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

The ratio of intercellular to ambient CO 2 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.