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Investigation into the productivity of single- and mixed-species, second-growth stands of western hemlock.. Klinka, Karel; Collins, D. Bradley; Chourmouzis, Christine 2001

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Scientia Silvica Extension Series, Number  36, 2001Investigation into the Productivity of Single- and Mixed-Species,Second-growth Stands of Western Hemlock and Western RedcedarIntroductionIn BC, it is required that harvested areas be regenerated with a mixture of tree species whenever appropriate to the site.This policy is based upon the assumption that increases in stand productivity, reliability, and/or biodiversity can be achievedin mixed-species stands. However, the knowledge justifying this policy is at best incomplete.Differences  in forest  productivity of  mixed-species stands have been attributed mostly to competition. However,  an increasingnumber of studies  are providing evidence to  support  alternate theories,  in  which positive plant interactions  play a major role.Positive plant interactions  are divided into  two  components: (i)  competitive reduction  through  structural  and physiologicaldifferences in above and below ground structures, and (ii) facilitation through any positive effect on the growing environment ofone plant species by another. These theories have yet to be tested in forest ecosystems. The objectives of this study, with respectto naturally established, unmanaged, second-growth stands of western hemlock (Hw) (Tsuga heterophylla (Raf.) Sarg.), westernredcedar (Cw) (Thuja plicata Donn ex D. Don in Lamb.), and their mixtures, were: (1) to review the mechanisms of positive plantinteractions and their potential to occur in these mixtures, and (2) to compare the productivity of these three stand types, usingrelative and absolute yield.Study Stands and MethodsStudy stands were located in three areas: Capilano River Valley (Capilano), University of British Columbia Malcolm Knapp ResearchForest east of Vancouver (Knapp),  and Mission Tree  Farm License  No. 26 (Mission).  18 stands were selected for  the  study: 7hemlock stands, 4 Cw stands, and 7 Hw-Cw stands. The stands were naturally regenerated, unmanaged, closed-canopied, and even-aged (53 to 65 years @ bh), and represented the end of the stem exclusion stage of stand development. All stands were within theSubmontane Very Wet Maritime Coastal Western Hemlock (CWHvm1) variant, and were located on fresh, nutrient-medium sites.Within each stand a 30 x 30 m (0.09 ha) sample plot uniform in topography, vegetation, and soil was established. Diameter at breastheight (dbh)  and height were measured. Each live  tree  was assigned into  one of four  crown classes: dominant, codominant,intermediate, or suppressed. Species, dbh, total height, and crown height (distance from the ground to the lowest live branch) wererecorded for all trees taller than 1.3 m. The breast height ages of three dominant trees of Cw and hemlock per plot were determinedfrom increment cores.  Site index of Cw and Hw was estimated using standard equations.The relative yields of the mixed- and single-species stands were compared using the mean annual increment of each stand, basedon the sum of individual tree volumes (inside bark, gross volume from stump to tree top) divided by the mean age at breast heightof three  dominant trees  of each predominant species  per stand.    Relative yield comparisons were made with and without thevolume contributions of non-study species. Comparisons were calculated for mixtures of species X and Y, relative yield (RY) andtotal relative yield (RYT).Productivity was compared among stand types using mean annual increment (MAI). These comparisons were augmented with standvolume and basal area. Productivity comparisons among stand types were made using regression and analysis of variance (ANOVA).Despite controlling as much as possible for tree age, site quality, and stand density, some variation in these factors could not beavoided and was addressed by adding covariates to  ANOVA models. To  help explain differences or similarities in  productivityamong stand types, the individual characteristics of both Cw and Hw trees were compared in the single- versus the mixed-speciesstands.  The mean dbh, height, height to live crown, and MAI of Cw trees in Cw stands were compared to those of the Cw in themixed stands.  