Abstract: Photosynthetically active radiation (PAR) is an important ecological factor affecting the energy transformation and matter production of plant ecosystem. Studies on the spatiotemporal distribution patterns of PAR in a plant ecosystem are helpful to understand the physiological and ecological characteristics and photosynthesis of the ecosystem. In this paper, the temporal and vertical distributions of PAR in a coniferous and subtropical evergreen forest in Ailaoshan Mountains of Southwest China were analyzed. At daily scale, the intensity of PAR during  passing through the forest canopy decreased gradually. At monthly scale, the PAR flux varied in each forest layer, with the maximum and minimum values above the canopy in February and January, respectively. The diurnal change of the PAR above the canopy (PAR₁) presented a single-peak curve, while that of the PAR at the bottom of the canopy (PAR₂) and in the arbor sub-layers (PAR₃ and PAR₄) and shrub layer (PAR₅) presented an irregular single-peak curve. The PAR₁, PAR₂, PAR₃, PAR₄ and PAR₅ fluxes at yearly scale were 8824.98, 682.20, 403.09, 216.62 and 208.52 mol·m^-2, and the ratios of yearly PAR₂, PAR₃, PAR₄ and PAR₅ to PAR₁ were 7.73%, 4.57%, 2.45% and 0.92%, respectively. The average PAR₁, PAR₂, PAR₃, PAR₄ and PAR₅ in rain season were 89.77%, 89.34%, 97.32%%, 116.36% and 115.09% of those in dry season, respectively.

Key words: Growth dynamics, Schumacher equation, Genetic algorithms, Modified model