Radiation interception by forest trees: a simulation study on effects of stand density and foliage clustering on absorption and transmission

As part of a study on growth of mixed-species forest stands, a spatial model has been developed that simulates the absorption of photosynthetically active radiation (PAR) by individual trees and the transmission through the forest canopy. Model performance was tested by comparing simulated transmissions with field measurements. The model was applied to analyze the role of crown characteristics on radiation transmission and on absorption (APAR) by individual trees, and to analyze the effects of stand density and foliage clustering on transmission. Simulated transmission patterns appeared largely comparable with field measurements. Discrepancies between field measurements and model estimates could be attributed to interception by stems and (dead) branches, which were not part of the model. Analysis of absorption by trees showed that APAR was closely related to tree leaf area in closed canopy stands, but that in the open stand the relationship was less clear. This indicates that in open stands spatial information on tree position and crown dimensions will strongly affect absorption by individual trees. Uncertainties in leaf area estimates, due to variability in allometric relationships, had only small effects on APAR of the trees. Analysis of the role of stand density on the extinction-coefficient (K) showed that model estimates of K were clearly lower than values calculated with the Lambert–Beer theory: simulated K increased with increasing LAI, varying between 0.17 and 0.33 in case of Douglas fir and 0.30–0.63 for Beech. The degree of clustering decreased with LAI, amounting to a multiplier of 2.2–4.2 and 1.1–2.4 for Douglas fir and Beech, respectively. The model was considered a suitable tool in simulating growth and development of heterogeneous forests like mixed-species stands, because it enables to account for the growing conditions of individual trees in the forest canopy.