On the ratio of intercellular to ambient CO₂ (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.