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]