Regional variability of species-specific crown openness for aspen and spruce in western boreal Canada

Abstract

In the Canadian boreal mixedwood region, there is currently increasing management desire to foster and release understory trees. Consequently, there is also an increased interest in modeling and predicting understory light levels. In this study, species-specific crown openness is defined as the fraction of sky that can be seen through the crown of an individual tree of a given species. Species-specific crown openness is of general interest for understanding light transmission through forest canopies, and is an essential part of the light submodel in SORTIE. The main objective of this study was to investigate whether regional differences in mean species-specific crown openness exist for aspen ( Populus tremuloides Michx) and white spruce ( Picea glauca (Moench) Voss) in western boreal Canada. To ensure a robust comparison of regional mean species-specific crown openness, we initially investigated the underlying assumption that crown openness is unaffected by dbh and angle of view. In our data, both aspen and spruce crown openness was found to be independent of angle of view. Crown openness was also independent of dbh in aspen, while weak indications of a correlation between crown openness and dbh was found for white spruce. However, this relationship has little actual effect on crown openness and its effect on predicted understory light level is judged to be small. We found significant regional differences in mean crown openness for both aspen and spruce. However, these regional differences are small and are likely to have relatively little effect on understory light levels predicted with SORTIE. The results from this study indicate that, although previous estimates of aspen and spruce crown openness in western boreal Canada varied greatly, this is more likely the result of different methodologies than actual differences in crown openness.