Changing scale in ecological modelling: A bottom up approach with an individual based vegetation model

Net primary production (NPP) depends crucially on the sun radiation absorption, and hence its prediction on the modelling of the radiation. In heterogeneous systems such as savannas, the photosynthetically active radiation (PAR) absorption is more difficult to predict than in homogeneous systems, due to the presence of tree clumps and open grass areas. Using a detailed 3D model, we propose new formulations of PAR absorption derived from the Beer–Lambert law for heterogeneous vegetation. To account for intra-plot vegetation heterogeneity we assume an intra-plot partition of space into two or three zones, based on tree location. We develop two models derived model from the Beer–Lambert law: (1) a simple partition of space below and outside trees, (2) a partition in three zones, including the tree shading area. The only model to predict PAR interception for all types of spatial patterns was the most complex one (including tree+tree shading zone+open grass areas). In this model, extinction coefficients can be derived from vegetation structural parameters like tree cover, and tree shading area. Tree shading area could be computed from geometric site parameters (latitude, longitude, tree height). With increasing satellite resolution it can be possible to obtain these structural parameters of vegetation (cover, overlap, tree height) needed by this Beer–Lambert derived model. Such an absorption model could then be used as a basis for a Monteith-style model of vegetation functioning for heterogeneous vegetation.