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

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 delta O-18 and delta C-13 of ecosystem respiration. The mean tree ring cellulose delta C-13 varied from -22.1 to -26.3% among sites and showed enrichment with decreasing water availability across the transect. The delta C-13 in cellulose varied across the precipitation transect in a similar pattern to the delta C-13 of leaf and root tissues as well as ecosystem respiration, although tree ring cellulose was enriched in C-13 by over 3% compared to other organic matter components. The mean tree ring cellulose delta O-18 varied from 28.1 to 30.3%. However, trends of cellulose delta O-18 change with water availability were obscured by differences in stem water delta O-18. When calculated as deviation from stem water (delta O-18(cellulose) - delta O-18(stem water)) the differences in evaporative enrichment between sites was more pronounced (range of 9.6%). The limited observed variation in tree ring cellulose delta O-18 of field grown trees despite large site difference in stem and leaf water delta O-18 across the transect agreed with predictions from a mechanistic model. Tree ring records of cellulose delta O-18 may provide useful proxy information regarding humidity and site water balance especially if combined with delta C-13 records that also vary with plant water status.