Reducing paper birch density increases Douglas-fir growth rate and Armillaria root disease incidence in southern interior British Columbia

Paper birch (Betula papyrifera Marsh.) is routinely chemically or mechanically weeded from Douglas-fir (Pseudotsuga menziesii var. glauca (Beissn.) Franco) plantations to increase conifer productivity, but these practices are controversial because of unknown effects on forest health and diversity. This study examined effects of paper birch density reduction treatments on productivity and disease incidence of Douglas-fir saplings in the southern interior of British Columbia. Five paper birch density treatments (0, 400, 1111, 4444 stems ha−1 and an un-thinned control of >7000 stems ha−1), where paper birch was manually cut at the root collar in 1999, were replicated on four sites in a randomized block design. After 2 years, Douglas-fir survival did not differ significantly among treatments, but where Armillaria ostoyae (Romagn.) Herink was present in untreated stands, mortality due to root disease increased significantly with increasing paper birch thinning intensity. Mean diameter increment and height:diameter ratio of Douglas-fir improved where paper birch was completely removed or partially thinned compared with the control. Mean light transmittance and soil moisture content increased with decreasing paper birch density, but Douglas-fir foliar N status was unaffected. Our results suggest that Douglas-fir growth can be improved and root disease incidence minimized by thinning paper birch to 4444 stems ha−1 in young mixed stands, but that subsequent release treatments may be necessary to maintain Douglas-fir growth rates.