TY - JOUR
AU - Kantavichai, Rapeepan
AB - Abstract This study examined the changes in response of first thinning in four Douglas-fir sites in the Coastal Pacific Northwest in multiple positions along stems. Four installations contain one control plot and four thinning plots that were first thinned between 17 to 34 years during 1987 to 1998, depending on plot relative densities. After being harvested in 2006, wood densitometry measurements were conducted to determine annual ring mass, annual earlywood mass, annual latewood mass, and percentage of latewood responses. We used repeated measures ANOVA to test the significant changes in the 12-year period after thinning between control and thinned plots. Thinning increases ring mass growth with a redistribution of ring mass toward the stem base that gradually reverses with time. The ring mass response to thinning is parabolic, starting low immediately following thinning, reaching a peak or stable level for a period of time, and then declining as pre-thinning competition is re-established. Patterns of increases in both earlywood and latewood ring mass are similar to that of whole ring mass. However, in the sites that commonly experience summer drought, thinning increases latewood as a percent of ring mass responding to reduced competition for water and extended growth during the time when latewood is normally produced. biomass, diameter growth, latewood, earlywood Introduction Many studies have related annual ring width or ring area to silvicultural treatments such as thinning, commonly by taking a pith to bark radial sample at breast height (BH) of trees. However, in the context of carbon or energy stored in tree stems, biomass (dry weight), not volume, is the basis for calculating carbon and energy content of wood. Biomass can be derived from the product of volume and wood density. The effects of thinning on biomass can be observed from changes in either volume growth or wood density or both. It is possible for wood density to increase, decrease, or have no change as volume growth increases, decreases, or does not change in response to stand development and treatments such as thinning (Bowyer et al. 2007). Thinning is a silviculture practice that affects both rate of growth and its distribution along the stem of a tree and can have positive, negative, or no effect on wood density (Bowyer et al. 2007). The main goals of thinning are to increase stem volume and improve quality. By reducing competition between trees within a stand, thinning provides each tree of the residual stand with access to more light, more soil nutrients and water, and more growing space. The response of the above-ground portion of the tree can be classified as crown effects and stem effects. With more light, water, and nutrients, the tree will increase its foliage mass over time, which in turn increases production of photosynthetic products, leading to more crown and stem growth. However, the heavier crown induces more mechanical load on the stem and accentuates mechanical stresses along the stem due to wind sway, so resistance to wind sway becomes important. The initial response of a thinned tree, compared to what it would do if left unthinned, is to reduce height growth, shift the point of maximum diameter growth down the stem, and produce relatively more diameter growth toward the stem base and relatively less diameter growth toward the top, that is, lay down wood faster where the stresses are greatest (Kozlowski 1971, Ennos 1995, Mattheck 1991, Mattheck and Kubler 1995). Consequently, the stem becomes more tapered since more wood is produced at the stem base than in the upper stem (Larson 1963, Kozlowski 1971, Barbour et al. 1992, Valinger 1992a, Tassissa and Burkhart 1997, Goudiaby et al. 2012). The diameter growth response at the stem base is immediate (Valinger 1992a, Tasissa and Burkhart 1997, Peltola et al. 2002), and the heavier the thinning, the greater this immediate effect (Hilt and Dale 1979, Tasissa and Burkhart 1997, Peltola et al. 2002, Newton and Cole 2015). The growth distribution response along the stem has been observed to change with time since thinning. With time, growth of the stand leads to a resumption of inter-tree competition. As trees return toward pre-thinning competition, height growth increases. The point of maximum diameter growth rises up the stem, relatively less diameter growth occurs at the stem base, and relatively more diameter growth occurs toward the upper stem (Larson 1963, Valinger 1992a, Tasissa and Burkhart 1997). This shift may in part be due to changes in stresses from wind sway and crown loading as stand closure returns (Jacobs 1954, Valinger 1992b). Peltola et al. (2002) compare percent diameter change at four height levels at BH, 4m, 6m, and 8m of thinned vs unthinned Scots pine (Pinus sylvestris) over 12 years following thinning at BH age 22. The thinning trial left three residual stand densities, and height was 12–14m when sample trees were harvested. They showed that the largest relative gain in diameter is at stem base and the smallest is toward the upper stem (8m). In the first three-year period only, there was a relative loss in diameter growth in the upper stem. The general patterns with increasing height were similar for all thinning intensities but were more extreme as thinning became heavier. The maximum responses were at 7–9 years after thinning except toward the top, where the maximum response was delayed to 10–12 years after thinning. In other words, with time, thinned trees tend to redistribute growth back up the stem as they return toward crown closure, competition, protection from wind sway, and mutual assistance in holding heavier crowns. Finally, although the thinning response has started to slow, it was sustained for the full 12-year assessment period. The overall pattern is suggestive of a concave-down parabolic response over time since thinning. Brix and