Simulating radiation distribution in a heterogeneous Norway spruce forest on a slope

Simulating the dynamics of heterogeneous forests calls for spatially explicit radiation transmission models at the scale of individual trees and requiring only a short computing time. Such a model was developed for spatially heterogeneous coniferous forest canopies. Based on the interception of light rays by parabolic crowns, it simultaneously calculates the radiation intercepted by each tree and the distribution of irradiance on the ground. For every sky direction, parallel rays aim at ground cell centres and are intercepted by tree crowns on their way. Slope and exposure are taken into account eliminating rays coming from above the slope. Optimisation in computing time is obtained by pre-selecting for each ray direction a rectangular cell neighbourhood around the target cell in such a way that potentially intercepting trees can only be found in this neighbourhood. Model evaluation was done by comparing hemispherical photographs with model predictions in a spatially heterogeneous Norway spruce (Picea abies, L. Karst) stand at the upper montane level in the Alps. Thanks to a satisfactory fit between model and data and a reasonably short computing time (about 2 min for 0.25 ha), the model can be considered for integration into a forest dynamics simulator. In the reference stand, 42% of incident radiation was intercepted by the highest trees (132 ha−1). Due to the clumped structure, 23% of incident radiation reached the soil. Interception by individual trees varied considerably as a function of tree size and location (from 0 to 60% of the potential interception of a tree growing in isolation). Irradiance at ground level was also extremely variable (between 0 and 50% of irradiance above the canopy), illustrating the need for spatial models in heterogeneous stands. The interception of radiation by trees was not affected by slope and exposure, unlike irradiance at ground level, which increased for southern exposures. Therefore, in terms of dynamics, regeneration should be the most sensitive to changes in these factors. Because the model produces precise radiation values at the ecosystem level, it allows analysis of the links between the structure of a forest stand and its energetic functioning synthetically.