The Southwest has experienced increased drought over the past several decades with further changes in the timing and magnitude of precipitation anticipated. Understanding how these changes affect foundation tree species lends insights into forest and woodland vulnerability to climate change. Thus, we focus on three foundation tree species (aspen [Populus tremuloides], piñon [Pinus edulis], Utah juniper [Juniperus osteosperma)]) and estimate their dependence on deep (winter-derived) versus shallow (summer/monsoon) soil moisture across 13 sites in Arizona, New Mexico, Colorado, and Utah. The sites spanned a 1500-m elevation gradient and a wide range of El Niño (winter 2015-2016) precipitation, which led to variable “deep” soil moisture availability. We inferred water sources via stable isotopes (δD and δ18O) in water extracted from soils (three depths) and stems (five trees per species per site). We conducted two field campaigns in 2016 (pre- and post-monsoon) to evaluate how tree- and species-level water sources changed over the 2016 growing season. We also obtained weekly isotope data from the US Network of Isotopes in Precipitation, and applied a simple linear mixing model to the isotope data (field and network) to evaluate the relative importance of deep (winter) versus shallow (summer/monsoon) water for these important Southwest tree species.
Isotopes in shallow (0-10 cm), intermediate (10-30 cm), and deep (30-60 cm) soil water were sufficiently different to constrain tree water sources. Differences were most pronounced in pre-monsoon, shallow soil water with average δ18O and δD approximately 6‰ and 22‰ higher, respectively, than water at intermediate depths. Post-monsoon shallow moisture exhibited less evaporative enrichment at all sites. Generally, juniper and piñon trees rely on deep moisture both pre- and post-monsoon, while aspen trees rely on intermediate to shallow moisture, reflecting greater dependence on monsoon inputs. Prior to the monsoon, juniper stem water (δ18O = -8.9‰, δD = -78‰) was consistent with deep soil water, while piñon stem water (δ18O = -6.3‰, δD = -67.5‰) was consistent with intermediate soil water. Post-monsoon juniper and piñon stem water was more consistent with intermediate-depth soil water. In aspen, pre-monsoon stem-water represented a mixture of intermediate (53%) and shallow (47%) soil water; post-monsoon water sources were more variable, with some trees accessing intermediate (61%) to deep (39%) soil water and others utilizing mostly shallow soil water, suggesting rooting distribution divergence. These data suggest aspen may be more sensitive to summer precipitation fluctuations, while piñon and juniper may be more vulnerable to decreased winter precipitation.