Mitchell (1980, 1986) and Brix (1981) reported on the effects of thinning that removed two-thirds of the basal area from a low-site-quality 24-year-old Douglas-fir stand at Shawnigan Lake in British Columbia. Compared to unthinned control, thinning benefited growth, especially in the lower crown, due to improved light conditions. Thinning decreased height growth for the first two years, but after year five thinning increased height growth, increased crown length (slowed crown recession), increased the number of needles per post-thinning shoot but did not increase the number of shoots, and lowered the point in the crown with maximum needle mass. Thinning increased soil water potential during summer dry periods. Due to the fact that thinning reduced stand leaf area, which reduced demand for water and increased soil water content until the pre-thinning leaf area was re-established (Zahner 1968, Sucoff and Honn 1974, Jarvis 1975). In terms of wood formation, thinning did not change the period over which tracheids were formed but increased the daily rate of tracheid production, tracheid radial wall diameter, and tracheid file length in both earlywood (EW) and latewood (LW). There was a tendency for thinning to slightly delay the transition to LW, and the LW% decreased to 35.4% compared to 38.4% in the controls. The tracheid production rate, which increased until mid-July, was positively correlated with temperature over the same period. Subsequently the rate tended to decrease in relation to water stress. Jozsa and Brix (1989) took samples for X-ray densitometry from BH and 25%, 50%, and 75% of the distance from BH to the top. Compared to controls over 12 years, thinned trees redistributed growth toward the stem base and increased stem taper. In the first period, there was a redistribution of ring specific gravity (SG) toward the stem base, about 7% denser at BH and unchanged at the 75% height level. This was composed of increasing EW SG and LW% (LW SG not changed) toward the BH level. In the last period, these patterns reversed, with the rings having relatively lower SG than controls toward the BH level and relatively higher SG than controls at the 75% level. Overall, thinning initially increased SG on this site, a result of relief of water stress in July and August that increased production of denser LW tracheids. Kantavichai et al. (2010) found similar responses by a 55-year-old Douglas-fir stand. They found that thinning increased percent LW and increased ring density by 4%. Ring density decreased with increased July soil moisture deficit, which curtailed LW production; alternatively, SG increased with increased July total precipitation, which continued LW production. Warmer mean March–May or August–November temperatures also increased ring density. The effect of a warmer March–May may be theresult of the increased rate of EW tracheid formation (Brix and Mitchell 1980) and an earlier transition to LW. A warmer August–November may signal a delay of the return of more winter-like storms in the fall, permitting a longer period of LW formation. Each year following thinning, trees will utilize improved available light, nutrients, and water to replace pre-thinning cohorts of older needles. In the case of coastal Douglas fir, which typically retains needles for 4–5 years (Turner and Olson 1976, Omdal and Ramsey-Kroll 2010) to 5–7 years (Monserud and Marshall 2001), complete replacement of pre-thinning needles would take a similar length of time. As replacement occurs, the greater mass and efficiency of post-thinning needles will increase production of photosynthate, which can be used partly to deposit stronger, stiffer LW and partly for storage for the next year’s EW growth. Since the primary function of EW is to conduct water and nutrients from the soil to the needles, increased demand as the crown enlarges would require more EW for efficient conduction. Zahner and Oliver (1962) found that thinning delays LW formation at both BH and in the crown. Smith (1980) reported that trees in wider spacing have lower percentage of LW. This agrees with studies in other species that have found that EW increases more than LW (Barbour et al. 1994, Koga et al. 2002). These studies suggest that, while the ring width and wood volume is increased by thinning, especially toward the stem base, there is a shift toward a lower proportion of LW, which implies lower ring density. This contradicts the Shawnigan Lake and Kantavichai et al. (2010) results for Douglas-fir on sites experiencing summer drought typical of the Pacific Northwest. These studies found that thinning increased wood density by improving water availability in summer, which allowed production of more LW. Annual biomass production will increase when thinning produces increased volume and wood density and may increase if the volume increase due to thinning overcomes the wood density decrease noted by other studies. The aim of this study was to examine the changes in response of thinning in Douglas-fir sites in the Coastal Pacific Northwest using dendrochronology techniques. Four installations located in western Oregon and Washington were used in this study. We hypothesize that Thinning increases ring mass growth with redistribution toward the stem base that gradually reverses with time. The ring mass response with time-since-thinning is parabolic, starting low immediately following thinning, reaching a peak or stable level for a period of time, and then declining as pre-thinning competition is re-established. Thinning increases both EW and LW ring mass in patterns along the stem and over time similar to that of whole ring mass. Thinning increases LW as a percent of ring mass in summer drought Douglas-fir sites. Materials and Method Study Site Descriptions Four Douglas-fir research installations located in western Oregon and Washington (Table 1, Figure 1) of the Stand Management Cooperative (SMC) that were originally