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Effect of paper birch density on the performance of interior douglas-fir when grown in mixture in the southern interior of British Columbia Baleshta, Karen Eileen


The objective of this study was to examine the effects of Betula papyrifera Marsh, (paper birch) density treatments on the productivity of Pseudotsuga menziesii var. glauca (Beissn.) Franco (interior Douglas-fir) saplings when the two species are grown in mixture in the ICHmw3 and IDFmw2 variants in the southern interior of British Columbia. Four study sites were selected in May 1999 in 10- to 15-year-old interior Douglas-fir plantations in the Adams Lake drainage, approximately 100 kilometers (km) northeast of Kamloops. Five paper birch (Ep) density treatments (0, 400, 1111, 4444 stems per hectare and control) were replicated four times in a randomized complete block design, where site location was used as the blocking factor. The effects of the paper birch density treatments on the performance of interior Douglas-fir were determined by comparing individual tree and environmental response variables. Interior Douglas-fir survival was high and there were no significant differences in mortality among treatments two-years following the initiation of the paper birch density treatments. Interior Douglas-fir mortality due to Armillaria ostoyae tended to be lower in the control (no thinning) versus the four thinned treatments combined, but the differences among treatments were not significant (P=0.2114). In this study, mean diameter increment of interior Douglas-fir was higher in the treatments where the broadleaf component was completely removed, heavily thinned and lightly thinned (e.g. 0 Ep, 400 Ep and 4444 Ep, respectively) compared with the control two years after the density treatments were implemented. On average, interior Douglas-fir in the lightly thinned treatment (4444 Ep) tended to be the tallest with the largest stem diameter, crown diameter, stem volume and canopy volume. Interior Douglas-fir height:diameter ratio tended to decrease in all treatments, with significant differences occurring among treatments in 2001 (P=0.0333). The control had a higher height:diameter ratio than (a) the four other thinning treatments combined, and (b) the treatment where the broadleaf component was completely removed (0 Ep)(P=0.0046 and P=0.0169, respectively). Mean net photo synthetic rate and mean stomatal conductance of the interior Douglas-fir measured in 2001, and mean foliar nitrogen concentration measured in 2000, were not significantly different among treatments (P=0.8157, P=0.8020 and P=0.8994, respectively). Mean specific leaf area of the interior Douglas-fir sampled in 2000 increased with increasing paper birch density and decreasing light availability (P=0.0018). Mean light transmittance measured in 2000 yielded differences among treatments (PO.0001), with the trend from highest to lowest as follows: 0 Ep > 400 Ep > 1111 Ep > 4444 Ep > control. Soil moisture content did not differ among treatments in 2000 (P=0.3922), but it did in 2001 (P=0.0086). In 2001, the lowest mean soil moisture content was in the control (10.12% ± 2.18%), whereas the 0 Ep treatment, where the broadleaf component was completely removed, had the highest (17.23% ± 3.31%). In this study, paper birch survival was high, averaging between 98% and 99%. Substantial main stem breakage (at 1.54 - 2.40 meter height) by moose occurred in the heavily thinned treatment (400 Ep)(excluding Burton Creek), whereas little occurred in the other treatments between 1999 and 2001, but differences among treatments were not significant (P=0.1889). The mean relative diameter growth rate of the dominant paper birch (tallest 250 stems per hectare) increased with decreasing birch density (P=0.0005). Mean diameter increment also tended to increase with decreasing birch density, although the treatment rankings were not systematic (P=0.0213). The trends in interior Douglas-fir growth across treatments in this study tend to support other studies showing that moderate densities of paper birch can be retained without detrimentally affecting interior Douglas-fir performance. While preliminary, these results provide valuable information toward identifying density regimes that are indicative of healthy conifer/broadleaf mixed-stands.

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