Evidence for ¹³C/¹²C fractionation between tree leaves and wood

An isotopic fractionation step between carbon fixed in the leaves and that assimilated into wood cellulose has not previously been included in atmosphere-plant fractionation models. Data presented here, however, show that the magnitude of such a fractionation step may be significant. In an experiment which involved 10 juniper trees from around Arizona, cellulose of leaves was found to be isotopically lighter than that of the corresponding tree rings in all trees by an average of 2‰. In a second experiment, the δ¹³C of leaf material from a juniper tree sampled at monthly intervals was compared with the δ¹³C of the corresponding ring. For cellulose, δ¹³C of the wood varied with changes in δ¹³C of the leaves and was persistently isotopically heavier by 3-4‰. Furthermore, the observed change in δ¹³C through the growing season suggests a temperature coefficient of about -0.27‰ °C^-1. The direction of this leaf-wood fractionation precludes diffusion processes as the primary cause.