Light absorption and bole volume growth of individual Douglas-fir trees

Empirical growth and yield models for forest management are evolving toward individual-tree models that are capable of simulating the growth of mixed and uneven-aged stands. Spatially explicit (i.e., distance-dependent) models usually modify the growth of trees by means of competition indices; however, these competition indices rarely simulate the light available for tree growth explicitly. We used tree growth data from an even-aged, unthinned, 50-year-old Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) stand in British Columbia to test the hypothesis that the amount of absorbed light is a good predictor of diameter at breast height, height, and bole volume growth of an individual tree. We also explored the relationships between these variables. A spatially explicit light model was used to simulate photosynthetically active radiation absorbed by individual trees during a growth period (APAR) based on detailed canopy architecture information. For the purpose, we used a weighted leaf area (WLA) that is linearly related to APAR. Because of the integration of light absorption by a tree crown, estimates of WLA were highly correlated with leaf area for dominant trees. For suppressed trees, leaf area was a poor estimator of WLA. The relationship between WLA and bole volume growth was nonlinear, indicating a higher light-use efficiency in suppressed trees than in dominant trees. This relationship was strong enough to be useful for growth modeling. Only height growth of suppressed trees was affected by WLA. We conclude that single-tree WLA can be used as a process-oriented competition index in growth models for forest management.