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.
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