How does drought affect the δ18O cellulose record? A Biosphere 2 experiment.

Wood cellulose records environmental conditions via its isotopic composition, which can be used to reconstruct different environmental events or patterns. However, it has been suggested that there can be a decoupling of the δ¹⁸O of cellulose and environmental conditions due to a lag from post-carboxylation processes. Thus, studying the dynamics of intra-seasonal tree growth provides a unique way to examine how the δ¹⁸O of cellulose responds to environmental and ecophysiological processes. There are two main factors that contribute to the δ¹⁸O signature of cellulose: the isotopic content of the source water and the leaf evaporative enrichment effect, both of which can vary under natural settings. Thus, separating the source water signal from the atmospheric humidity signal in the δ¹⁸O of cellulose can be difficult. In this study, we took advantage of a highly controlled ecosystem scale study at the University of Arizona Biosphere 2 tropical forest biome, where a drought treatment was implemented with a deep re-wetting component followed by a shallow re-wetting component. Continuous measurements of δ¹⁸O of atmospheric water vapor, soil water and xylem water as well as targeted gas exchange measurements of stomatal conductance and transpiration were made throughout the study. We also collected the δ¹⁸O of phloem sugars and cellulose to address how well the Roden et. al. (2001) cellulose model estimated observed δ¹⁸O values. One main objective was to examine how the fraction of carbonyl oxygen atoms that exchange at the cambium during cellulose biosynthesis, or P_ex, is altered, since recent studies suggest that P_ex can very among species, across aridity gradients, and throughout the growing season. Thus, this highly instrumented experiment allows us to look at variations in P_ex at a high temporal scale. By examining potential shifts in P_ex throughout the formation of a tree ring, we can increase the robustness of reconstructions by targeting specific woody anatomy to capitalize on the different signals of source water and the evaporative effect laid down in wood cellulose.