The purpose of this study was to examine the effects of varying amounts of paper birch (Betula papyrifera Marsh.) on growth of 7- to 11-year-old Douglas-fir (Pseudostuga menziesii var. glauca (Mrib.) Franco) in the interior Cedar-Hemlock (ICH) zone of the southern interior of British Columbia. Competitive interactions in five birch dominated Douglas-fir plantations were retrospectively examined using a neighborhood approach, where the size of target Douglas-fir was related to the density, size and proximity of neighboring birch. The neighborhood measures were chosen based on the assumption that paper birch affects Douglas-fir growth primarily through competition for light. Two ecological competition thresholds were identified at each site: the first threshold was the birch density at which competition began and Douglas-fir growth was limited; the second was the birch density at which Douglas-fir growth became independent of birch density.