Operational birch thinning treatments prescribed to reduce the effects of paper birch (Betula papyrifera Marsh.) and other associated broadleaf trees on the growth performance of planted Douglas-fir (Pseudotsuga menziesii [Mirb.] Franco) were followed for 19 years post treatment on four sites in the Interior Cedar–Hemlock biogeoclimatic zone (ICH) of British Columbia. Treatments ncluded 600 and 2500 stems per hectare (sph) of retained birch, as well as untreated controls. These treatments were established at each site, where planted Douglas-fir and retained birch were tagged and repeatedly measured over a 19-year post-treatment period in combination with vegetation assessments and silviculture surveys. Significant early increases in Douglas-fir diameter and crown length associated with birch thinning were not found > 5 years post treatment, although Douglas-fir were visually larger through to 19 years. Retained birch diameter and crown size were significantly larger within the 600 sph thinning plots (B600) compared to both the 2500 sph retained birch and the untreated control plots from years 3 to 19. Douglas-fir and retained birch tree volume increments over the 5- to 19-year post-treatment period were 150% and 260% larger, respectively, in the B600 treatment plots than in the controls.
Over the 19-year post-treatment period, Douglas-fir/birch allometric ratios for height and crown size indicated that the height and crown growth rate of Douglas-fir was greater than that of birch after year 5, which indicated that Douglas-fir canopy dominance increased over time. Stand-level densities indicated by year 19 total density were significantly less in the thinned treatments due to significant declines in broadleaf density. Even though by year 19 post treatment, conifer densities were 3300–4800 sph across all treatments, only 600 sph were free-growing. Conifer ingress, noted since treatment establishment (9–14 years post planting), together with birch densities of 1100–2700 sph has resulted in overstocked stands that contain, on average, between 4500 and 7600 total sph at year 19, depending on the treatment. As a result, prediction of Douglas-fir tree growth in relation to neighbourhood competition was influenced by large variations in local density, species composition, and cover across the treatments. Douglas-fir crop tree size distribution was explored across the treatments using diameter–frequency plots, density functions, and cumulative stem volume growth patterns.
Harper, G.J.. 2023. Effects of Paper Birch Retention on Douglas-fir Stand Development 19 Years Post Treatment in Southern Interior British Columbia. Ministry of Forests. Technical Report (FLNRORD). TR147
Topic: FLNRORD Research Program
Scientific Name: Betula papyrifera var. papyrifera, Pseudotsuga menziesii var. menziesii
ISBN: ISBN 978-1-0399-0
English Name: Paper Birch, Coast Douglas-fir
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