Combining sap flow measurement-based canopy stomatal conductance and 13 C discrimination to estimate forest carbon assimilation

The available methods for studying C uptake of forest and their problems in practices are reviewed, and a new approach to combining sap flow and 13 C techniques is proposed in this paper. This approach, obtained through strict mathematic derivation, combines sap flow measurement-based canopy stomatal conductance and 13 C discrimination to estimate instantaneous carbon assimilation rate of a forest. Namely the mean canopy stomatal conductance ( g c ) acquired from accurate measurement of sap flux density is integrated with the relationship between 13 C discrimination (Δ) and C i /C a (intercellular/ambient CO 2 concentrations) and with that between A net (net photosynthetic rate) and g co2 (stomatal conductance for CO 2 ) so that a new relation between forest C uptake and Δ as well as g c is established. It is a new method of such kind for studying the C exchange between forest and atmosphere based on experimental ecology.