-old loblolly pine (Pinus taeda L.) plantation. For some nutrient-limited forested sites, selected herbicide applications can also alter competitive interaction between the overstory and understory, resulting in changes in both overstory productivity (Lauer et al. 1993, Jokela et al. 2010) and understory plant community dynamics (Swindel et al. 1989, Litt et al. 2001, Jones et al. 2009). For example, Miller et al. (2003) documented a compositional shift in the vegetative community (e.g., reduction in mean tree taxa with woody vegetation control and shrub taxa with herbaceous vegetation control) of 15-year-old loblolly pine plantations after herbicide applications over the first three to five growing seasons. Swindel et al. (1989) reported a reduction in understory vegetation cover from 100% to less than 10% after 5 years of repeated herbicide applications in an intensively managed southern pine stand. Although the effects of herbicide applications on understory vegetation generally tend to be short lived (Brockway et al. 1998, Miller et al. 1999), factors related to frequency and intensity of application as well as the selective nature of herbicides can affect understory plant community reinitiation and recovery (Jose et al. 2010). However, little is known regarding the inter-rotational effects of these treatments on understory vegetation reinitiation and recovery, especially in managed southern pine ecosystems. In this study, we hypothesize that long-term silvicultural treatments from the previous rotation will influence understory reinitiation in the successive rotation. For previously fertilized plots, woody species are expected to be a dominant component of the resulting understory community. For plots receiving sustained weed control treatments in the previous rotation, the reinitiated understory community in the next rotation is expected to be less diverse and dominated by graminoids. Specifically, we used two long-term replicated field experiments established on nutrient-limited Spodosols in north central Florida to answer two questions: Do historical silvicultural treatments (i.e., intensive fertilization and weed control) applied in the first rotation affect understory reinitiation and vegetative composition in a subsequently untreated second-rotation loblolly pine stand? Does the continuation of first-rotation treatments in the second rotation exhibit similar shifts in functional groups of understory vegetation as in the untreated second rotation? These questions were addressed by examining and comparing aboveground biomass accumulation, species richness, and diversity of the understory vegetation community over a range of silvicultural treatment histories and varying levels of soil nutrient availability. Methods Study Area The Intensive Management Practices Assessment Center (IMPAC) study was established by the University of Florida, the US Forest Service, and forest industry partners in 1983 to evaluate factors limiting the productive potential of southern pines (Swindel et al. 1988). The experiment is located approximately 10 km north of Gainesville, Florida (29°30′N latitude and 82°20′W longitude) at an elevation of 45 m from the mean sea level. The climate is warm and humid with a mean annual temperature of 20.6 °C and total annual rainfall of 1,178 mm (National Oceanic and Atmospheric Administration 2012). Pomona fine sand (sandy siliceous hyperthermic Ultic Alaquods) is the predominant soil series at the study site. Loblolly and slash pine (Pinus elliottii var. elliottii Engelm.) growing on these soils are typically responsive to fertilizer additions, including N, P, and micronutrients (Colbert et al. 1990, Jokela et al. 1991, Vogel and Jokela 2011). Understory vegetation was typical of lower Coastal Plain flatwoods ecosystems. Wiregrass (Aristidia strictus (Michx.)), gallberry (Ilex glabra (L.) A. Gray), saw palmetto (Serenoa repens (W. Bartram) Small), fetterbush (Lyonia ferruginea (Walter) Nutt), blueberries (Vaccinium spp.), broomsedges (Andropogon spp.), panic grasses (Dichanthelium spp. and Panicum spp.), and wax myrtle (Myrica cerifera (L.) Small) were the predominant understory species found at the study site (Neary et al. 1990a). Study Design The original IMPAC experiment was designed as a 2 × 2 × 2 factorial consisting of species (loblolly and slash pine), complete and sustained weed control, and annual fertilization arranged in a randomized split-plot (species as whole plots) design. This resulted in four treatments within each species' whole plot: control (C), weed control only (W), fertilizer only (F), and both fertilizer and weed control (FW). The entire experimental area was site prepared using a single-pass bedding treatment. Genetically improved (first-generation, open-pollinated) 1–0 bareroot stock of both loblolly and slash pine were hand planted in Jan. 1983 (Swindel et al. 1988, Colbert et al. 1990). A balanced blend of macro- and micronutrients were applied for the first 10 years to the F and FW treatments; it was stopped in May 1993 and then resumed during the 16th to 18th growing seasons (1998–2000; Jokela and Martin 2000; Table 1). Fertilizers were applied in narrow bands (30-cm semicircle) around the base of each tree or planting location. A combination of chemical and mechanical methods was used annually to control competing understory vegetation in the W and FW treatments for the first 10 years until canopy closure impeded further understory development (Colbert et al. 1990, Neary et al. 1990a, Dalla-Tea and Jokela 1994). Rotation-long production and stand dynamics in response to these silvicultural treatments for the IMPAC experiment were documented by Jokela et al. (2010). Treatments received by the understory vegetation community growing on Spodosols in north Florida at the IMPAC II study. Table 1. Treatments received by the understory vegetation community growing on Spodosols in north Florida at the IMPAC II study. View Large Table 1. Treatments received by the understory vegetation community growing on Spodosols in north Florida at the IMPAC II study. View Large After the IMPAC experiment was harvested in May 2009, the original plots in the first rotation were reestablished and those plots were used to examine both the “untreated carryover” and “actively managed retreatment” effects on the understory vegetation community responses in the second-rotation experiment. The IMPAC II experiment now consists of two randomized complete block designs (RCBDs; three replications each) with four treatments for the untreated carryover design (CC, CF, CFW, CW) and four treatments (C, F, FW, and W) for the actively managed retreatment design (Figure 1 and Table 1). The untreated carryover experiment, established on the previous slash pine plots, received no treatments in the second rotation. However, the actively managed retreatment experiment, established on the previous loblolly pine plots, continued to receive similar treatments as in the first rotation (Subedi et al. 2014). Figure 1. View largeDownload slide Layout of the actively managed retreatment and the untreated carryover experiments at the IMPAC II experiment near