Spatially explicit modeling of PAR transmission and growth of Picea glauca and Abies balsamea in the boreal forests of Alberta and Quebec

To investigate the feasibility of a spatially explicit, radiation-based regeneration model for the boreal forest, we tested the predictions of a three-dimensional simulator of photosynthetically active radiation transmission (%PAR), MIXLIGHT, and the growth response of understory Abies balsamea (L.) Mill. (balsam fir) and Picea glauca (Moench) Voss (white spruce) to %PAR in two large (>1 ha) mixed-species forest sites, one in eastern Canada at Lac Duparquet, Quebec, and one in western Canada at Calling Lake, Alberta. Overstory tree locations and dimensions were obtained from aerial photographs or ground measurements and allometric relationships. Seasonal %PAR calculated by MIXLIGHT for the Calling Lake site was very similar to seasonal %PAR measured by quantum sensors (n = 5, %PAR range = 15%-33%, r = 0.93). Daily measurements of %PAR were also predicted well by simulations at both sites (n = 34-36, %PAR range = 1%-45%, r => 0.76). Functional relationships, designed to saturate at the maximum height growth potential of these sites, were developed to predict sapling height growth from simulated seasonal %PAR and initial height (R^2 => 0.74). These results demonstrate the potential of the MIXLIGHT simulator for estimating PAR at microsites within heterogeneous forests and for modeling understory tree growth.