Carbon and oxygen isotope ratios of tree ring cellulose along a precipitation transect in Oregon, United States

[1] The carbon and oxygen isotopic compositions of tree ring cellulose were examined for trees along a precipitation gradient in western Oregon, United States. Two years of cellulose from four sites dominated by coniferous forests ranging in precipitation from 227 to 2129 mm were sampled in conjunction with studies that measured the dO and dC of ecosystem respiration. The mean tree ring cellulose dC varied from 22.1 to 26.3% among sites and showed enrichment with decreasing water availability across the transect. The dC in cellulose varied across the precipitation transect in a similar pattern to the dC of leaf and root tissues as well as ecosystem respiration, although tree ring cellulose was enriched in C by over 3% compared to other organic matter components. The mean tree ring cellulose dO varied from 28.1 to 30.3%. However, trends of cellulose dO change with water availability were obscured by differences in stem water dO. When calculated as deviation from stem water (dOcellulose dOstem water) the differences in evaporative enrichment between sites was more pronounced (range of 9.6%). The limited observed variation in tree ring cellulose dO of field grown trees despite large site difference in stem and leaf water dO across the transect agreed with predictions from a mechanistic model. Tree ring records of cellulose dO may provide useful proxy information regarding humidity and site water balance especially if combined with dC records that also vary with plant water status.