sampled for an acoustic velocity–product recovery study (Briggs et al. 2008) were used for this study. Harvested sample trees for the recovery study provided cross-section disks from the ends of each log that provided data for this study. Three sites are <175 m elevation, and the other is >750m elevation with slight slope. The site index ranged from 39 to 44m, indicative of the ownership of higher site land by forest industry. Installation 807, in the foothills of the Oregon Cascades, had the highest average annual temperature and lowest annual precipitation. Installation 808 in the Oregon Coast range had the highest elevation, lowest average annual temperature, and highest annual precipitation (Figure 2). Table 1. Characteristics of the four study sites. Installation  803  805  807  808  Location  Shelton, WA  Mt Vernon, WA  Estacada, OR  Dallas, OR  Planting date  Jan 1955  Jan 1970  Jan 1974  Dec 1960  Year established  1987  1988  1989  1989  Density at est.(tph)  780  840  1630  700  Age at 2006 (yr)  51  36  32  45  SI (m)  44  47  38  39  QMD (cm)  36  30  28  33  HT40 (m)  38  32  25  31  Elevation (m)  175  168  152  762  Slope  0  15  0  5  Aspect  0  90  0  360  Yearly precipitation (mm)  3,172  1,376  1,291  4,046  Avg temperature (C)  9.58  9.26  11.26  8.85  Installation  803  805  807  808  Location  Shelton, WA  Mt Vernon, WA  Estacada, OR  Dallas, OR  Planting date  Jan 1955  Jan 1970  Jan 1974  Dec 1960  Year established  1987  1988  1989  1989  Density at est.(tph)  780  840  1630  700  Age at 2006 (yr)  51  36  32  45  SI (m)  44  47  38  39  QMD (cm)  36  30  28  33  HT40 (m)  38  32  25  31  Elevation (m)  175  168  152  762  Slope  0  15  0  5  Aspect  0  90  0  360  Yearly precipitation (mm)  3,172  1,376  1,291  4,046  Avg temperature (C)  9.58  9.26  11.26  8.85  View Large Table 1. Characteristics of the four study sites. Installation  803  805  807  808  Location  Shelton, WA  Mt Vernon, WA  Estacada, OR  Dallas, OR  Planting date  Jan 1955  Jan 1970  Jan 1974  Dec 1960  Year established  1987  1988  1989  1989  Density at est.(tph)  780  840  1630  700  Age at 2006 (yr)  51  36  32  45  SI (m)  44  47  38  39  QMD (cm)  36  30  28  33  HT40 (m)  38  32  25  31  Elevation (m)  175  168  152  762  Slope  0  15  0  5  Aspect  0  90  0  360  Yearly precipitation (mm)  3,172  1,376  1,291  4,046  Avg temperature (C)  9.58  9.26  11.26  8.85  Installation  803  805  807  808  Location  Shelton, WA  Mt Vernon, WA  Estacada, OR  Dallas, OR  Planting date  Jan 1955  Jan 1970  Jan 1974  Dec 1960  Year established  1987  1988  1989  1989  Density at est.(tph)  780  840  1630  700  Age at 2006 (yr)  51  36  32  45  SI (m)  44  47  38  39  QMD (cm)  36  30  28  33  HT40 (m)  38  32  25  31  Elevation (m)  175  168  152  762  Slope  0  15  0  5  Aspect  0  90  0  360  Yearly precipitation (mm)  3,172  1,376  1,291  4,046  Avg temperature (C)  9.58  9.26  11.26  8.85  View Large Figure 1. View largeDownload slide Location of study sites. Figure 1. View largeDownload slide Location of study sites. Figure 2. View largeDownload slide Monthly minimum temperature, maximum temperature, and total precipitation by sites in last ten years of study period (1997–2006). Figure 2. View largeDownload slide Monthly minimum temperature, maximum temperature, and total precipitation by sites in last ten years of study period (1997–2006). Each site had five plots; a control plus four plots following thinning regimes based on Curtis’s relative density (RD, Curtis 1982); Table 2 presents the thinning regimes and date and RD before and after each thinning on each plot. One plot, on installation 808, was not used due to prior storm damage to the trees. Most plots received their first thinning within a five-year period in the late 1980s and early 1990s, and those scheduled for a repeat thinning did not reach their second RD thinning trigger by the time of harvest. Table 2. Thinning regimes, thinning dates, and relative densities before and after thinning by plot and site. (RD, Curtis 1982). Site  Plot  1st thinned  2nd thinned  Regime Description  803  1  1987  -  Repeated thinning: RD55-->RD35, RD55-->RD40, subsequent RD60-->RD40  803  2  ctl  ctl  No treatments (defined as control)  803  3  1987  -  Repeated thinning: RD55-->RD30, subsequent RD50-->RD30  803  4  1987  -  Minimal thinning: RD55-->RD30, no further thinning  803  5  1995  -  Delayed thinning: RD65-->RD35, no further thinning  805  1  1990  -  Minimal thinning: RD55-->RD30, no further thinning  805  2  1990  2004  Repeated thinning: RD55-->RD30, subsequent RD50-->RD30  805  3  1998  -  Delayed thinning: RD65-->RD53, no further thinning  805  4  ctl  ctl  No treatments (defined as control)  805  5  1996  -  Repeated thinning: RD55-->RD35, RD55-->RD40, subsequent RD60-->RD40  807  1  ctl  ctl  No treatments (defined as control)  807  2  1989  2001  Repeated thinning: RD55-->RD30, subsequent RD50-->RD30  807  3  1989  -  Repeated thinning: RD55-->RD35, RD55-->RD40, subsequent RD60-->RD40  807  4  1989  -  Minimal thinning: RD55-->RD30, no further thinning  807  5  1993  -  Delayed thinning: RD65-->RD35, no further thinning  808  1  1991  -  Repeated thinning: RD55-->RD35, RD55-->RD40, subsequent RD60-->RD40  808  2  1993  -  Repeated thinning: RD55-->RD30, subsequent RD50-->RD30  808  3  1991  -  Minimal thinning: RD55-->RD30, no further thinning  808  4  1993  -  Delayed thinning: RD65-->RD35, no further thinning  808  5  ctl  ctl  No treatments (defined as control)  Site  Plot  1st thinned  2nd thinned  Regime Description  803  1  1987  -  Repeated thinning: RD55-->RD35, RD55-->RD40, subsequent RD60-->RD40  803  2  ctl  ctl  No treatments (defined as control)  803  3  1987  -  Repeated thinning: RD55-->RD30, subsequent RD50-->RD30  803  4  1987  -  Minimal thinning: RD55-->RD30, no further thinning  803  5  1995  -  Delayed thinning: RD65-->RD35, no further thinning  805  1  1990  -  