Gainesville, Florida. Figure 1. View largeDownload slide Layout of the actively managed retreatment and the untreated carryover experiments at the IMPAC II experiment near Gainesville, Florida. Before harvesting, all treatment plot corners were physically remonumented. The understory vegetation within the C and the F treatments was mulched in place (Apr. 2009). Mulching was not necessary for the W and FW treatments because of the sustained weed control treatment history from the previous rotation. Each plot was whole-tree harvested and processed off the treatment plots. After harvest, the entire study area was later bedded in June, with a second bedding pass conducted in August of the same year. In Oct. 2009, the W and FW treatments were treated using a broadcast application of 0.84 kg a.e. ha−1 imazapyr in the form of Chopper (BASF Corporation, Research Triangle Park, NC), 1.12 kg a.e. ha−1 triclopyr in the form of Garlon 4 (Dow AgroSciences LLC, Indianapolis, IN), and 0.14 kg ha−1 of metsulfuron methyl in the form of Escort (E.I. du Pont de Nemours and Company, Inc., Wilmington, DE). In Dec. 2009, the entire study was regenerated using containerized seedlings from a single, full-sub loblolly family. Similar to the last rotation, loblolly pines were planted in each plot at a 1.8 m by 3.0 m spacing, with measurement plots (0.02 ha) consisting of 40 trees per plot (8 trees each in five beds). A treated buffer consisting of three trees and two beds surrounded each measurement plot, resulting in a 0.08-ha treatment plot. Six tree spaces of untreated buffer were provided between two adjacent treatment plots. Across the treatment plots, an untreated buffer of four beds was maintained (Figure 1). All treatments (actively managed retreated and untreated carryover) received a single application of Fipronil (9.1%) in the form of PTM (BASF Corporation, Research Triangle Park, NC) in Mar. 2010 to control Nantucket pine tip moth (Rhyacionia frustrana). In Oct. 2010, the W and FW treatments received a directed spray application of triclopyr (3%) and imazapyr (1%) to control I. glabra (L.) and other understory competitors. Later in Sept. 2011, a directed spray of glyphosate (3%) was also applied to the actively managed W and FW treatments to maintain a weed-free environment. The untreated carryover experiment did not receive any additional chemical treatments (herbicide or fertilizer), with the exception of a banded 0.2-kg a.e. ha−1 imazapyr application in May 2010 to control Dichanthelium sp. and aid seedling survival in all treatment plots. This same banded herbicide treatment was also applied to the beds of actively managed C and F treatments. One year after the banded imazapyr application (2011), a clip-plot survey was done in the bed and interbed positions of the C, CC, F, and CF plots. A multiresponse permutation procedure (MRPP) analysis of understory community composition in the bed and interbed position of these plots revealed that the understory community composition did not differ significantly between the positions (For C and CC: agreement statistic [AS] = 0.01, P = 0.34; F and CF: AS < 0.01, P = 0.52). As a result, we assumed that the single banded imazapyr application in the C, CC, F, and CF during site regeneration had no significant effect on understory vegetation composition. The actively managed retreated (F and FW) experiment received fertilizer at the end of July 2011 and beginning of Sept. 2012. Consistent with the last rotation treatments, the total nutrient additions over the first three growing seasons for the F and FW treatments were (kg ha−1) 120 N, 53 P, 99 potassium [K], 40 calcium [Ca], 19 magnesium [Mg], 56 sulfur [S], 1.3 manganese [Mn], 0.5 iron [Fe], 0.2 copper [Cu], 0.5 zinc [Zn], and 0.2 boron [B]. As done in the first-rotation experiment, the fertilizer was applied in narrow bands (30-cm semicircle) around the base of each tree or planting location. Sampling During Aug. and Sept. 2011, the understory vegetation community was assessed by conducting a clip-plot survey. Six 1-m2 quadrats were randomly placed in each measurement plot, stratified equally between bed and interbed positions. All standing vegetation that fell within the quadrats was clipped at ground level and sorted separately by species or genus. All samples were oven-dried at 65 °C to a constant weight. All samples were weighed to estimate the proportion of biomass accumulated by each species in each measurement plot. The biomass data were used to estimate species diversity in each treatment. In addition, plants were grouped into three categories: graminoids (grasses and grasslike species), woody (shrubs, vines, and trees), and other herbaceous (forbs and ferns). Overstory (pine) biomass estimates for this study were reported by Subedi et al. (2014). In brief, the height of all 2-year-old pine trees in the measurement plots were measured in 2012. Allometric equations developed by Adegbidi et al. (2002) were used to estimate aboveground pine biomass for intensively managed loblolly pine plantations. Logarithmic corrections were made on all estimates of overstory biomass (Baskerville 1972, Sprugel 1983). Soil nutrient availability was assessed in the actively managed and untreated carryover experiments using PRS-probes (Western Ag Innovations, Inc., Saskatoon, SK, Canada) ion-exchange membranes. Four cation and four anion PRS-probes were buried randomly in the beds of all measurement plots in Aug. 2011. After 8 weeks of burial, these probes were removed from the soil and rinsed free of adhering soil particles with deionized and distilled water. All probes were eluted using a 0.5- N hydrochloric acid (HCl) solution for 1 h. The eluate was then analyzed colorimetrically using an automated flow injection analysis system for nitrate as nitrogen (NO3-N) and ammonium as nitrogen (NH4-N) to obtain total N supply. An inductively coupled plasma atomic emission spectroscopy system was used to estimate P, K, Ca, Mg, S, B, Cu, Mn, Zn, and Fe. All analyses were done at the Western Ag Innovations, Inc., in Saskatoon, SK, Canada. Data Analysis Using understory species biomass as the measure of species abundance (Chiarucci et al. 1999, Zhu et al. 2016), two indices of diversity were calculated: the Simpson index and the Shannon-Weiner index. The Simpson's index (Simpson 1949) is a widely used measure of diversity and was estimated by the following function:   where s is the total number of species and pi is the relative abundance (i.e., biomass for each species) of species i. D represents the probability of two individuals chosen at random from a community to be different. The Shannon-Weiner index, which is sensitive to rare species, was also used as a measure of diversity. The Shannon-Weiner index (H′), associated with the degree of uncertainty in predicting an occurrence of species chosen at random in a community (Shannon and Weaver 1949), was estimated by the following function:   where s is the total number of species and pi is the relative abundance (i.e., biomass for each species) of species i. The value of H′ varies from 0, for a community with no diversity, to a maximum of ln(s). Species richness was