WINTER VS MONSOON WATER SOURCES FOR FOUNDATION TREE SPECIES ACROSS THE COLORADO PLATEAU

Vegetation plays important roles in semiarid water cycles, but plant water use is often uncertain due to complex rooting structures, landscape hydrology, and seasonality of precipitation. Understanding water sources for major tree species is important for evaluating present-day ecohydrology and for predicting vegetation responses to changing precipitation regimes. We used stable isotope ratios in water (δD and δ¹⁸O) extracted from 633 stems and soils at 13 sites across AZ, NM, UT, and CO to quantify the soil depths recharged by monsoon precipitation and the relative importance of winter versus summer (monsoon) sources for each of three foundation tree species common to the Colorado Plateau (aspen [Populus tremuloides], piñon [Pinus edulis], Utah juniper [Juniperus osteosperma)]). The sites span a 1500-m elevation gradient and a wide range of 2015-2016 winter precipitation. We used weekly isotope data from the US Network of Isotopes in Precipitation (USNIP) to characterize seasonal precipitation signatures. We then applied a Bayesian mixing model to the isotope data to evaluate (1) relative contributions of winter and summer precipitation to soil water at different depths (shallow = 0-10 cm, intermediate = 0-30 cm, deep = 30-60 cm), and (2) depths from which trees extracted water during the 2016 growing season. We found that differences in isotope ratios between soil water at different depths were most pronounced prior to monsoon onset, when shallow soil moisture showed strong signs of evaporative enrichment, especially at lower elevation piñon-juniper sites, and deep soil water had an isotopic signature consistent with winter precipitation. At all sites, differences in the isotope ratios in shallow and deep soil water were less pronounced post-monsoon. Modeling results indicate that monsoon moisture comprised up to 50% (20%) of post-monsoon deep soil water at piñon-juniper (aspen) sites. During the growing season, trees at all sites decreased their reliance on deep soil water and increased their use of shallow to intermediate soil water by 25% in aspen, 35% in piñon, and 45% in juniper. Our results suggest that monsoon precipitation can infiltrate 30 cm or more at all sites and that these foundation tree species can shift their water sources to take advantage of changing regional precipitation regimes.