Leaf and twig (delta)13C during growth in relation to biochemical composition and respired CO2

In deciduous trees, the (delta)13C values of leaves are known to diverge during growth from those of woody organs. The main purpose of this study is to determine whether the divergence in (delta)13C between leaves and current-year twigs of Fagus sylvatica (L.) is influenced by changes (i) in the relative contents of organic matter fractions and (ii) in the (delta)13C of respired CO2. The (delta)13C values of bulk matter, extractive-free matter, lignin, holocellulose, starch, soluble sugars, water-soluble fraction and respired CO2, as well as their relative contents in bulk matter were determined. The (delta)13C values of biochemical fractions and respired CO2 showed very similar temporal variations for both leaves and twigs. Variations in bulk matter (delta)13C during growth were, therefore, poorly explained by changes in biochemical composition or in respiratory fractionation and were attributed to the transition from (delta)13C-enriched reserves (mainly starch) to (delta)13C-depleted new photoassimilates. The divergence between leaves and twigs was related to higher values of soluble sugar (delta)13C in twigs. However, the difference between lignin and holocellulose (delta)13C varied during growth. This phenomenon was attributed to the delay between holocellulose and lignin deposition. These results may have implications for analysis of organic matter (delta)13C in trees and forest ecosystems.