Combining d13C and d18O analyses to unravel competition, CO2 and O3 effects on the physiological performance of different-aged trees.

Combined d13C and d18O analyses of leaf material were used to infer changes in photosynthetic capacity (Amax) and stomatal conductance (gl) in Fagus sylvatica and Picea abies trees growing under natural and controlled conditions. Correlation between gl and d18O in leaf cellulose (d18Ocel) allowed us to apply a semi-quantitative model to infer gl from d18Ocel and also interpret variation in d13C as reflecting variation in Amax. Extraction of leaf cellulose was necessary, because d18O from leaf organic matter (d18OLOM) and d18Ocel was not reliably correlated. In juvenile trees, the model predicted elevated carbon dioxide (CO2) to reduce Amax in both species, whereas ozone (O3) only affected beech by reducing CO2 uptake via lowered gl. In adult trees, Amax declined with decreasing light level as gl was unchanged. O3 did not significantly affect isotopic signatures in leaves of adult trees, reflecting the higher O3 susceptibility of juvenile trees under controlled conditions. The isotopic analysis compared favourably to the performance of leaf gas exchange, underlining that the semi-quantitative model approach provides a robust way to gather time-integrated information on photosynthetic performance of trees under multi-faced ecological scenarios, in particular when information needed for quantitative modelling is only scarcely available