Minimal thinning: RD55-->RD30, no further thinning  805  2  1990  2004  Repeated thinning: RD55-->RD30, subsequent RD50-->RD30  805  3  1998  -  Delayed thinning: RD65-->RD53, no further thinning  805  4  ctl  ctl  No treatments (defined as control)  805  5  1996  -  Repeated thinning: RD55-->RD35, RD55-->RD40, subsequent RD60-->RD40  807  1  ctl  ctl  No treatments (defined as control)  807  2  1989  2001  Repeated thinning: RD55-->RD30, subsequent RD50-->RD30  807  3  1989  -  Repeated thinning: RD55-->RD35, RD55-->RD40, subsequent RD60-->RD40  807  4  1989  -  Minimal thinning: RD55-->RD30, no further thinning  807  5  1993  -  Delayed thinning: RD65-->RD35, no further thinning  808  1  1991  -  Repeated thinning: RD55-->RD35, RD55-->RD40, subsequent RD60-->RD40  808  2  1993  -  Repeated thinning: RD55-->RD30, subsequent RD50-->RD30  808  3  1991  -  Minimal thinning: RD55-->RD30, no further thinning  808  4  1993  -  Delayed thinning: RD65-->RD35, no further thinning  808  5  ctl  ctl  No treatments (defined as control)  View Large Table 2. Thinning regimes, thinning dates, and relative densities before and after thinning by plot and site. (RD, Curtis 1982). Site  Plot  1st thinned  2nd thinned  Regime Description  803  1  1987  -  Repeated thinning: RD55-->RD35, RD55-->RD40, subsequent RD60-->RD40  803  2  ctl  ctl  No treatments (defined as control)  803  3  1987  -  Repeated thinning: RD55-->RD30, subsequent RD50-->RD30  803  4  1987  -  Minimal thinning: RD55-->RD30, no further thinning  803  5  1995  -  Delayed thinning: RD65-->RD35, no further thinning  805  1  1990  -  Minimal thinning: RD55-->RD30, no further thinning  805  2  1990  2004  Repeated thinning: RD55-->RD30, subsequent RD50-->RD30  805  3  1998  -  Delayed thinning: RD65-->RD53, no further thinning  805  4  ctl  ctl  No treatments (defined as control)  805  5  1996  -  Repeated thinning: RD55-->RD35, RD55-->RD40, subsequent RD60-->RD40  807  1  ctl  ctl  No treatments (defined as control)  807  2  1989  2001  Repeated thinning: RD55-->RD30, subsequent RD50-->RD30  807  3  1989  -  Repeated thinning: RD55-->RD35, RD55-->RD40, subsequent RD60-->RD40  807  4  1989  -  Minimal thinning: RD55-->RD30, no further thinning  807  5  1993  -  Delayed thinning: RD65-->RD35, no further thinning  808  1  1991  -  Repeated thinning: RD55-->RD35, RD55-->RD40, subsequent RD60-->RD40  808  2  1993  -  Repeated thinning: RD55-->RD30, subsequent RD50-->RD30  808  3  1991  -  Minimal thinning: RD55-->RD30, no further thinning  808  4  1993  -  Delayed thinning: RD65-->RD35, no further thinning  808  5  ctl  ctl  No treatments (defined as control)  Site  Plot  1st thinned  2nd thinned  Regime Description  803  1  1987  -  Repeated thinning: RD55-->RD35, RD55-->RD40, subsequent RD60-->RD40  803  2  ctl  ctl  No treatments (defined as control)  803  3  1987  -  Repeated thinning: RD55-->RD30, subsequent RD50-->RD30  803  4  1987  -  Minimal thinning: RD55-->RD30, no further thinning  803  5  1995  -  Delayed thinning: RD65-->RD35, no further thinning  805  1  1990  -  Minimal thinning: RD55-->RD30, no further thinning  805  2  1990  2004  Repeated thinning: RD55-->RD30, subsequent RD50-->RD30  805  3  1998  -  Delayed thinning: RD65-->RD53, no further thinning  805  4  ctl  ctl  No treatments (defined as control)  805  5  1996  -  Repeated thinning: RD55-->RD35, RD55-->RD40, subsequent RD60-->RD40  807  1  ctl  ctl  No treatments (defined as control)  807  2  1989  2001  Repeated thinning: RD55-->RD30, subsequent RD50-->RD30  807  3  1989  -  Repeated thinning: RD55-->RD35, RD55-->RD40, subsequent RD60-->RD40  807  4  1989  -  Minimal thinning: RD55-->RD30, no further thinning  807  5  1993  -  Delayed thinning: RD65-->RD35, no further thinning  808  1  1991  -  Repeated thinning: RD55-->RD35, RD55-->RD40, subsequent RD60-->RD40  808  2  1993  -  Repeated thinning: RD55-->RD30, subsequent RD50-->RD30  808  3  1991  -  Minimal thinning: RD55-->RD30, no further thinning  808  4  1993  -  Delayed thinning: RD65-->RD35, no further thinning  808  5  ctl  ctl  No treatments (defined as control)  View Large Tree Sampling Twelve trees per plot were selected for harvesting in October–November 2006 using a stratified random sample. Stratification was based on acoustic velocity measured over 1m at BH on standing trees for the product recovery study (for details, see Briggs et al. 2008). The log preference in the woods was to cut 10.1m and 10.7m lengths, respectively, for lumber and veneer conversion. Cross-section disks were cut from the base of the butt log (stump level, at approximately 0.3m) and the top of adjacent logs in each tree. Subsequently, at the mill woods logs were cut into 4.9m or 5.1m logs for processing into lumber or veneer, respectively, and a disk was taken at the cut point. Thus, there were disks from the stump level and every 4.9m or 5.1m along the merchantable bole of each tree; most trees were utilized to a minimum top diameter of about 15–20cm. However, logs from site 807 were not sent to the mill due to flooding of the site, so disk data from 5m is missing. Two hundred twenty-eight trees were harvested. Some disks were lost or became unidentifiable. In total, 217 trees and 562 disks were used in this study (Table 3). Table 3. Number disk samples in control plot and thinning plots by site and height level. Height  803  803  805  805  807  807  808  808  (m)  Ctl  Thin  Ctl  Thin  Ctl  Thin  Ctl  Thin  21.0  8  32  8  7  n/a  n/a  9  21  15.8  6  24  8  6  4  17  3  15  10.7  12  35  10  11  12  34  10  21  5.5  10  23  5  9  1  23  11  17  0.3  12  36  10  12  12  35  11  22  Height  803  803  805  805  807  807  808  808  (m)  Ctl  Thin  Ctl  Thin  Ctl  Thin  Ctl  Thin  21.0  8  32  8  7  n/a  n/a  9  21  15.8  6  24  8  6  4  17  3  15  10.7  12  35  10  11  12  34  10  21  5.5  10  23  5  9  1  23  11  17  0.3  12  36  10  12  12  35  11  22  View Large Table 3. Number disk