Abstract:Photosynthetic characteristics in leaves of Castanopsis hystrix saplings in Longyandong Forestry, Guangzhou, were studied during early summer and autumn under natural conditions by leaf gas exchange and chlorophyll fluorescence techniques. The light saturation point （LSP） of sun leaves in canopy was 400 μ mol photons m-2s-1, and the maximum net photosynthetic rate was 4-6 μ mol m-2s-1. These data showed that C. hystrix saplings are sun plants. The values of chlorophyll a/b in sun leaves and shade leaves were both upto 2.4, and the content of chlorophyll b in shade leaves was higher than that in sun leaves, indicating that shade leaves in canopy had the potential ability to utilize low light intensity. The daily changes in chlorophyll fluorescence parameters recorded that maximum photochemical efficiency of PSⅡ(Fv/Fm) and photochemical quantum yield of PS Ⅱ (Yield) in sun leaves of C. hystrix canopy declined slightly at the midday, suggesting the occurrence of reversible photoinhibition. Owing to the lower maximum net photosynthetic rate in sun leaves and the ability to effectively and consistently utilize the scattered light and/or transmitted light, C. hystrix saplings could survive low light intensity, grow well at the edge of the forest and in the understorey, and eventually become one of the dominant species in upper layer of the community.