Comparison of leaf gas exchange and stable isotope signature of water-soluble compounds along canopy gradients of co-occurring Douglas-fir and European beech

Abstract

Combined δ 13C and δ 18O analyses of water-soluble leaf and twig phloem material were used to determine intrinsic water-use efficiency (iWUE) and variability of stomatal conductance at different crown positions in adult European beech (Fagus sylvatica) and Douglas-fir (Pseudotsuga menziesii) trees. Simultaneous gas exchange measurements allowed evaluation of the differences in calculating iWUE from leaf or phloem water-soluble compounds, and comparison with a semi-quantitative dual isotope model to infer variability of net photosynthesis (A n) between the investigated crown positions. Estimates of iWUE from δ 13C of leaf water-soluble organic matter (WSOM) outperformed the estimates from phloem compounds. In the beech crown, δ 13C of leaf WSOM coincided clearly with gas exchange measurements. The relationship was not as reliable in the Douglas-fir. The differences in δ 18O between leaf and phloem material were found to correlate with stomatal conductance. The semi-quantitative model approach was applicable for comparisons of daily average A n between different crown positions and trees. Intracanopy gradients were more pronounced in the beech than in the Douglas-fir, which reached higher values of iWUE at the respective positions, particularly under dry air conditions. © 2012 Blackwell Publishing Ltd.