D/H ratios of cellulose nitrate in white pine (Pinus strobus L.) from a site in southeastern New York have been used to test a model for isotopic fractionation in tree rings. Both intra-ring (six samples per year) and inter-ring samples were collected. All of the parameters currently thought to influence D/H ratios in tree rings were also measured: D/H ratios in source water and air vapor, temperature, and humidity. The model incorporates leaf water fractionation and fractionation during photosynthesis. In the context of the model, D/H ratios in tree rings respond most strongly to changes in D/H ratios of source water. This is due to negative feedback pairs which cancel the impacts of other variables. Potential effects of changes in humidity are canceled by accompanying changes in the D/H ratio of air vapor. The net biological fractionation factor is apparently a function of temperature. Its change with temperature cancels the change with temperature of the phase change fractionation. The model predicts a relationship between cellulose nitrate and source water D/H ratios which fits very well with the data. This predicted relationship is not applicable to all areas, however, as it depends partly on the relationship between D/H ratios in air vapor and relative humidity, which can vary with location. Despite the success of the model, a less complicated model in which re-equilibration of starches with source water negates the effects of leaf water and biochemical fractionations is also compatible with the data.
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