estimated as the total number of species present in the measurement plot. Species evenness was estimated using the following function (Pielou 1969):   where J is species evenness, H′ is the Shannon-Weiner index, and S is the average species richness. Analysis of variance (ANOVA) for a RCBD was used to test the effects of fertilizer and weed control treatments on species richness, evenness, and diversity for both the untreated carryover and actively managed retreated experiments. To ensure that the data met assumptions of normality and homoscedasticity, Kolmogorov-Smirnov and equal variance tests were used, respectively (Massey 1951). For species biomass data that did not meet the assumptions of normality and homoscedasticity, square root transformations were made before conducting ANOVA in SAS (SAS Institute 2007). Tukey's studentized test was used to separate differences among treatment means at an α level of 0.05 unless noted otherwise. A nonmetric multidimensional scaling (NMS) ordination method in PC-ORD 6.19 was used to investigate the relationships among the understory vegetation community and environmental gradients such as soil nutrient supply, weed control, fertilizer addition, and overstory biomass (loblolly pine biomass). NMS not only offers an advantage compared with other univariate methods in identifying complex relationships between treatments and numerous environmental variables, but it also works well in non-normal data or even in data that are on an arbitrary or discontinuous scale (Clarke 1993, McCune et al. 2002). Species biomass data were normalized for each site before NMS analysis. A Sorensen (Bray-Curtis) distance measure was used for ordination in this study. Choice of the number of ordination axes depended both on the Monte Carlo test and on the plot of stress versus iteration for the stability of a solution at the chosen axes (McCune et al. 2002). To test if the understory vegetation community in the second-rotation, 2-year-old loblolly pine plantation differed with respect to long-term applications of herbicide and fertilizer, a MRPP analysis (Mielke 1984) in PC-ORD 6.19 (McCune and Mefford 2011) was conducted using the species biomass data. Because rare species and outliers create noise in the data and influence MRPP results, outliers (samples with biomass >2 standard deviations from the block means) and rare species occurring in just 1 (8%) of the 12 plots were removed before MRPP analysis (Marchant 2002, McCune et al. 2002, Sickle et al. 2007). Unlike multivariate analysis of variance (MANOVA), MRPP does not require the assumptions of normality and homoscedasticity to be met. The Sorensen (Bray-Curtis) distance measure and natural weighting suggested by Mielke (1984) was used in the MRPP analysis. MRPP calculates within-group distance (delta), separation between groups or effect size (T statistic), and within-group homogeneity (AS) in species space as well as a P value (probability of smaller or equal delta). When MRPP revealed significant treatment differences, separate post hoc MRPP analyses were used to identify the significantly different treatment pairs. Indicator species (Dufrene and Legendre 1997) were then identified for treatments after the MRPP revealed a significant difference between treatment pairs. Indicator species for each treatment or group were identified using indicator values (IVs), which represented the percentage of perfect indication by species. The greatest IV for each species was tested against the random expectation obtained by 10,000 Monte Carlo permutations at a 95% significance level. Indicator species were obtained using the Indicator Species Analysis module in PC-ORD 6.19 (McCune and Mefford 2011). Results Species Richness and Evenness In the untreated carryover experiment, nutrient additions and herbicide application from the first rotation did not significantly affect the understory species richness in the second rotation (Figure 2). However, the effects of treatments on the richness of woody components were weakly significant (P = 0.06). The richness of woody species was relatively higher in the CC (6.0) treatment, followed by the CF (4.0), CFW (3.3), and CW (2.7) treatments. Likewise, the treatment effects on the richness of graminoids were weakly significant (P = 0.08). The richness of graminoids was relatively higher in the CW (6.0) treatment, followed by the CC (5.0), CF (3.3), and CFW (3.7) treatments. Figure 2. View largeDownload slide Mean richness, evenness, Shannon, and Simpson species diversity for understory vegetation growing in the untreated carryover experiment of a second-rotation, 2-year-old loblolly pine plantation in north Florida. Within a given index, treatments followed by similar letters were not significantly different among treatments at α = 0.05. Figure 2. View largeDownload slide Mean richness, evenness, Shannon, and Simpson species diversity for understory vegetation growing in the untreated carryover experiment of a second-rotation, 2-year-old loblolly pine plantation in north Florida. Within a given index, treatments followed by similar letters were not significantly different among treatments at α = 0.05. Species richness was affected by silvicultural treatments in the actively managed retreated experiment. The FW treatment had significantly lower species richness compared with the F and C treatments (Figure 3). The species richness in the C (13.3) and F (11.7) treatments was almost 3 times higher than in the FW (4.3) treatment. The richness of woody components (shrubs, vines, and trees) was almost 3 times lower (P ≤ 0.001) in the FW (1.5) and W (2.0) treatments compared with the C (5.7) and F (6.0) treatments. The richness of both graminoids and other herbaceous species was not influenced by treatments in the actively managed retreated experiment. Figure 3. View largeDownload slide Mean richness, evenness, Shannon, and Simpson species diversity for understory vegetation growing in the actively managed retreated experiment of a second-rotation, 2-year-old loblolly pine plantation in north Florida. Within a given index, treatments followed by similar letters were not significantly different among treatments at α = 0.05. Figure 3. View largeDownload slide Mean richness, evenness, Shannon, and Simpson species diversity for understory vegetation growing in the actively managed retreated experiment of a second-rotation, 2-year-old loblolly pine plantation in north Florida. Within a given index, treatments followed by similar letters were not significantly different among treatments at α = 0.05. In the untreated carryover experiment, the CFW treatment had significantly lower understory vegetation evenness compared with the CC treatment (0.20 versus 0.51; Figure 2). However, treatments did not significantly influence the evenness of understory vegetation in the actively managed retreated experiment (Figure 3). Species Diversity The first-rotation silvicultural treatments significantly influenced the second-rotation understory vegetation diversity in the untreated carryover experiment (Figure 2). The Shannon-Weiner diversity in the CFW treatment was reduced by almost 3-fold compared