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

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.