samples in control plot and thinning plots by site and height level. Height  803  803  805  805  807  807  808  808  (m)  Ctl  Thin  Ctl  Thin  Ctl  Thin  Ctl  Thin  21.0  8  32  8  7  n/a  n/a  9  21  15.8  6  24  8  6  4  17  3  15  10.7  12  35  10  11  12  34  10  21  5.5  10  23  5  9  1  23  11  17  0.3  12  36  10  12  12  35  11  22  Height  803  803  805  805  807  807  808  808  (m)  Ctl  Thin  Ctl  Thin  Ctl  Thin  Ctl  Thin  21.0  8  32  8  7  n/a  n/a  9  21  15.8  6  24  8  6  4  17  3  15  10.7  12  35  10  11  12  34  10  21  5.5  10  23  5  9  1  23  11  17  0.3  12  36  10  12  12  35  11  22  View Large X-Ray Densitometry In the field, fresh disks were measured for green weight, green volume, and other characteristics and transported to the University of Washington to obtain oven-dried weight. Dry disks were then transported to the Weyerhaeuser Technology Center in Federal Way, WA. A random 6mm x 6mm pith-to-bark sample was sawn from each disk, attached to a mounting stick, sawn to a thickness of 1.8mm on a table saw with a tandem blade setup, and soaked overnight at room temperature in acetone to remove extractive materials. The samples were conditioned to equilibrium moisture content ambient with the laboratory, and scanned on a custom X-ray densitometer using a step resolution of 0.13mm. X-ray scans were calibrated to SG using a series of wood samples of known SG that were scanned simultaneously with the samples. A SG value of 0.50 was used to differentiate EW from LW. This value was determined by visual identification of the EW-LW transition of a random sample of mature wood rings on the sample strips, marking the corresponding location on the X-ray profile, averaging the corresponding SG values. Output data included EW width, LW width, whole ring width, EW specific gravity, LW specific gravity, and percent late wood. Ring mass was calculated by the product of cross-section area and specific gravity for each EW mass, LW mass, and whole ring mass. The percent change due to thinning was calculated as follows:  % mass change of thinned in a time period = 100 (thinned mass growth in period - unthinned mass growth in periodunthinned mass growth in period) Analysis of Thinning Effect Effects of thinning were assessed with repeated measures ANOVA of the three years of ring mass for four periods since thinning. These time intervals were chosen to correspond to those used in other studies, allowing more comparability of results. Pre-thinning repeated measures ANOVA was also tested for three years mass before thinning to exclude bias from pre-existing growth. If it exists, the plot would be removed for post-thinning test. Due to small test power, tests were not conducted when sample size was small; for example, in site 807, control plot has less than five trees in 5m levels (Table 3). Results Change in Whole Ring Mass of Thinned vs. Unthinned In years 1–3 since thinning, ring mass growth of thinned trees are statically significantly greater (p <=0.05) than control trees at 0.3m in WA sites 803 and 805 (Figure 3). Ring mass in OR sites 807 and 808 also show produced increase in stem mass toward stem base. In years 4–6 since thinning, the percent change in ring mass of thinned tress are greater than in years 1–3 and significant from control trees in all sites at 0.3m. There are marginally significant (0.05 < p <=0.1) in percent ring mass gain of thinned over unthinned at 5.5m and 10.7m for sites 803, 805, and 807 and significant at site 808. In years 7–9 since thinning, the percent change in ring mass is positive and statistically significant with the same pattern as years 4–6. This suggests more overall ring mass growth and continued the redistribution of growth toward the stem base. In years 10–12 since thinning, the percent change in ring mass is still positive but not significant at site 803 in all heights. For other sites, ring mass of thinned trees are still significantly more than control. However, the positive changes are often smaller at the stem base and negative changes are often smaller at the upper stem than in years 7–9 for each site. This suggests that the ring mass growth redistribution toward the stem base following thinning is both reversing and diminishing as stand leaf area and competition return to pre-thinning conditions. Figure 3. View largeDownload slide Relative ring mass change of thinning vs unthinned for years 1–3, 4–6, 7–9, and 10–12 after thinning by height level and site. Big symbols represent p-value <0.10, and big with outline symbols represent p-value < 0.05 from the repeated measures ANOVA. Figure 3. View largeDownload slide Relative ring mass change of thinning vs unthinned for years 1–3, 4–6, 7–9, and 10–12 after thinning by height level and site. Big symbols represent p-value <0.10, and big with outline symbols represent p-value < 0.05 from the repeated measures ANOVA. Change in Latewood Mass of Thinned vs. Unthinned In years 1–3 since thinning, the percent change in LW ring mass at 0.3m is positive and significant at 803, 805, and 807 (Figure 4). At 5.5m the percent change is positive at all sites and marginally significant in site 803. The percent changes in LW are greater than percent changes in ring mass. These results suggest that, as part of the process of redistributing growth toward the stem base in response to stresses following thinning, there is a corresponding, but small, tendency to increase production of strong, stiff LW tracheids. Examining patterns of LW mass over all four growth periods, there is a tendency for thinned trees to produce more LW mass at the stem base and less LW mass toward the top than unthinned counterparts. LW production seems to generally increase until period 10–12, when relative gains are less than in the previous period and appear to be reversing toward the upper stem. Figure 4. View largeDownload slide Relative latewood mass change of thinning vs unthinned for