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) model s usuary modify the growth of trees by means of competition indices; howeve r, these competition indices rarely simulate the light available for tree g rowth explicitly. We used tree growth data from an even-aged, unthinned, 50 -year-old Douglas-fir (Pseudotsuga menziesii (Mirb.) France) stand in Briti sh Columbia to test the hypothesis that the amount of absorbed light is a g ood predictor of diameter at breast height, height, and bole volume growth of an individual tree. We also explored the relationships between these var iables. A spatially explicit light model was used to simulate photosyntheti cally 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. Becaus e of the integration of light absorption by a tree crown, estimates of WLA were highly correlated with leaf area for dominant trees. For suppressed tr ees, leaf area was a poor estimator of WLA. The relationship between WLA an d bole volume growth was nonlinear, indicating a higher light-use efficienc y in suppressed trees than in dominant trees. This relationship was strong enough to be useful for growth modeling. Only height growth of suppressed t rees 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.