Aspen competition affects light and white spruce growth across several boreal sites in western Canada

The effectiveness of competition indices for predicting light transmittance and white spruce (Picea glauca (Moench) Voss) growth were examined across trembling aspen (Populus tremuloides Michx.) density gradients using sites from a long-term study of mixedwood growth and development in Alberta and Saskatchewan. Competition indices based on density (number of trees, basal area, and spacing factor), distance-dependent and -independent size ratio (Hegyi's and Lorimer's), and crown characteristics (crown volume, surface area, and cross-sectional area) were tested. Transmittance was effectively predicted by crown competition indices followed closely by aspen basal area and size ratio indices. Models of spruce growth indicated better results for stem volume compared with diameter or height. Competition alone accounted for less than 60% of stem growth variation, with basal area and transmittance providing some of the best models. The predictive ability of spruce growth was increased up to 93% by adding initial size as a second explanatory variable. In this respect, initial diameter was superior to initial height, crown volume, and surface area. Relationships between competition, transmittance, and spruce growth were found to differ significantly between geographical locations. These results suggest the need for local development of models relating tree growth to competition.