with the CC treatment. However, the CF and CW treatments had no significant effect on the understory diversity compared with the CC treatment. Shannon-Weiner diversity was highest for the CC (H′ = 1.31) treatment followed by CF (H′ = 1.24), CW (H′ = 0.99), and CFW (H′ = 0.45) treatments. Simpson diversity followed the same trend as the Shannon-Weiner diversity (Figure 2). Although woody species diversity (H′) was not significantly affected by treatments (P = 0.38), the CF (H′ = 0.10), the CFW (H′ = 0.12), and CW (H′ = 0.79) treatments reduced graminoid diversity compared with the CC treatment (H′ = 1.04) (P < 0.001; Figure 4A). Figure 4. View largeDownload slide Shannon-Weiner diversity for the life-forms (woody and graminoids) of the understory community in the (A) untreated carryover and (B) actively managed retreated experiments on Spodosols in north Florida. Within a given life-form, treatments followed by similar letters were not significantly different among treatments at α = 0.1. Figure 4. View largeDownload slide Shannon-Weiner diversity for the life-forms (woody and graminoids) of the understory community in the (A) untreated carryover and (B) actively managed retreated experiments on Spodosols in north Florida. Within a given life-form, treatments followed by similar letters were not significantly different among treatments at α = 0.1. Continuation of the fertilization and weed control treatment in the second rotation significantly affected understory diversity in the actively managed experiment. The Shannon-Weiner diversity was almost 2.6 times lower in the FW compared with the F treatment (Figure 3). Diversity in the FW (H′ = 0.50) treatment was reduced by 2.5-fold compared with the C (H′ = 1.23) treatment. The diversity of understory vegetation in the F (H′ = 1.33) and W (H′ = 0.99) treatments were not different compared with the C treatment. Simpson diversity (D′) followed the same trend as the Shannon-Weiner diversity (Figure 3). Although differences in the graminoid diversity (H′) were not significant among treatments (P = 0.37), woody species diversity was weakly significant (P = 0.06; F [H′ = 0.98] > FW [H′ = 0.15]). Nevertheless, the graminoid diversity (H′ = 0.42 in the FW and 0.74 in the W treatments) was higher (in absolute values) than the woody species diversity (H′ = 0.15 in the FW and 0.51 in the W treatments) for those plots that received the herbicide treatment (Figure 4B). Biomass Historical fertilizer and weed control treatments in the untreated carryover experiment significantly influenced the life-form biomass of understory vegetation (Figure 5A). The first-rotation weed control treatment significantly reduced the biomass of woody vegetation in the CFW (0.26 Mg ha−1) and CW (0.08 Mg ha−1) treatments compared with the CC treatment (4.2 Mg ha−1) in the second rotation. In contrast, historical nutrient addition alone (CF) did not increase shrub biomass in the second rotation. The CF treatment also did not significantly increase graminoid biomass compared with the CC treatment. In contrast, the CFW and CW treatments increased the graminoid biomass component by approximately 6- and 3-fold, respectively, compared with the CC treatment (0.5 Mg ha−1). Other herbaceous biomass constituted a small fraction of total understory biomass (2% in the CC treatment to 0.2% in the CW treatment) and, as in the actively managed experiment, was not influenced by treatments. Figure 5. View largeDownload slide Life-form biomass for the understory vegetation community in the (A) untreated carryover and (B) actively managed retreated experiments on Spodosols in north Florida. Within a given life-form, components followed by similar letters were not significantly different among treatments at α = 0.05. Figure 5. View largeDownload slide Life-form biomass for the understory vegetation community in the (A) untreated carryover and (B) actively managed retreated experiments on Spodosols in north Florida. Within a given life-form, components followed by similar letters were not significantly different among treatments at α = 0.05. Although graminoids were the major contributor of understory biomass in the plots that received historical weed control (95% in the CW and 91% in the CFW treatments), woody species were the major contributors to understory biomass in the CC (87%) and the CF (84%) treatments. Chalky bluestem (Andropogon virginicus L. var. glaucus Hack.) accounted for approximately 89% of the total understory biomass in the CFW treatment and 67% in the CW treatment. In the CW treatment, tapered witchgrass (Dichanthelium acuminatum (Sw.) Gould & C.A. Clark) also accounted for 20% of the total understory biomass. In contrast, woody species such as gallberry and saw palmetto accounted for approximately 46% and 25% of the total understory biomass in the CF treatments and 49% and 26% in the CC treatments, respectively (Table 2). Mean aboveground biomass (kg ha−1) of understory species in the untreated carryover and actively managed experiments in north Florida. Table 2. Mean aboveground biomass (kg ha−1) of understory species in the untreated carryover and actively managed experiments in north Florida. Note: The word spp. after a genus is used to refer to several species of the same genus that were not differentiated. View Large Table 2. Mean aboveground biomass (kg ha−1) of understory species in the untreated carryover and actively managed experiments in north Florida. Note: The word spp. after a genus is used to refer to several species of the same genus that were not differentiated. View Large As in the untreated carryover experiment, life-form biomass was affected by the nutrient additions and weed control treatments in the actively managed experiment (Figure 5B). The weed control treatment significantly reduced the woody biomass components in the FW (0.01 Mg ha−1) and W (0.02 Mg ha−1) treatments compared with the C and F treatments. However, fertilizer additions did not significantly increase the biomass of woody understory vegetation in the F (5.8 Mg ha−1) treatment compared with the C (5.3 Mg ha−1) treatment. Graminoid biomass was also not significantly affected by the fertilizer additions or weed control treatments. Likewise, other herbaceous biomass, which constituted a small fraction of total understory biomass (7% in the W treatment to 0.2% in the FW treatment), was not affected by silvicultural treatments. Although the major contributor of understory biomass in the plots receiving the weed control treatment were the graminoids (98% in the FW and 91% in the W treatments), woody species were the dominant contributors of understory biomass in the C (91%) and the F (75%) treatments. Of the total understory biomass, graminoids such as chalky bluestem and tapered witchgrass accounted for approximately 94% and 4% in the FW treatment and 58% and 27% in the W treatment, respectively (Table 2). In contrast, woody species such as gallberry and saw palmetto accounted for almost 42% and 23% of the total understory biomass in the C treatment and 49% and 8% in the F treatment, respectively (Table 2). Understory Vegetation Communities The fertilization and weed