The same comparison was made among Hw trees in Hw and mixed stands.Results and DiscussionPositive plant interactions and their potential in hemlock-redcedar mixturesAlthough naturally occurring mixtures of Hw and Cw are very common in the CWH and southern ICH, the productivity and growthdynamics of these mixtures have been little studied. Theory on the mechanisms of competitive reduction suggests that Hw and/orCw may benefit in mixtures through: (1) belowground physiological separation via the preferential uptake of different forms ofnitrogen, and (2) aboveground spatial and temporal separation of canopies via differences in growth habit, spatial arrangement, andlongevity.  Theory on the  mechanisms of facilitation  suggests  that  Hw and Cw may benefit in  mixtures through:  (1)  resourcemodification whereby Cw increases  forest floor base cation concentrations, pH levels,  and microbial populations, (2) substratestabilization where Cw can enhance the  windfirmness of Hw,  and (3) herbivory reduction where each species shields the  otherfrom their common, specific defoliators. Other studies suggest that Hw and Cw may experience positive interactions through: (1)vertical canopy separation and a random spatial pattern of trees, (2) preferential uptake of different nitrogen forms, and (3) Cwincreasing forest floor pH levels, Ca and K concentrations, and microbial populations.The productivity of single- and mixed-species hemlock and redcedar stands using relative and absolute yield.Uncorrected MAI and quadratic mean diameter increased,  while stocking decreased with increasing  presence of Hw (Table 1),suggesting greater annual volume growth per tree in Hw stands. The highest mean basal area was also found in Hw stands, althoughthis may be misleading considering the influence of stand H7, which has substantially higher basal area than the other Hw stands.Excluding this stand, the mean basal area of the Hw stands was intermediate (80.0 ? 5.5 m2 ha-1) between the mixed and Cw stands.Table 1. Selected characteristics of study stands, stratified according to stand type (H - hemlock, M (mixed) - hemlock-redcedar,C - western redcedar; se is the stand type standard error of the mean.The relative yields of Hw and Cw were higher in mixtures when the effect of non-study species was removed. Both species had RY> 0.5, implying  that  mixtures are more productive than  single-species stands of either species. However,  when this  effect  wasaccounted for, the relative yields of Hw and Cw were lower in mixtures compared to single-species stands, due to the effects ofintraspecific competition. Although Hw still achieves the same yield in mixture with Cw as it would in alone, Cw achieves loweryield in mixture with Hw than it would in a monoculture.  This impact of competition on Cw yield decreases total relative yield inHw-Cw mixtures below 1.0, indicating that the mixtures are experiencing competitive inter-species interactions.Increasing Hw presence in a stand was associated with increasing MAI (Table 2). Differences in MAI among stand types becameincreasingly prominent with the complexity of the density covariate used in ANOVA models (complexity:  sph < qdbh < relativedensity). The model using relative  density explained the  most variation in  stand  type  mean annual increment,  and significantdifferences in increasing productivity were found for each stand type, with increasing presence of Hw. Wood volume also increasedStand   Dominant tree height (m) Dominant tree age at 1.3 m Site index (m) (Cw/Hw) Stems per hectare  Quadratic mean diameter (cm) Relative density  Basal area (m2 ha-1)  Volume  (m3 ha-1)  MAI  (m3 ha-1 yr-1) H1 37.3 65 29.5 1000 32.0 20.1 80.6 999.8 15.4 H2 35.2 57 28.4 989 33.2 21.0 85.4 1002.6 17.6 H3 38.7 61 31.7 767 37.4 19.7 84.0 1061.0 17.4 H4 46.9 65 35.0 744 35.3 17.5 72.7 1044.0 16.1 H5 39.3 66 31.9 1033 30.1 18.9 73.8 1037.4 15.7 H6 38.8 61 34.5 822 36.0 20.0 83.8 1105.4 18.1 H7 47.8 70 39.8 500 63.5 30.1 158.3 1619.0 23.1 mean 40.6 64 33.0 837 38.2 21.1 91.2 1121.2 17.6 se (4.8) (4) (3.9) (190) (11.4) (4.1) (30.0) (221.1) (2.6)          M1 33.2 60 26.6/31.1 978 32.2 19.9 79.6 850.7 14.2 M2 31.8 51 24.6/28.9 944 35.0 21.9 90.8 935.0 15.5 M3 37.6 55 24.4/31.0 1356 26.5 20.1 74.5 830.1 15.1 M4 33.5 50 30.0/32.0 822 34.3 18.5 76.1 730.1 14.6 M5 32.2 51 27.6/32.6 989 29.7 17.6 68.4 680.8 13.4 M6 33.2 58 27.2/29.0 1000 32.7 20.8 84.0 935.3 16.1 M7 32.9 58 26.3/29.9 944 30.8 17.9 70.5 713.6 12.3 mean 33.5  56 26.7 30.6 1005  31.6  19.5  77.7 810.8 14.4 se  (1.9) (4) (1.9/1.4) (166) (2.9) (1.6) (7.8) (104.7) (1.3)          C1 28.0 58 23.6 1100 29.7 19.6 76.0 699.9 12.1 C2 31.8 55 26.3 1478 27.4 23.1 87.0 796.3 14.5 C3 38.0 55 26.3 767 39.6 21.7 94.7 762.6 13.9 C4 32.4 57 28.5 967 33.1 20.5 83.2 786.5 13.8 mean 32.6 56 26.2 1078 32.5 21.2 85.2 761.3 13.6 se (4.1) (2) (2.0) (300) (5.4) (1.5) (7.8) (43.3) (1.0) G3with increasing presence of Hw in progression from the Cw to the Hw-Cw to the Hw stand types (Table 2). Increasing proportionsof the  variation in  volume was explained as the  complexity of the  density covariate increased,  although significant differencesamong stand types were only found using relative density.Productivity measure  Density covariate  Stand productivity according to stand type     Hemlock (n = 7)  Hemlock-redcedar (n = 7)  Redcedar (n = 4) Stand MAI (m 3 ha-1 yr-1)  Stems per hectare  16.3 a (0.7) 14.9a (0.6) 14.3a (0.8)   Quadratic mean diameter  16.5 a (0.6) 15.1a,b (0.5) 14.1b (0.7)  Relative density 17.0a (0.4) 15.1b (0.3) 13.5c (0.4) Stand volume (m 3 ha-1)  Stems per hectare  954.8 a (72.1) 911.6a (41.0) 874.2a (46.1)   Quadratic mean diameter  949.1 a (62.6) 915.2a (37.5) 876.3a (37.9)   Relative density   1053.3 a (38.0) 881.2b (21.0) 766.4c (28.1) G3Table 2. Least squares means and standard errors (in parentheses) of MAI and volume of 18 study stands, stratified according todensity covariate and stand type. Values in the same row with different superscripts are significantly different (p <0.05).The Cw in the Cw stands were similar in most respects to those in the mixed stands (Table 3). No significant differences (a = 0.05)in mean dbh, tree height, height to live crown, and MAI were found in Cw of either stand type. The mean height to live crown of Cwin  the  Hw-Cw stands was significantly greater than  in  the  Cw stands. In contrast, Hw in  the  Hw stands had greater MAI, andsignificantly greater mean diameter, height, and height to live crown, than the Hw in mixed stands (Table 3).Table 3. Mean individual tree characteristics, and standard errors of the means (in parentheses), stratified according to speciesand stand type. Values in the same row with the same superscript are not significantly different (p >0.05).Silvicultural ImplicationsForesters need to know whether a mixture of two or more tree species will be more productive than single-species stands of thesespecies. Taking into account the lack of canopy stratification in single-cohort, Hw-Cw mixtures, and the improved growth of Hwwith decreasing presence of Cw, we propose that pure Hw stands appear to be the most productive tree species option on the studysites.ConclusionsContrary to  other conclusions arrived on the  mechanisms of positive plant interactions  and their  potential to  occur in  Hw-Cwmixtures, the relative yields of each species in single- versus mixed-species stands appear to suffer from competitive interactions.MAI and wood volume increased with increasing presence of Hw, while basal area increased with increasing presence of Cw. TheHw stands were most productive because Hw height growth was greater than Cw, and its height and diameter growth were greaterin Hw stands. This greater diameter growth may be due to the superior ability of Hw to self-thin on these sites.  The Cw stand typewas the least productive due to dense stocking of slender, short stems. Although Hw was shorter and of smaller diameter in mixed-species stands, Cw grew equally well in mixed- and single-species stands.  Maximum wood volume production on similar siteswould be achieved by establishing Hw stands with a minimal component of other species.Tree species  Redcedar  Hemlock  Stand type  Redcedar  Hemlock-redcedar   Hemlock  Total height (m)  22.2 a (5.7) 22.8 a (5.3) 26.6 b (6.5) 31.0 a (6.6) Height to live crown (m)  13.0 a (3.4) 14.4b (3.5) 15.3 b (4.5) 18.1 a (4.1) Mean annual increment (m3ha-1yr-1)  0.012 a (0.013) 0.011 a (0.011) 0.017 a (0.013) 0.022 a (0.017) Diameter at 1.3 m (cm)  28.4a (14.1) 28.1 a (11.6) 29.4 b (10.9) 33.3 a (11.5) G3ReferenceCollins, D.B.  2000. Investigation  into  theproductivity of single-and  mixed-species,second  growth-growth stands  of westernhemlock and western redcedar.  M.Sc. Thesis,University of British Columbia, Vancouver, BC.Scientia Silvica is published by the Forest Sciences Department,The University of British Columbia, ISSN 1209-952XEditor: Karel Klinka (klinka@interchange.ubc.ca)Research: Brad Collins (brad@locin.com)Production and design: Christine Chourmouzis (chourmou@interchange.ubc.ca)Financial support: Natural Science and Engineering Council of Canada, Canadian Forest ProductsLtd., Timber West Forest Ltd., Weyerhauser of Canada Ltd., and BC Ministry of ForestsFor more information contact: Brad CollinsCopies available from: www.forestry.ubc.ca/klinka orK. Klinka, Forest Sciences Department, 3036-2424 Main Mall, UBC, Vancouver, BC  V6T 1Z4

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