Based on measurements of δO and δC in organic matter of C-plants, we have developed a conceptual model that gives insight into the relationship between stomatal conductance (g ) and photosynthetic capacity (A) resulting from differing environmental constraints and plant-internal factors. This is a semi-quantitative approach to describing the long-term effects of environmental factors on CO and HO gas exchange, whereby we estimate the intercellular CO concentration (c ) from δC and the air humidity from δO. Assuming that air humidity is an important factor influencing g , the model allows us to distinguish whether differences in c are caused by a response of g or of A. As an application of the model we evaluated the isotope data from three species in plots differing in intensity of land use (hay meadows and abandoned areas) at three sites along a south north transect in the Eastern Alps. We found three different δO-δC response patterns in native and planted grassland species (cultivated in the greenhouse). After preliminary confirmation by gas-exchange measurements we conclude that the proposed model is a promising tool for deriving carbon water relations in different functional groups from δO and δC isotope data.

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