control treatments from the previous rotation affected the understory vegetation communities early in the second rotation (Table 3). The understory vegetation in the CW or CFW treatment shifted to a graminoid-dominated community when compared with the woody vegetation-dominated understory community in the CC treatment. There were also significant understory community differences between the CW and CFW treatments. Although tapered witchgrass and nutrushes (Scleria spp.) were the indicator species for the CW treatment, chalky bluestem was the indicator for the CFW treatment (Table 4). In contrast, no significant influence of the CF treatment was observed in the understory vegetation composition compared with the CC treatment (Table 3). Summary of the MRPP analysis of understory vegetation communities following different treatments in a second rotation, 2-year-old loblolly pine plantation in north Florida. Table 3. Summary of the MRPP analysis of understory vegetation communities following different treatments in a second rotation, 2-year-old loblolly pine plantation in north Florida. a Within an experiment, treatments followed by the same letter indicate no significant difference in understory vegetation composition at α = 0.05. bP values denote the probability of a smaller or equal within-group distance (δ) resulting from the MRPP. The Sorensen distance measure was used for the MRPP. View Large Table 3. Summary of the MRPP analysis of understory vegetation communities following different treatments in a second rotation, 2-year-old loblolly pine plantation in north Florida. a Within an experiment, treatments followed by the same letter indicate no significant difference in understory vegetation composition at α = 0.05. bP values denote the probability of a smaller or equal within-group distance (δ) resulting from the MRPP. The Sorensen distance measure was used for the MRPP. View Large IVs of indicator species for the understory vegetation communities in a second-rotation, 2-year-old loblolly pine plantation growing in north Florida. Table 4. IVs of indicator species for the understory vegetation communities in a second-rotation, 2-year-old loblolly pine plantation growing in north Florida. Note: Grouping of treatments in the actively managed experiment was based on the MRPP. Treatment pairs that were not significantly different were paired together as groups. Only significant indicators are presented (P < 0.05). IVs represent the percentage of perfect indication by a given species. P values are based on 10,000 Monte Carlo randomizations of species abundance data. View Large Table 4. IVs of indicator species for the understory vegetation communities in a second-rotation, 2-year-old loblolly pine plantation growing in north Florida. Note: Grouping of treatments in the actively managed experiment was based on the MRPP. Treatment pairs that were not significantly different were paired together as groups. Only significant indicators are presented (P < 0.05). IVs represent the percentage of perfect indication by a given species. P values are based on 10,000 Monte Carlo randomizations of species abundance data. View Large In the actively retreated experiment, continuation of the fertilization and weed control treatments also altered the understory community composition in the second rotation (Table 3). At age 2 years, the composition of understory vegetation was influenced more by the W than the F treatment. Although understory vegetation composition in the C and F treatments was similar, they differed significantly from the FW and W treatments (Table 3). Indicator species analysis revealed that woody species were the indicators of plots with no historical herbicide treatment. For example, gallberry, saw palmetto, and fetterbush (Lyonia lucida (Lam.) K. Koch) were the indicators for the C and F treatment. In contrast, graminoid species such as chalky bluestem and tapered witchgrass were the indicators for the FW and W treatments (Table 4). In the untreated carryover experiment, almost 96% of the variation in the understory vegetation data set was explained by the two ordination axes; 84% of the variation was explained by the first axis and 12% of the variation was explained by the second axis (Table 5). The ordination plot of understory vegetation showed that the CW and CFW treatments were markedly different from the CF and CC treatments (Figure 6). The first-rotation weed control treatment was the most influential variable for both ordination axes (r = 0.94 with axis 1 and –0.93 with axis 2). Axis 1 was also influenced by both the overstory biomass (r = –0.50) and the growing season soil nutrient supply of NH4-N (r = –0.64), and P (r = –0.54). Likewise, axis 2 was highly correlated with the soil NH4-N supply (r = 0.76), overstory biomass (r = 0.55), and P supply (r = 0.46; Table 6). Species distribution in the ordination plot revealed that graminoids, such as chalky bluestem and tapered witchgrass, were abundant on plots receiving the historical weed control treatment. On the other hand, woody species such as gallberry and saw palmetto were characteristic of plots that had no prior weed control history. Coefficients of determination (R2) for the relationships between ordination distances and distances in the original n-dimensional space for the untreated carryover and actively managed retreated experiments in north Florida. Table 5. Coefficients of determination (R2) for the relationships between ordination distances and distances in the original n-dimensional space for the untreated carryover and actively managed retreated experiments in north Florida. Note:R2 for an axis represents the proportion of variance explained by that ordination axis. The distance was measured using the Sorensen distance measure for the NMS ordination. View Large Table 5. Coefficients of determination (R2) for the relationships between ordination distances and distances in the original n-dimensional space for the untreated carryover and actively managed retreated experiments in north Florida. Note:R2 for an axis represents the proportion of variance explained by that ordination axis. The distance was measured using the Sorensen distance measure for the NMS ordination. View Large Figure 6. View largeDownload slide A two-dimensional ordination plot derived from NMS for the untreated carryover experiment in north Florida using understory abundance data. Species abbreviations are as follows: an, Andropogon virginicus L. var. glaucus Hack.; ca, Carex spp.; cy, Cyperus spp.; di, Dichanthelium acuminatum (Sw.) Gould & C.A.; el, Eleocharis baldwinii (Torr.) Chapm.; el, Eleocharis baldwinii (Torr.) Chapm.; er, Erechtites hieraciifolius (L.) Raf. ex DC.; eu, Eupatorium capillifolium (Lam.) Small; hy, Hypericum spp.; il, Ilex glabra (L.) A. Gray; ju, Juncus spp.; la, Lachnanthes caroliniana (Lam.) Dandy; lyf, Lyonia ferruginea (Walter) Nutt.; pt, Pteridium aquilinum (L.) Kuhn; qu, Quercus nigra L.; rh, Rhus copallinum L.; sc, Scleria spp.; se, Serenoa repens (W. Bartram) Small; sm, Smilax rotundifolia L.; va, Vaccinium myrsinites Lam.; vi, Vitis rotundifolia Michx.; wo, Woodwardia virginica (L.) Sm. The angles