On the ratio of intercellular to ambient CO 2 (c i /c a ) derived from ecosystem flux.

The ratio of intercellular to ambient CO2 concentrations (c i /c a ) plays a key role in ecophysiology, micrometeorology, and global climatic change. However, systematic investigation on c i /c a variation and its determinants are rare. Here, the c i /c a was derived from measuring ecosystem fluxes in an even-aged monoculture of rubber trees (Hevea brasiliensis). We tested whether c i /c a is constant across environmental gradients and if not, which dominant factors control c i /c a variations. Evidence indicates that c i /c a is not a constant. The c i /c a 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 i /c a variations. c i /c a consistently decreases with increasing D. c i /c a decreases with square root of D as predicted by the optimal stomatal model. The D-driving single-variable model could simulate c i /c a 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 i /c a model prediction ability. Ecosystem flux can be effectively used to calculate c i /c a and use it to better understand various natural cycles.