USING STABLE ISOTOPE ANALYSES TO DETERMINE PONDEROSA PINE WATER SOURCES IN NORTHERN ARIZONA

Understanding the dynamic relationships between seasonal water use, stand density, and tree size in semi-arid forests of the southwestern U.S. is important for predicting climate change impacts and for tailoring forest restoration prescriptions to mitigate such impacts. Using stable isotope ratio (δD and δ¹⁸O) analyses of precipitation, soil water, stem water, and tree-ring alpha cellulose over a two-year sampling period, we found that winter precipitation was the dominant water source for ponderosa pines (Pinus ponderosa Dougl.) in northern Arizona in all seasons. Soil and stem waters were isotopically more enriched in high- than low-density stands. Isotopic analyses indicated large trees were more reliant on deep soil water than small trees. We also show that while trees do not appear to directly use monsoon rainwater for summer wood production, they do benefit from the increased humidity associated with this season. Our results indicate that management actions that maintain or create low-density stands of large deeply-rooted trees increase tree access to winter precipitation via deep soil storage and thus may help mitigate impacts of climate warming on tree health. Our findings provide new understanding of the complex relationships among seasonal water use, stand density, and tree size in a region where a drying climate puts increasing stress on forests.