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The ratio of intercellular to ambient CO concentrations ( c / c ) plays a key role in ecophysiology, micrometeorology, and global climatic change. However, systematic investigation on c / c variation and its determinants are rare. Here, the c / c was derived from measuring ecosystem fluxes in an even-aged monoculture of rubber trees ( Hevea brasiliensis). We tested whether c / c is constant across environmental gradients and if not, which dominant factors control c / c variations. Evidence indicates that c / c is not a constant. The c / c exhibits a clear ' V'-shaped diurnal pattern and varies across the environmental gradient. Water vapor pressure deficit ( D) is the dominant factor controls over the c / c variations. c / c consistently decreases with increasing D. c / c decreases with square root of D as predicted by the optimal stomatal model. The D-driving single-variable model could simulate c / c as well as that of sophisticated model. Many variables function on longer timescales than a daily cycle, such as soil water content, could improve c / c model prediction ability. Ecosystem flux can be effectively used to calculate c / c and use it to better understand various natural cycles. [ABSTRACT FROM AUTHOR]

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 (Peco,opt), respiration at daytime (Reco,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 (Ta) 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 Ta and VPD increased, inducing a decrease of ecosystem α and Peco,opt. The net CO2 exchange of the ecosystem strongly depended on the Ta above 20 °C and the VPD above 1 kPa. [ABSTRACT FROM AUTHOR]

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 (alpha) averaged by 0.813 mg x m(-2) x s(-1), 0.238 mg x m(-2) x s(-1), and 0.0023 mg x micromol(-1), respectively. As affected by the interaction of air temperature (Ta) 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 Ta and VPD increased, inducing a decrease of ecosystem alpha and P(eco,opt). The net CO2 exchange of the ecosystem strongly depended on the Ta above 20 degrees C and the VPD above 1 kPa.