Linking stable oxygen and carbon isotopes with stomatal conductance and photosynthetic capacity: a conceptual model

Based on measurements of δ¹⁸O and δ¹³C in organic matter of C₃-plants, we have developed a conceptual model that gives insight into the relationship between stomatal conductance (g _l) and photosynthetic capacity (A _max) 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 CO₂ and H₂O gas exchange, whereby we estimate the intercellular CO₂ concentration (c _i) from δ¹³C and the air humidity from δ¹⁸O. Assuming that air humidity is an important factor influencing g _l, the model allows us to distinguish whether differences in c _i are caused by a response of g _l or of A _max. 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 δ¹⁸O-δ¹³C 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 δ¹⁸O and δ¹³C isotope data.