Abstract: By using eddy covariance technique, this paper quantitatively analyzed the photosynthetic characteristics of tropical seasonal rainforest ecosystem and related environmental controlling factors in Xishuangbanna in 2003-2006. In the study period, less interannual difference was observed in the net photosynthesis of the ecosystem, with the maximum photosynthesis rate (P_eco,opt), respiration at daytime (R_eco,d), and apparent quantum yield (α) averaged by 0.813 mg·m^-2·s^-1, 0.238 mg·m^-2·s^-1, and 0.0023 mg·μmol^-1, respectively. As affected by the interaction of air temperature (T_a) and vapor pressure deficit (VPD）, the photosynthetic characteristics had some seasonal differences. In rainy season, the ecosystem had the strongest photosynthetic capacity because of the higher precipitation and warmer air temperature; in foggy and cool season, fog drip played an important role in the water relations of plants, and thereby, the ecosystem photosynthetic capacity was still higher; in dry and hot season, due to the limited precipitation and high temperature, the T_a and VPDincreased, inducing a decrease of ecosystem α and P_eco,opt. The net CO₂ exchange of the ecosystem strongly depended on the T_a above 20 ℃ and the VPD above 1 kPa.

Key words: photosynthesis, ecosystem, tropical seasonal rainforest, Xishuangbanna, Tianmu Mountain, old-growth subtropical forest, eddy covariance, CO₂ flux, carbon sink.