Temporal Isotopic Variations of Leaf Water in Pine Needles

Understanding the isotopic variations in a plant's leaf water is important for a number of climatological and biogeochemical studies. Leaf water isotopic composition is affected by the isotopic composition of the source water and the relative humidity of the air, both of which are related to climate. This dependency is the basis for climate reconstruction using isotopic compositions of tree-ring cellulose. The isotopic composition of leaf water is also important for the assessment of terrestrial biological productivity and the quantification of the Dole effect. We have studied the oxygen and hydrogen isotopic variations in leaf water of biennial needles from red pine (Pinus resinosa) and white pine (Pinus strobes) in Hanover, New Hampshire, USA. We have examined the leaf water δD and δ18O values along pine needles from base to tip, and the isotopic differences between young and old leaves. Within a needle, progressive enrichments of both oxygen-18 and deuterium were observed toward the tip, ranging for δD from -60.1 to 9.4 permil in white pine and -67.1 to -34.9 permil in red pine, and for δ18O from -3.1 to 19.1 for white pine and -7.3 to 5.5 permil in red pine. For both species, δD and δ18O were higher in old leaves than in young leaves. The isotopic difference between old and young leaves was most pronounced earlier in the growing season; the gap narrowed with time and finally disappeared in early fall. Early in the growing season, the δD values of young needles were -21 and -30 permil in white and red pine, respectively, and that of old needles were -3.0 and -8.0 permil, respectively. The δ18O values showed similar trends, and the δD vs. δ18O slope for the young leaves decreased from 3.6 in spring to ~1 in early autumn. Our observations can be simulated using the progressive isotopic enrichment model proposed by Barnes and Farquhar for monocotyledoneous leaves. Two variables, the transpiration rate and length of the needle, can explain the observed isotopic variations. These two variables can be combined into one parameter in the model as the longitudinal Peclet number of the leaf. In addition, the model can also explain the change in the slope of the δD vs. δ18O relationship in leaf water.