HR: 1340h
AN: B33D-1580    [Abstracts]
TI: Revisiting the Boundary Layer Leaf Water Isotopic Model
AU: * Feng, X
EM: xiahong.feng@dartmouth.edu
AF: Dartmouth College, Department of Earth Sciences 6015 Fairchild, Hanover, NH 03755, United States
AU: Shu, Y
EM: Yong.Shu.Adv07@Alum.Dartmouth.ORG
AF: Dartmouth College, Department of Earth Sciences 6015 Fairchild, Hanover, NH 03755, United States
AU: Posmentier, E S
EM: Posmentier@dartmouth.edu
AF: Dartmouth College, Department of Earth Sciences 6015 Fairchild, Hanover, NH 03755, United States
AU: Sonder, L J
EM: leslie.j.sonder@dartmouth.edu
AF: Dartmouth College, Department of Earth Sciences 6015 Fairchild, Hanover, NH 03755, United States
AU: Yakir, D
EM: dan.yakir@weizmann.ac.il
AF: Weizmann Institute of Science, Department of Environmental Sciences & Energy Research, Rehovot, 76100, Israel
AB: The boundary layer (BL) model for oxygen or hydrogen isotopic composition of leaf water has been widely used in the past four decades, and has been incorporated into models that require information about leaf water isotopic variations. However, since its introduction, model predictions of the bulk leaf water have often exceeded observed isotopic enrichments. There are also cases in which the model yielded lower than observed isotopic enrichments of bulk leaf water. In general, underpredictions occur under relatively high humidity. In order to explain why the BL model overpredicts the isotopic composition, several modifications of the model have been proposed. However, no explanation exists for why the BL model underestimates observed isotopic enrichments. We recently developed a 2D model that successfully simulates the observed along-leaf ¹⁸O enrichment of pine needles, and can explain why the BL model could have over- or under-predicted the bulk leaf water δ¹⁸O values. In the BL model, bulk leaf water is isotopically equivalent to water at the evaporation site, fed directly by stem water. In a real leaf, however, stem water enters the base of the leaf and becomes progressively enriched in ¹⁸O towards the tip due to fractionation by transpiration, consistent with both our observations and behavior of the 2D model. Therefore, at least part of the leaf water, that near the base, would have isotopic values lower than water at transpiration sites predicted by the BL model, which might thus overestimate bulk isotope values. On the other hand, as water moves through a leaf, it becomes increasingly enriched in ¹⁸O, and the leaf water near the tip may have δ¹⁸O values well above the BL model prediction. Therefore, it is also possible for the BL model to underestimate the bulk leaf water δ¹⁸O. The actual isotopic composition of the bulk leaf water is a combination of these two effects. It is clear then that the BL model may not accurately predict the δ¹⁸O value of the bulk leaf water, because to do so would require the volumetric average of the δ¹⁸O in the depleted and enriched parts of the leaf to exactly equal the BL model prediction. Furthermore, if leaves are assumed to transpire fast under low humidity, our 2D model can also reproduce the humidity dependence of the discrepancy between observation and the BL model prediction. This suggests an interaction between environmental conditions and the physiological behavior of plants. If the simplicity of the BL model justifies its continued use, then it is important to investigate its accuracy further under different environmental conditions and for leaves with different morphologies and water transport pathways.
DE: 0414 Biogeochemical cycles, processes, and modeling (0412, 0793, 1615, 4805, 4912)
DE: 0426 Biosphere/atmosphere interactions (0315)
DE: 0476 Plant ecology (1851)
DE: 1813 Eco-hydrology
DE: 1843 Land/atmosphere interactions (1218, 1631, 3322)
SC: Biogeosciences [B]
MN: 2007 Fall Meeting