Static and dynamic maximum size–density relationships for mixed trembling aspen and white spruce stands in western Canada

We examine maximum size–density relationships (MSDR) of pure and mixed stands of trembling aspen (Populus tremuloides Michx.) and white spruce (Picea glauca (Moench) Voss.) in the Boreal Forest Natural Region of Alberta, Canada. Stochastic frontier function regression was used to estimate the MSDR species boundary or static line and mixed models were used to investigate how individual stands self-thin (dynamic thinning line). Effects of age, stand composition, soil nutrient regime and soil moisture regime were also evaluated. A steeper slope was obtained for the dynamic than for the static thinning line, and both MSDR lines slopes are statistically different from the theoretical value proposed by Reineke (1933). The deciduous component (percent of stand total basal area that is deciduous) has a negative effect on the slope and a positive effect on the intercept of the static line. Composition (increasing aspen basal area) also has a negative effect on the intercept of the dynamic line although no effect was detected on the slope. Soil nutrient regime has a positive effect on the intercept and a negative effect on the slope of the dynamic thinning line. Results suggest that local differences such as site quality and stand composition are important factors in determining maximum size–density relationships for these mixedwoods stands and how individual stands develop and self-thin.

► We examine size–density relationships for pure and mixed stands of trembling aspen and white spruce in western Canada.
► Stochastic frontier functions and mixed models were used to define static and dynamic thinning lines.
► Significantly different slopes than the value proposed by Reineke (1933) were obtained for both limits.
► Local differences are key factors in determining maximum size–density relationships of boreal mixedwood stands.