Modelling the interception of solar radiant energy 209

A model has been constructed for predicting the interception of solar radiant energy by an array of plants with ellipsoidal crowns. The applicability of the model to stands of Pinus radiata D. Don was investigated by comparing model predictions with measurements of instantaneous irradiance below tree crowns in a 9-year-old stand. The agreement between actual and predicted trans-mittances was generally good. However, small errors in measuring crown shape and leaf area can cause large variations in predicted instantaneous trans-mittances. When predicting the interception of solar radiant energy on a daily basis, small errors in measuring crown shape or leaf area did not cause large variations in intercepted radiant energy. Model simulations indicated that there is a linear relationship between intercepted photosynthetically active radiant energy (PAR) and above-ground dry matter production in stands of P. radiata growing on a fertile site. This relationship was unaffected by thinning and light pruning. Assuming that the relationship is unaffected by more extreme thinnings and primings, simulations indicated that unthinned-pruned stands can intercept up to 25 % more PAR than unpruned-thinned stands with the same leaf area index