Carbon isotopic composition and oxygen isotopic enrichment in phloem and total leaf organic matter of European beech (Fagus sylvatica L.) along a climate gradient

This study investigated the influence of climate on the carbon isotopic composition (delta C-13) and oxygen isotopic enrichment (Delta O-18) above the source water of different organic matter pools in European beech. In July and September 2002, delta C-13 and Delta O-18 were determined in phloem carbohydrates and in bulk foliage of adult beech trees along a transect from central Germany to southern France, where beech reaches its southernmost distributional limit. The data were related to meteorological and physiological parameters. The climate along the transect stretches from temperate [subcontinental (SC)] to submediterranean (SM). Both delta C-13(leaf) and Delta O-18(leaf) were representative of site-specific long-term environmental conditions. delta C-13 of leaves collected in September was indicative of stomatal conductance, vapour pressure deficit (VPD) and radiation availability of the current growing season. Delta O-18 was mainly correlated to mean growing season relative humidity (RH) and VPD. In contrast to the leaves, delta C-13(phloem) varied considerably between July and September and was well correlated with canopy stomatal conductance (G(s)) in a 2 d integral prior to phloem sampling. The relationship between delta C-13 and Delta O-18 in both leaves and phloem sap points, however, to a combined influence of stomatal conductance and photosynthetic capacity on the variation of delta C-13 along the transect. Delta O-18(phloem) could be described by applying a model that included O-18 fractionation associated with water exchange between the leaf and the atmosphere and with the production of organic matter. Hence, isotope signatures can be used as effective tools to assess the water balance of beech, and thus, help predict the effects of climatic change on one of the ecologically and economically most important tree species in Central Europe.