Isotopic studies of leaf water. Part 2: Between-age isotopic variations in pine needles

We studied between-age oxygen and hydrogen isotopic variations of two pine species, red pine (Pinus resinosa Ait) and white pine (Pinus strobus L.), with particular attention paid to variations associated with needle maturity. The 2D model by Shu et al. (this volume) was applied to simulate oxygen isotopic ratios of the pine needles; calculated results were compared with data collected from trees in the Hanover, NH area. The calculations indicate that the mechanism most responsible for age-dependent isotopic variations is the variation in transpiration rate along the needle. Because stomata mature progressively from needle tip to needle base, young needles transpire most of their water near the tip. Mature needles, on the other hand, transpire water more evenly along their lengths. We cannot successfully simulate hydrogen isotopic variations, and it remains to be investigated what mechanism or physical process is not included in the model that affects hydrogen isotopic compositions in leaf water.