and lengths of the vectors of environmental variables represent the direction and strength of relationships of the variables with the ordination scores. Vectors with r2 > 0.2 are shown. Figure 6. View largeDownload slide A two-dimensional ordination plot derived from NMS for the untreated carryover experiment in north Florida using understory abundance data. Species abbreviations are as follows: an, Andropogon virginicus L. var. glaucus Hack.; ca, Carex spp.; cy, Cyperus spp.; di, Dichanthelium acuminatum (Sw.) Gould & C.A.; el, Eleocharis baldwinii (Torr.) Chapm.; el, Eleocharis baldwinii (Torr.) Chapm.; er, Erechtites hieraciifolius (L.) Raf. ex DC.; eu, Eupatorium capillifolium (Lam.) Small; hy, Hypericum spp.; il, Ilex glabra (L.) A. Gray; ju, Juncus spp.; la, Lachnanthes caroliniana (Lam.) Dandy; lyf, Lyonia ferruginea (Walter) Nutt.; pt, Pteridium aquilinum (L.) Kuhn; qu, Quercus nigra L.; rh, Rhus copallinum L.; sc, Scleria spp.; se, Serenoa repens (W. Bartram) Small; sm, Smilax rotundifolia L.; va, Vaccinium myrsinites Lam.; vi, Vitis rotundifolia Michx.; wo, Woodwardia virginica (L.) Sm. The angles and lengths of the vectors of environmental variables represent the direction and strength of relationships of the variables with the ordination scores. Vectors with r2 > 0.2 are shown. Pearson correlation coefficients (r) for environmental variables associated with axes 1 and 2 in the NMS ordination for the untreated carryover and actively managed retreated experiments in north Florida. Table 6. Pearson correlation coefficients (r) for environmental variables associated with axes 1 and 2 in the NMS ordination for the untreated carryover and actively managed retreated experiments in north Florida. Note: Coefficients of correlation (r) > 0.4 represent strong correlations of variables with ordination axes 1 or 2. View Large Table 6. Pearson correlation coefficients (r) for environmental variables associated with axes 1 and 2 in the NMS ordination for the untreated carryover and actively managed retreated experiments in north Florida. Note: Coefficients of correlation (r) > 0.4 represent strong correlations of variables with ordination axes 1 or 2. View Large Almost 93% of the variation in the understory vegetation data set in the actively managed experimental treatments was partitioned to the first two ordination axes; the first axis explained approximately 40% and the second explained 53% of the total variation (Table 5). In a two-dimensional species ordination space, the C and F treatments were clustered together compared with the FW and W treatments (Figure 7). In the actively managed retreated experiment, the correlation between the ordination axes and environmental variables such as weed control, fertilizer addition, soil nutrient supply (NO3-N, NH4-N, P, K, S, B, Cu, Mn, Zn, and Fe) rate at a 0- to 15-cm depth, and overstory biomass revealed that weed control had the most influence (r = –0.79 with axis1 and –0.86 with axis 2). The effect of the overstory species biomass (i.e., loblolly pine [r = –0.24 with axis 1 and r = –0.59 with axis 2]) on the ordination axes was second to weed control. Although soil supply rates of NO3-N, NH4-N, and S in the growing season influenced axis 2 (r = –0.51, –0.47, and –0.56), fertilizer additions did not have a strong influence on the ordination axes (Table 6). The ordination plot also clearly revealed that graminoids such as chalky bluestem and tapered witchgrasses were present along the weed control gradient. On the contrary, woody species such as gallberry, saw palmetto, and dwarf huckleberry (Gaylussacia dumosa (Andrews) Torr. & A. Gray) were present on plots that did not receive the weed control treatment (Figure 7). Figure 7. View largeDownload slide A two-dimensional ordination plot derived from NMS for the actively managed retreated experiment in north Florida using understory abundance data. Species abbreviations are as follows: an, Andropogon virginicus L. var. glaucus Hack.; cy, Cyperus spp.; di, Dichanthelium acuminatum (Sw.) Gould & C.A.; el, Eleocharis baldwinii (Torr.) Chapm.; eu, Eupatorium capillifolium (Lam.) Small; ga, Gaylussacia dumosa (Andrews) Torr. & A. Gray; hy, Hypericum spp.; il, Ilex glabra (L.) A. Gray; la, Lachnanthes caroliniana (Lam.) Dandy; lyf, Lyonia ferruginea (Walter) Nutt.; lyl, Lyonia lucida (Lam.) K. Koch; pa, Paspalum notatum Flügge; pe, Persea borbonia (L.) Spreng.; qu, Quercus nigra L.; rh, Rhus copallinum L.; ru, Rubus cuneifolius Pursh.; sc, Scleria spp.; se, Serenoa repens (W. Bartram) Small; sh, Sorghastrum secundum (Elliott) Nash; sm, Smilax rotundifolia L.; va, Vaccinium myrsinites Lam.; vi, Vitis rotundifolia Michx. The angles and lengths of the vectors of environmental variables represent the direction and strength of relationships of the variables with the ordination scores. Vectors with r2 > 0.2 are shown. Figure 7. View largeDownload slide A two-dimensional ordination plot derived from NMS for the actively managed retreated experiment in north Florida using understory abundance data. Species abbreviations are as follows: an, Andropogon virginicus L. var. glaucus Hack.; cy, Cyperus spp.; di, Dichanthelium acuminatum (Sw.) Gould & C.A.; el, Eleocharis baldwinii (Torr.) Chapm.; eu, Eupatorium capillifolium (Lam.) Small; ga, Gaylussacia dumosa (Andrews) Torr. & A. Gray; hy, Hypericum spp.; il, Ilex glabra (L.) A. Gray; la, Lachnanthes caroliniana (Lam.) Dandy; lyf, Lyonia ferruginea (Walter) Nutt.; lyl, Lyonia lucida (Lam.) K. Koch; pa, Paspalum notatum Flügge; pe, Persea borbonia (L.) Spreng.; qu, Quercus nigra L.; rh, Rhus copallinum L.; ru, Rubus cuneifolius Pursh.; sc, Scleria spp.; se, Serenoa repens (W. Bartram) Small; sh, Sorghastrum secundum (Elliott) Nash; sm, Smilax rotundifolia L.; va, Vaccinium myrsinites Lam.; vi, Vitis rotundifolia Michx. The angles and lengths of the vectors of environmental variables represent the direction and strength of relationships of the variables with the ordination scores. Vectors with r2 > 0.2 are shown. Discussion Over the past few decades, consistent progress has been made to advance sustainable forest management systems, which aim to ensure long-term site productivity and the maintenance of the ecosystem services provided by healthy forests (MacDicken et al. 2015). Understory diversity and productivity modify ecosystem services, for example, through the provision of nontimber products; habitats suitable for endangered, game, or pollinator species; and rates of nutrient cycling and retention. These services could vary with overall changes in diversity, the loss or gain of individual species, or in response to changes in whole functional groups (e.g., grasses versus shrubs). Several assessments exist on how silvicultural prescriptions affect a current rotation's understory diversity (Duguid and Ashton 2013); however, the unique aspect of the study described here is how the understory community dynamics in loblolly pine stands respond both to the legacy effects of past treatments and the continued application of prior treatments in the next rotation. Legacy Effects of Intensive Silviculture on the Second-Rotation Understory Vegetation