Tree size, stand density, and the source of water used across seasons by ponderosa pine in northern Arizona

Understanding the dynamic relationships between seasonal water use, stand density, and tree size in semi-arid forests of the southwestern US is important for predicting climate change impacts and for tailoring forest restoration prescriptions to mitigate such impacts. Using hydrogen stable isotope ratio (δD) analyses of precipitation, soil water, and stem water over a 2-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. 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.