Hydrogen isotope fractionation in wood-producing avocado seedlings: Biological constraints to paleoclimatic interpretations of delta D values in tree ring cellulose

Climatic reconstructions from the delta D values of wood cellulose nitrate have been compromised because it is unclear whether the isotopic ratios are affected only by temperature or by temperature and humidity. To quantify the effect of humidity on the delta D values of leaf and wood cellulose nitrate, we grew avocados (Persea americana Mill. cv. Mexican) from seed at high and low humidities until they set wood. The source water for seed production was isotopically the same as the source water for seedling propagation. The delta D values of leaf cellulose nitrate were related to those of leaf water, which were, in turn, influenced by humidity (P < 0.01). The delta D values of wood cellulose nitrate were unrelated to those of leaf water or any other indicator of humidity, but were related to the delta D values of water in wood (P less than or equal to 0.05). The delta D values of wood cellulose nitrate were identical in three out of five pairs of low and high humidity treatments. These results suggest humidity cannot be reliably inferred from delta D values in wood cellulose nitrate. The delta D values of cellulose nitrate in both leaves and wood appear to have been influenced by the incorporation of stored metabolites into cellulose. Trees, like avocado seedlings, have considerable post-photosynthetic organic reserves that can be tapped for growth. Conditions that stimulate use of post-photosynthetic carbon reserves are varied for trees. Significant contributions from these reserves could lead to erroneous temperature inferences from delta D values of wood cellulose nitrate.