years 1–3, 4–6, 7–9, and 10–12 after thinning by height level and site. Big symbols represent p-value <0.10, and big with outline symbols represent p-value < 0.05 from the repeated measures ANOVA. Figure 4. View largeDownload slide Relative latewood mass change of thinning vs unthinned for years 1–3, 4–6, 7–9, and 10–12 after thinning by height level and site. Big symbols represent p-value <0.10, and big with outline symbols represent p-value < 0.05 from the repeated measures ANOVA. Change in Earlywood Mass of Thinned vs. Unthinned In years 1–3 since thinning, the percent change in EW ring mass at 0.3m is only marginally significant in site 803 (Figure 5). Also, at in-crown length 10.7m the percent change is positive and marginally significant at OR sites 807 and 808. These results suggest that, as part of the process of responding to thinning, EW growth has redistributed growth toward the stem base to accommodate increased demand for water and nutrients by increasing the crown mass as well as the need to respond to stresses from wind sway. EW mass changes in periods 4–6 and 7–9 are generally larger, as would be expected with continued replacement of pre-thinning cohorts of needles with a greater mass of post-thinning needles with a greater demand for conduction of water and nutrients from the soil. EW production seems to generally increase until period 10–12, when relative gains are less than in the previous period and appear to be reversing toward the upper stem. Figure 5. View largeDownload slide Relative earlywood mass change of thinning vs unthinned for years 1–3, 4–6, 7–9, and 10–12 after thinning by height level and site. Big symbols represent p-value <0.10, and big with outline symbols represent p-value < 0.05 from the repeated measures ANOVA. Figure 5. View largeDownload slide Relative earlywood mass change of thinning vs unthinned for years 1–3, 4–6, 7–9, and 10–12 after thinning by height level and site. Big symbols represent p-value <0.10, and big with outline symbols represent p-value < 0.05 from the repeated measures ANOVA. Change in Latewood Mass Percent of Thinned vs. Unthinned In years 1–3 since thinning, the percent changes in LW% of ring mass are positive and significant toward base height at sites 803, 805, and 807 (Figure 6). At 10.7m the percent change is negative and significant at site 808. Percentage of LW production at the stem base is smaller and not significant in periods 4–6 and 7–9. At higher height positions 5.5m and 10.7m, the changes in LW% are positive at WA sites 803 and 805 but negative at OR sites 807 and 808. At the highest height, 21m, the LW% change is positive and significant at site 805. In years 10–12, change in LW% is still positive at the upper stem at site 805 while change in LW% at site 808 is negative along the stem. Figure 6. View largeDownload slide Relative LW% of ring mass change of thinning vs unthinned for years 1–3, 4–6, 7–9, and 10–12 after thinning by height level and site. Big symbols represent p-value <0.10, and big with outline symbols represent p-value < 0.05 from the repeated measures ANOVA. Figure 6. View largeDownload slide Relative LW% of ring mass change of thinning vs unthinned for years 1–3, 4–6, 7–9, and 10–12 after thinning by height level and site. Big symbols represent p-value <0.10, and big with outline symbols represent p-value < 0.05 from the repeated measures ANOVA. Examining patterns of LW mass as a percent of ring mass over all four growth periods, there is a tendency for thinned trees to respond to wind sway by immediately increasing the LW mass proportion at the stem base and less toward the top than unthinned counterparts. This seems to weaken with time, with reapportioning toward the upper stem. It may reflect the building of a larger crown adding a new source of stress and instability requiring the tree to build resistance higher in the stem. The Oregon and Washington sites appear to behave differently; thinned trees on the Oregon sites exhibit smaller LW gain in the lower stem and more LW loss relative to unthinned counterparts than on the Washington sites. The foothill of the Washington Cascades site, 805, indicates much greater LW mass proportion in the upper stem. Discussion Thinning led to increased total ring mass by increasing both EW mass and LW mass. This result agrees with studies in loblolly pine (Tasissa and Burkhart 1997), Jack pine (Zahner and Oliver 1962, Schneider et al. 2008), and Scots pine (Peltola et al. 2002, Goudiaby et al. 2012). As thinning increases the crown size, more light, water, and nutrients, more photosynthates are derived and transported downward. Some of the transported materials are used to increase LW production, and some are stored for increased EW production in the next season. Since EW functions for water conduction, increased demand by the enlarging post-thinning crown requires more EW for efficient water and nutrient conduction from the soil to the crown. Since thinning increases wind sway and crown mass, trees must also respond to the altered mechanical stresses and would also be expected to produce more of the denser stiff and strong LW to resist the stresses. The balance between EW and LW production could be that LW percent is unchanged, increased, or decreased compared to unthinned trees. Zahner and Oliver (1962) found that thinning delays LW formation at both BH and in the crown positions. Others have found that EW increases more than LW, resulting in reduction in the percentage of LW and lower overall ring wood density (Barbour et al. 1994, Koga et al. 2002, Liu et al. 2003). In this study, LW mass and LW% increased especially toward the stem base in the early years since thinning. The study sites commonly experience drought, soil moisture deficits, and induced dormancy in the summer when LW is formed. By reducing competition for water among the residual trees, thinning relieves or delays the onset of water deficit