A light model for spatially explicit forest stand models

There is an increasing demand for detailed simulation of light in forest stand models. This is particularly true for spatially explicit growth models that represent individual trees. A large number of light models has been developed for forest stands, but only a few are spatially explicit and none has become part of a forest growth model for management applications. This paper describes a light model called tRAYci that can be used with models including a three-dimensional canopy representation calculated from crown shapes. The unobstructed irradiation of PAR above the canopy is simulated by combining submodels for the spatial distribution of direct and diffuse light. For a given paint within a forest stand, up to 32,400 sample rays are traced in the upper hemisphere, and for each sample ray, its path length in the simulated canopy space is calculated. PAR attenuation in the modelled crown space is simulated according to Bouguer's law using leaf area density for each species as the only variable. The results are given as percentage of the above canopy Light (PACL) for a defined growth period no shorter than a few weeks. PACL can be calculated for every point in a simulated stand. Light maps, profiles, and average PACL over the crown surface for individual tree crowns can be generated. The model was validated using light measurements and estimates from hemispherical photographs from a Douglas-fir stand in British Columbia. Sensitivity analyses for all model variables are presented and demonstrate the importance of crown representations for light models. (C) 1998 Elsevier Science B.V.