Study examining the effects of varying amounts of paper birch on growth of 7-11-year-old Douglas-fir in the Interior Cedar-Hemlock zone of the southern interior of B.C. Competitive interactions in five birch-dominated Douglas-fir plantations were retrospectively examined using a neighbourhood approach, where the size of target Douglas-fir was related to the density, size and proximity of neighbouring birch. The neighbourhood measures were chosen based on the assumption that paper birch affects Douglas-fir growth primarily through competition for light. Ecological competition thresholds identified at each site were the birch density at which competition began and Douglas-fir growth was limited, and the birch density at which Douglas-fir growth became independent of birch density.