Abstract: Photosynthetically active radiation (PAR) is a main factor controlling primary productivity, which influences the structure, function and dynamics of forest ecosystems. Here, we analyzed the variations of PAR in different community layers (77.7 m of above canopy, 61.3 m of upper canopy, 24.3 m of middle canopy, 16 m of lower canopy, and 2 m of shrub layer) and different seasons (rainy season and dry season) in a tropical seasonal rain forest of Xishuangbanna. The key factors affecting the distribution of PAR were determined using linear mixed model. Results showed that: (1) The diurnal dynamic trends of PAR in various vertical layers were generally similar in both seasons, with the maximum value appearing around noon, and PAR in the dry season was slightly higher than that in the rainy season. (2) The maximum monthly PAR in various vertical layers usually appeared in the dry-hot period (March to April) of the dry season, while the minimum mostly appeared in the fog-cool period of the dry season (November to December). (3) The annual total amount of PAR in the above canopy, upper canopy, middle canopy, lower canopy, and shrub layer were 7533.63, 5327.93, 1346.69, 1036.87 and 58.63 mol·m^-2, respectively, which was roughly equally distributed between dry season and rainy season. Due to different degrees of shading, the annual total amount of PAR in upper canopy, middle canopy, lower canopy and shrub layer accounted for 70.72%, 17.88%, 13.76% and 0.78% of the total, respectively. (4) The results of mixed linear model showed that both temporal factor (hour, season) and spatial factor (PAR of the upper canopy) affected the PAR of each subsequent layer in the community. PAR gradually decreased toward the forest floor, which subsequently created a complex light environment and sustained diverse groups of organisms in the tropical seasonal rain forest in Xishuangbanna.

Key words: tropical rainforest, light, canopy, vertical gradient, habitat heterogeneity.