Reducing Paper Birch Density Increases Douglas-Fir Growth Rate and Armillaria Root Disease Incidence in Southern British Columbia

In four 10 15-yr-old interior Douglas-fir Pseudotsuga menziesii plantations in southern British Columbia, Canada, the effect of a reduction in paper birch Betula papyrifera density on the growth, nutrition, and Armillaria ostoyae infection of understory Douglas-fir was studied. At the time of treatment, all sites were dominated by mixed stands of Douglas-fir and paper birch. Five treatments were initiated to represent a gradient in paper birch density, decreasing from a control of more than 7000 stems/ha to 4444, 1111, 400, and 0 stems/ha, while Douglas-fir density was not manipulated. Douglas-fir survival was not affected by the thinning treatments, but Douglas-fir mortality due to A. ostoyae increased with increasing birth thinning intensity. Over the 3-yr measurement period, height:diameter ratio decreased in all treatments, but more so with increasing thinning intensity. Complete paper birch removal resulted in a 30 and 32% higher relative diameter growth rate than the control and lightly thinned treatments, respectively.