The potential of the first rotation competition control and nutrient addition treatments to persist and affect the understory abundance, diversity, and composition in the early stages of the second rotation highlights the role that intensive forest management systems play on understory community dynamics. The residual effect of the first-rotation herbicide applications altered the successional pathway of the understory vegetation in the CW and CFW treatments to that of an earlier seral stage dominated by grasses. Our findings were consistent with Jose et al. (2010), who reported a significant reduction in shrub abundance after herbicide application in longleaf pine (Pinus palustris (Mill.)) stands of Florida. A grass-dominated understory could be a desirable attribute if the attempt is to restore a more pine-grass-dominated understory. Especially in savannas, where woody encroachment has a negative effect on plant diversity and wildlife habitat (Walker and Silletti 2007), herbicides have been used to restore a grass-dominated understory (Welch et al. 2004, Freeman and Jose 2009). In addition, the shift to graminoid understory may also potentially reduce the severity of wildfires by supporting low-intensity ground fires and reducing the risk of crown fires from the woody mid-story and other ladder fuels in these plantations (Brose and Wade 2002). However, declines in understory species diversity, shifts in the understory community, and loss of functional groups such as woody vegetation associated with historical intensive management treatments may be a cause for concern if the attempt is to maintain multiple ecosystem functions, including nutrient cycling (Tilman et al. 1997, Isbell et al. 2011, Byrnes et al. 2014). For example, lack of woody understory vegetation in the plots with a weed control treatment history may affect nutrient cycling because historical P in the upper solum can leach to lower horizons in the absence of understory vegetation, which can act as a nutrient sink (Subedi et al. 2014). More importantly, the absence of species such as gallberry, a dominant Mn accumulator (Jokela et al. 1991, Subedi 2013), in these treatments may also affect the recycling of important micronutrients in these plantations. Reduced understory diversity in the CFW treatment was expected given the repeated herbicide application and fertilization treatment history because only few species can recover after frequent herbicide applications (Kaeser and Kirkman 2010). In addition, the overall decline in diversity was likely due to the strong dominance of chalky bluestem (89% of the total understory biomass) over other understory species in the CFW treatment. In the CFW treatment, historical nutrient additions likely exacerbated the decline in understory diversity by creating conditions favorable to a few species such as chalky bluestem, leading to their colonization (Hobbs and Atkin 1988, Suding et al. 2005). Roots associated with arbuscular mycorrhizal fungi (Ning and Cumming 2001) and the allelopathic interaction (Rice 1972, Peters and Lowance 1974) of chalky bluestem with other understory vegetation will likely lead to its abundance in the understory community in the CW treatment before the onset of light limitations due to canopy closure. As a result, transient difference in understory community composition and diversity between the CW and CFW observed in this study will likely diminish with time. Historical fertilizer additions did not change the understory community composition and response in our study. Woody species such as gallberry, saw palmetto, winged sumac (Rhus copallinum L.), and dwarf huckleberry were dominant in the CC and CF treatments. It is interesting to note that pioneer species such as chalky bluestem and Panicum spp., documented by Conde et al. (1983) as the dominant species 2 years after mechanical site preparation on a flatwoods site in north Florida, were no longer dominant in the CC and CF treatments in this study. Retention of first-rotation woody understory vegetation in the CC and CF treatment as mulch, while establishing the second-rotation stand, may have served as a source of propagules and thereby favored the initiation of woody species in those treatments. Woody species such as gallberry and saw palmetto have rhizomes (underground storage organs) and are able to sprout readily from these root structures (Van Deelen 1991, Anderson 2001). Retention of rhizomes likely increased the abundance of woody species in plots with a “no weed control” history. On the other hand, mulching has the potential to serve as a physical barrier for the establishment of herbaceous species with low seed reserves. Mulching may also limit light below the compensation point and facilitate the exhaustion of the energy reserves before seedling roots penetrate the mulch and duff layer (Teasdale and Mohler 1993). However, the double bedding operation after mulching minimized such an effect in this study. Furthermore, understory mulch and the forest floor, which served as an important sink for soil nutrients such as N and P in the first rotation (Vogel et al. 2011, Subedi et al. 2014), likely served as a source of nutrients in this rotation and thereby favored the growth of woody species in the CC and CF treatments. Nevertheless, a significantly more diverse understory community in the CF compared with the CFW treatment suggests that if forest management objectives include enhanced understory diversity for ecosystem functions (Nilsson and Wardle 2005), fertilization alone may be preferred over the combination of fertilization and intensive weed control in these plantations. Continued Management Effects of Intensive Silviculture on the Second-Rotation Understory Vegetation The potential of weed control treatments, especially herbicides, to affect understory vegetation richness and diversity has been previously reported for forest ecosystems across different regions (Miller et al. 1995, Jose et al. 2010, Boateng et al. 2000, Bell and Newmaster 2002). Continuation of weed control treatments in the second rotation reduced the richness of woody components but did not influence the richness of graminoids in this study. Similar herbicide effects on richness have also been observed in other conifer dominated forests (Sullivan et al. 1996). Similar to Jones et al. (2009), the nonsignificant effect of continued herbicide treatments on the aboveground biomass of graminoids in the current study suggests that they are less responsive to release from woody competition at this stage of stand development. Nevertheless, repeated control of a functional group, usually hardwoods, has the potential to shift the understory vegetation community by preventing the recovery of understory abundance and diversity (Neary et al. 1990b). In the FW treatment, the suppression of woody vegetation and higher soil nutrient supply (NH4-N, NO3-N, and S; Table 6) may have favored the growth of chalky bluestem in the second rotation, leading to a decline in overall species richness and diversity. Swindel et al. (1989) reported for the same site as the current study a dramatic reduction in understory richness with