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Hydrogen and Oxygen Isotope Ratios of Tree Ring Cellulose for Field-Grown Riparian Trees

John S. Roden and James R. Ehleringer
Oecologia
Vol. 123, No. 4 (2000), pp. 481-489
Published by: Springer in cooperation with International Association for Ecology
Stable URL: http://www.jstor.org/stable/4222646
Page Count: 9
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Hydrogen and Oxygen Isotope Ratios of Tree Ring Cellulose for Field-Grown Riparian Trees
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Abstract

The isotopic composition of tree ring cellulose was obtained over a 2-year period from small-diameter riparian-zone trees at field sites that differed in source water isotopic composition and humidity. The sites were located in Utah (cool and low humidity), Oregon (cool and high humidity), and Arizona (warm and low humidity) with source water isotope ratio values of -125/-15‰ ($\delta \text{D}/\delta {}^{18}\text{O}$), -48/-6‰, and -67/-7‰, respectively. Monthly environmental measurements included temperature and humidity along with measurements of the isotope ratios in atmospheric water vapor, stream, stem, and leaf water. Small riparian trees used only stream water (both δD and δ 18O of stem and stream water did not differ), but δ values of both atmospheric water vapor and leaf water varied substantially between months. Differences in ambient temperature and humidity conditions between sites contributed to substantial differences in leaf water evaporative enrichment. These leaf water differences resulted in differences in the δD and δ 18O values of tree ring cellulose, indicating that humidity information was recorded in the annual rings of trees. These environmental and isotopic measurements were used to test a mechanistic model of the factors contributing to δD and δ 18O values in tree ring cellulose. The model was tested in two parts: (a) a leaf water model using environmental information to predict leaf water evaporative enrichment and (b) a model describing biochemical fractionation events and isotopic exchange with medium water. The models adequately accounted for field observations of both leaf water and tree ring cellulose, indicating that the model parametrization from controlled experiments was robust even under uncontrolled and variable field conditions.

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