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Modeling growth of understory aspen and spruce in western boreal Canada Astrup, Rasmus

Abstract

For aspen and spruce in western sub-boreal and boreal Canada this study investigated regional variation in: (1) species-specific crown openness for mature trees, and (2) light-growth relationships for juvenile trees. Regional variation in species-specific crown openness was found for both species, but the magnitude of the variation was judged to only cause small variation in understory light levels. Regional variation in the light-growth relationship was found for both species but no indications of shifts in the successional dynamics between the two species were observed. The regional variation in the two investigated quantities did not follow the same regional pattern and their combined effect cannot be expected to strengthen regional differences in understory tree performance. The regional shapes of the light-growth relationship were similar for the two species. The main observed difference between the two species was the initial fast growth of aspen. For spruce in aspen-dominated areas of the boreal mixedwood region, asymptotic light growth relationships were found. In western conifer-dominated regions, approximately linear light-growth relationships were observed. This regional variation was best explained by the different effect of aspen-dominated and conifer-dominated canopies on the light-growth relationship. It is hypothesized that the different effects of canopy-type can be caused by several effects, including nutrient availability, soil temperature, and leaf-off period. The findings of this study were incorporated in the stand level simulation model SORTIE-ND. An evaluation of SORTIE-ND illustrated that the model generally produces realistic predictions and can be considered as a management tool. A comparison of model predictions with permanent plot data for mixed stands in the Sub-Boreal Spruce Zone of British Columbia indicated that the model has problems with prediction of aspen mortality rates and tends to overestimate growth rates especially for spruce.

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