and dormancy, permitting prolonged formation of LW. Furthermore, the LW that does form may be in the form of compression wood, a reaction to the stresses from wind and increased crown mass, which is typically wider and denser than normal LW. This would tend to further increase the mass of LW and its percentage of the ring mass. This study found that thinned trees produced relatively more ring mass toward the stem base and relatively less toward the top than unthinned trees. This shift is present in years 1–3 following thinning, becomes more pronounced in years 4–6 and 7–9, and is reversing in years 10–12. The immediate shift toward the stem base is likely a reaction to wind sway resulting from the wider spacing in the residual stand. This reallocation of resources along stem or “thinning shock” is noted in other Douglas-fir stands (Worthington et al. 1962, Mitchell 2000). Thinned trees build a larger crown by replacing pre-thinning needle cohorts with denser post-thinning cohorts and retain a longer crown due to improved lighting reaching lower branches (Maguire et al. 1991). With time, the distribution of ring mass along the stem reverses toward the pre-thinning distribution as pre-thinning leaf area and competition for light, water, and nutrients is re-established. A similar pattern was observed for stem diameter changes in Scot pines by Peltola et al. (2002). Thinning shows similar trends on the four study sites. The mass increase is in only the lowest part of the stem in period 1–3, and the increase progresses up the stem in the second three-year period. Reukema (1964) found that stem growth of released trees was less than growth of unreleased trees at the point of maximum growth near the crown base. However, released trees had a less sharp decline downward and more gain below BH. The reduction growth on the top part and more growth in lower part agrees with Jozsa and Brix (1989). In Balsam fir, Koga et al. (2002) found significant growth increase from thinning in the lower stem (<5 m), mostly in the form of EW. In loblolly pine, Tasissa and Burkhart (1997) found increased growth in the lower stem, with increases in both EW and LW but with LW% unchanged. When the height-in-bole and time-since-thinning patterns are examined for the EW and LW components of ring mass, the overall patterns for each are quite similar to that described for whole ring mass. Initially relatively more of each is produced toward the stem base and relatively less toward the top, with an eventual reversal over time. Unlike others who found that thinning led to delayed and presumably less LW production and reduced ring density for a period of time following thinning (Zahner and Oliver 1962, Barbour et al. 1994, Koga et al. 2002), this study found that ring mass increased due to the overall increase in both EW and LW. Compared to unthinned trees, thinned trees in this study formed relatively more LW as a percent of ring mass toward the stem base and relatively less toward the top, again gradually reversing with time. This tendency to produce relatively more LW may be explained by the common occurrence of summer drought in the Douglas-fir region, which frequently produced water stress, soil moisture deficits, and induced dormancy on the study sites. By reducing competition for water, thinning extended the soil moisture reserves for the residual stand, which likely delayed water stress, soil moisture deficit, and induced dormancy. This allowed residual trees to continue growth during the time of year when LW is normally produced. Therefore, a relatively low proportion of dense LW in each ring is increased, which contributes toward increasing overall ring density. This effect was also noted in other studies of Douglas-fir thinning (Brix and Mitchell 1980, Jozsa and Brix 1989). The increase in stem growth following thinning can be viewed as having three phases over time that follow a parabolic curve form. The first immediate phase is a period when height growth is reduced, photosynthetic capacity of needles may be reduced by the sudden increase in sunlight, and photosynthate production is reallocated to the lower crown and new foliage (Brix 1983, Donner and Running 1986). In the second phase, more photosynthate production is available for stem growth, with continued replacement of pre-thinning needles and crown expansion. In the third phase, the phase-two level may be sustained for a time but will diminish as pre-thinning competition is re-established (Thomson and Barclay 1984, Tasissa and Burkhart 1997). Sites 805 and 808 followed this pattern; the initial response in period 1–3 is small, increases to a peak at periods 4–6 and/or 7–9, and decreases in periods 10–12. Achievement of peak response corresponds reasonably well with needle retention of coastal Douglas fir of about 4–5 years of needles (Turner and Olson 1976, Omdal and Ramsey-Kroll 2010). After thinning, it would take about this much time for replacement of pre-thinning needle cohorts with post-thinning needle cohorts that may be more numerous and more productive. However, as time goes on, this would reach a steady state of post-thinning cohort replacement, but efficiencies will decline as the stand returns to crown closure and increased competition for resources by the larger trees. As this occurs, the decline from the peak, resulting in the parabolic response curve form, would be expected. In contrast, sites 803 and 807 only show the immediate increase followed by the declining phase. Site 803 has a shallow soil over a glacial hardpan. Roots of these trees likely fully exploit the shallow soil for water, and while thinning reduces competition for water, the root systems of the residual trees may take advantage so quickly that there is only an immediate gain with a rapid return to severe competition for water and a declining response. Site 807 is the hottest and driest and has the most frequent, severe, and