Competitive interactions between aspen and white spruce vary with stand age in boreal mixedwoods

Abstract

Effects of aspen ( Populus tremuloides Michx.) competition on growth of white spruce ( Picea glauca (Moench) Voss) were examined using a neighbourhood approach across an age sequence of nine boreal mixedwood stands in Alberta, Canada. Three similar natural stands were selected for each of the three age classes considered (10–20, 20–40 and 40–60 years). Several 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 examined for their effectiveness at predicting light transmittance and spruce growth. Transmittance was poorly predicted by competition indices, except for some indices based on density, with relationships generally explaining less than 40% of variation. Models of spruce growth (diameter and height increment) indicated variable performance among stands and indices tested. Competition accounted for up to 70% of growth variation, with some of the best results provided by Lorimer's, Hegyi's and crown volume indices. The addition of initial size as a second explanatory variable increased the predictive ability of growth models. Best models of spruce growth were selected and tested for differences between stands and age classes using indicator variable tests. Results indicated that data from stands within the 20–40 and 40–60 year age classes could be pooled across sites and across the two age classes into a single model. Data from the youngest age class (10–20 years) could not be pooled with those from the two older age classes. Results from this study indicate that relationships between growth and levels of competition may differ between young and older stands and suggest it may be necessary to parameterize growth models separately for different age classes when growth is predicted as a function of competition.