Trembling aspen competition and climate effects on white spruce growth in boreal mixtures of Western Canada

We investigated the combined effect of trembling aspen competition and climate on white spruce growth using data from a long term sutdy with matching treatments spread across the boreal mixedwood forests of Alberta and Saskatchewan (Canada). Results indicate that competition (i.e., aspen basal area), initial size of the tree and mean annual temperature can account for 88% of the year to year variation in spruce volume growth for these six locations. Based on the model that we developed, spruce growth, in the absence of competition, is estimated to increase by up to 17% compared with an increase in mean annual temperature from 2 C to 3.3 C, while at high levels of competition (aspen basal area = 27 m2/ha) spruce growth increases by only 8%. Moreover, effects of aspen on spruce growth increase more than proportionally as temperature increases. This outcome indicates that abundant aspen competition limits the spruce responses to rising temperature, presumably due to competition for light and potentially increased competition for soil resources. Results also show that competition and climate effects vary between locations, indicating that spruce growth is strongly influenced by local factors such as micro-climate, topography, and soil properties.