long-lasting soil moisture deficits. While thinning reduced competition for water among the residual trees, which showed an immediate response, the droughtiness and water deficits may continue to limit growth. There are also notable differences in the height-in-bole and time-since-thinning patterns between the four study sites, which were different in age when thinned and exist on different soils. These are subject to different temperature/precipitation regimes, and consequently differ in the frequency, severity, and duration of soil moisture deficits (Figure 2). The distribution of ring mass on site 805 in the Washington North Cascade foothills is more extreme than on the other sites. Compared to unthinned, the highest relative ring mass gain is at the stem base (0.3m) and the highest relative ring mass loss is at 15m, with the high LW% gain at upper bole. Given its location, this site may experience stronger winds and receive more snow than the others so, once the plots were thinned, wind sway and snow loading of expanding crowns may be causing more extreme redistribution in growth. Site 807 in the Oregon Cascade foothills, the warmest and driest of the sites with the most frequent, severe, and longest-lasting soil moisture deficits, seems to have a more delayed response pattern than the other sites, as effects still persist in later years. Trees in this site showed immediate react by LW increase at stem base and then gradually increase in EW, which results in greater LW%, especially at the stem base in years 1–3, and then lower LW% compared to unthinned trees in the upper part of stems. Site 808, in the Oregon Coast range, has the highest elevation with the coolest and wettest climate, had the least frequent, severe, and lasting soil moisture deficits, and again exhibits a different thinning response pattern. Two variables that may help understanding of these different site reactions to thinning are exposure to wind and occurrence of snow. Local data for these variables could not be obtained, but differences in the frequency and severity of each may produce different stresses on the trees, which underlie the ring mass distribution patterns. Conclusion This study examined the following hypotheses of response of Douglas-fir ring mass production by Douglas firs on sites west of the Cascades in Oregon and Washington. Thinning increases ring mass growth with a redistribution of ring mass toward the stem base that gradually reverses with time. This was confirmed, but the magnitude of the initial redistribution varied by height position and the timing of reversal varied from site to site. The ring mass response to thinning is parabolic, starting low immediately following thinning, reaching a peak or stable level for a period of time and then declining as pre-thinning competition is re-established. This was confirmed. Ring mass increased immediately during years 1–3 following thinning on each site but, depending on height position and site, the peak response was not reached until 4–6 or 7–9 years after thinning. Decreased production was underway after the peaks became more noticeable, 10–12 years after thinning. The parabolic form appears to follow an increasing trend as trees replace pre-thinning by post-thinning foliage; eventually they produce more photosynthate for stem wood rather than for new foliage production. However, with time, the enlarging trees re-establish competition for light, water, and nutrients and ring mass production declines. While decline was underway at all four sites, it is apparent that they are still sustaining response. Thinning increases both EW and LW ring mass in patterns similar to that of whole ring mass. This was confirmed. The patterns of EW and LW mass formation along the stem were quite similar to that of whole ring mass on all sites. Thinned trees must respond to increased demand by expanding foliage for water and nutrients that are conducted by EW from the roots to the crown but must also respond to increased mechanical stresses imposed by wind sway and by crown expansion. While expanding the mass of EW may partially address these stresses, production of more dense, stiff and strong LW, and compression wood in the more stressed regions of the stem would be more efficient. Thinning increases LW as a percent of ring mass since thinning Douglas-fir sites that commonly have summer drought will respond to reduced competition for water with extended growth during the time when LW is normally produced. This was confirmed. LW as a percent of ring mass was greater, especially at the stem base where relatively more ring mass was being produced. This is unlike many reports for other species where percent LW decreased. There are two key limitations of this study. First, it is limited to thinning response by these Douglas-fir sites with the thinning regimes indicated in Table 2. Although overall general patterns are similar, suggesting a qualitative model of how Douglas-fir growing on drought-prone sites responds to thinning, the degree and timing of response varies between the sites. Differences can also be expected if the thinning was less or more severe or if the ratio of post-thinning vs. pre-thinning dbh, d/D = 0.9 for the study sites, was substantially different, indicating stronger thinning from below or from above. Extending this study to more sites and thinning conditions will be needed to develop a general quantitative model of response. The second limitation is the lack of complete local growing environment data. 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TI - Effects of Thinning on Ring Mass Growth along Stem of Douglas Fir in Four Coastal Pacific Northwest Sites
JF - Forest Science
DO - 10.1093/forsci/fxx003
DA - 2018-04-01
UR - https://www.deepdyve.com/lp/springer-journals/effects-of-thinning-on-ring-mass-growth-along-stem-of-douglas-fir-in-AINUo1vOaw
SP - 139
EP - 148
VL - 64
IS - 2
DP - DeepDyve
ER -