Combining δ¹³C and δ¹⁸O analyses to unravel competition, CO₂ and O₃ effects on the physiological performance of different-aged trees

Combined δ¹³C and δ¹⁸O analyses of leaf material were used to infer changes in photosynthetic capacity (A_max) and stomatal conductance (g_l) in Fagus sylvatica and Picea abies trees growing under natural and controlled conditions. Correlation between g_l and δ¹⁸O in leaf cellulose (δ ¹⁸O_cel) allowed us to apply a semi-quantitative model to infer g_l from δ¹⁸O_cel and also interpret variation in δ¹³C as reflecting variation in A_max. Extraction of leaf cellulose was necessary, because δ¹⁸O from leaf organic matter (δ¹⁸O_LOM) and δ¹⁸O_cel was not reliably correlated. In juvenile trees, the model predicted elevated carbon dioxide (CO₂) to reduce A_max in both species, whereas ozone (O₃) only affected beech by reducing CO₂ uptake via lowered g_l. In adult trees, A_max declined with decreasing light level as g_l 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.