Simulating radiation distribution in a heterogeneous Norway spruce forest on a slope
Résumé
Simulating the dynamics of heterogeneous forests calls for spatially explicit radiation transmission models at the scaleof individual trees and requiring only a short computing time. Such a model was developed for spatially heterogeneousconiferous forest canopies. Based on the interception of light rays by parabolic crowns, it simultaneously calculates theradiation intercepted by each tree and the distribution of irradiance on the ground. For every sky direction, parallel rays aimat ground cell centres and are intercepted by tree crowns on their way. Slope and exposure are taken into account eliminatingrays coming from above the slope. Optimisation in computing time is obtained by pre-selecting for each ray direction arectangular cell neighbourhood around the target cell in such a way that potentially intercepting trees can only be found in thisneighbourhood. Model evaluation was done by comparing hemispherical photographs with model predictions in a spatiallyheterogeneous Norway spruce (Picea abies, L. Karst) stand at the upper montane level in the Alps. Thanks to a satisfactoryfit between model and data and a reasonably short computing time (about 2min for 0.25 ha), the model can be considered forintegration into a forest dynamics simulator. In the reference stand, 42% of incident radiation was intercepted by the highesttrees (132 ha.1). Due to the clumped structure, 23% of incident radiation reached the soil. Interception by individual treesvaried considerably as a function of tree size and location (from 0 to 60% of the potential interception of a tree growingin 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 treeswas not affected by slopeand 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 valuesat the ecosystem level, it allows analysis of the links between the structure of a forest stand and its energetic functioningsynthetically.