Hydrogen and oxygen isotope ratios of tree ring cellulose for field-grown riparian trees

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 Arizo na (warm and low humidity) with source water isotope ratio values of -125/- 15 parts per thousand (delta D/delta(18)O), -48/-6 parts per thousand, and -67/-7 parts per thousand, respectively. Monthly environmental measurements included temperature and humidity along with measurements of the isotope r atios in atmospheric water vapor, stream, stem, and leaf water. Small ripar ian trees used only stream water (both delta D and delta(18)O of stem and s tream water did not differ), but delta values of both atmospheric water vap or and leaf water varied substantially between months. Differences in ambie nt temperature and humidity conditions between sites contributed to substan tial differences in leaf water evaporative enrichment. These leaf water dif ferences resulted in differences in the delta D and delta(18)O values of tr ee 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 delta D and delta(18)O values in tree ring cellulose. The model was tested in two part s: (a) a leaf water model using environmental information to predict leaf w ater evaporative enrichment and (b) a model describing biochemical fraction ation events and isotopic exchange with medium water. The models adequately accounted for field observations of both leaf water and tree ring cellulos e, indicating that the model parameterization from controlled experiments w as robust even under uncontrolled and variable field conditions.