the FW treatment, which resulted in a larger growth response of pines in the first rotation. Compared to this study, most operational fertilization and weed control treatments in southern pine plantations are less intensive and are usually applied during the first year of stand establishment. As a result, recovery of understory diversity may be possible on operationally treated areas within a few years after site preparation or herbicide release treatments (Boyd et al. 1995, Miller et al. 1999). In N- and P-limited pine stands, inter-rotational applications of fertilizer did not influence the measures of understory vegetation diversity in the second rotation at age 2 years. Ostertag and Verville (2002) reported similar understory diversity responses in N- and P-limited Hawaiian montane forests. Gilliam et al. (2006) also observed no changes in plant diversity after 6 years of aerial application of N in a montane hardwood forest in West Virginia. However, findings of this study are different from the generally observed trend of diversity decline in productive ecosystems (Hautier et al. 2009, Ceulemans et al. 2011, Dickson and Foster 2011). Such declines in diversity are thought to be the result of competitive exclusion of some species for light (Hautier et al. 2009) and other site resources (Dickson and Foster 2011). However, the absence of such a diversity decline in understory vegetation in our study likely resulted because light was not a limiting factor (Morris et al. 1993) and belowground competition from pine trees was less severe (Adegbidi et al. 2004) at this stage of stand development. More importantly, nonsignificant differences in diversity between the F and C treatments were likely due to the retention of the first-rotation woody understory as a mulch, which may serve as propagules, during site preparation in the second rotation. In addition, the nonsignificant differences between the woody vegetation biomass in the C and F treatments were presumably due to the similar growing season soil N and P supply rates observed for this site (Subedi et al. 2014). As in the first rotation, where Neary et al. (1990a) observed a dominance of gallberry and saw palmetto after 6 years of nutrient additions, the F treatment is expected to accumulate higher woody understory biomass compared with the C treatment before canopy closure. Overstory vegetation effects on understory vegetation community responses observed in other studies (Harrington and Edward 1999, McGuire et al. 2001, Harrington 2011) would be less likely to occur at the stage of stand development associated with our study. Strong correlations observed between overstory biomass and the ordination axes in this study were presumably confounded by the strong influence of the weed control treatments (highly correlated with both ordination axes). For instance, although the overstory biomass was relatively high in the FW treatments (10.3 Mg ha−1; Subedi et al. 2014), the lower relative abundance and diversity of understory vegetation associated with those treatments was not likely due to shading or competition for nutrients but rather due to the sustained elimination of woody species by the herbicide treatments. This interpretation is further supported by the relatively higher understory abundance and diversity observed in the F treatment, which had a comparable overstory biomass (6.9 Mg ha−1), and the negligible correlation between fertilizer addition and the ordination axes in the actively managed experiment. Nevertheless, it is likely that at later ontogenetic stages the nonlimiting environment created by fertilizer additions will accelerate canopy closure in these stands, potentially leading to a change in the understory vegetation response. Fertilizer additions could also reduce the understory diversity in pine stands as overstory leaf area accrues and thereby reduces the light received by understory plants (Thomas et al. 1999, Gilliam 2006, Lu et al. 2010). Changes in land management practices used in intensively managed southern pine plantations, including exclusion of fire, are thought to affect subsequent understory community composition and responses (Gilliam and Platt 1999, Hiers et al. 2007). Fire exclusion in these plantations may have led to the absence of some species dependent on regular fire for germination, growth, flowering, and seed production (Jaynes 1968, Outcalt 1994). For example, graminoids such as wiregrass (Aristida spp.) and beak-sedge (Rhynchospora spp.), observed as the important species of the grass community for the same site as reported by Swindel and Smith (1989) (unpublished field survey data) at age 2 years in the first rotation, were not observed for all treatments in this study. Other species such as hairy laurel (Kalmia hirsuta Walter), clustered mille graines (Oldenlandia uniflora L.), vanillaleaf (Trilisa odoratissima (J.F. Gmel.) Cass.), and yelloweyed grasses (Xyris spp.) (Swindel and Smith 1989, unpublished field survey data) were also absent across all treatments in the second rotation. In addition to fire exclusion, factors such as site preparation, shading influences by the overstory, physical barrier to germination from forest floor, and adjacency to native vegetation are also likely to influence understory vegetation community responses in plantations managed beyond a single rotation (Newmaster et al. 2007, Barbier et al. 2008, Zamora et al. 2010). Summary and Conclusions Our results suggest that common but intensive silvicultural practices such as fertilization and herbicide control of competing vegetation imposed in the first rotation will influence the understory vegetation community reinitiation, diversity, and response in the second rotation. For this N- and P-limited site, historical nutrient amendments had little influence on the second-rotation understory community composition, at least during these early stages of stand development. In contrast, long-term and intensive weed control treatments were associated with an understory community dominated by graminoids, at least until canopy closure. When both herbicide and nutrients were added in the first rotation, the shifts in understory community composition became more pronounced; woody species were suppressed and a few herbaceous species like chalky bluestem were favored in the second rotation. Similar understory community responses were observed on continuation of these treatments into the second rotation. Because silvicultural treatments used in this study were more intensive than typically used in southern pine forest management, the shifts in understory community composition would likely be less pronounced in stands receiving operational herbicide treatments. 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TI - Inter-Rotational Effects of Fertilizer and Herbicide Treatments on the Understory Vegetation Community in Juvenile Loblolly Pine (Pinus taeda L.) Stands
JF - Forest Science
DO - 10.5849/fs-2016-127
DA - 2017-10-01
UR - https://www.deepdyve.com/lp/springer-journals/inter-rotational-effects-of-fertilizer-and-herbicide-treatments-on-the-hZIE41jP0o
SP - 459
EP - 473
VL - 63
IS - 5
DP - DeepDyve
ER -