The isotopic composition of soil water used by trees is affected by multiple ecohydrological processes, and the relative abundance of stable isotopes in plant tissue is determined by subseasonal hydroclimatic conditions. We measured δ 18O in precipitation (δ 18O PPT), xylem water (δ 18O XW), and cellulose of tree-rings (δ­ 18O CELL and δ­ 13C CELL) in dominant and codominant ponderosa pines for the 2015 and 2016 warm seasons during drought conditions. Quantitative wood anatomy, including tracheid lumen diameter (LD) and cell wall thickness (CWT), provided phenological dates of wood formation. False ring formation was measured in multiple trees in response to precipitation from a large remnant storm of Hurricane Dolores that reached southern Nevada . We utilized the opportunity to test if the α-cellulose in false rings had a different isotopic signal between dominant and codominant trees. Indeed, we measured a different isotopic ratio (δ­ 18O CELL) in codominant trees associated with higher soil moisture at shallower soil depths. Contrariwise, the δ­ 18O CELL in dominant trees formed during the 2016 warm season reflected fluctuations in soil moisture at deeper soil depths, accessing water stored in the macropores of bedrock after a wetter early warm-season and drier monsoon season. No subseasonal differences between codominant and dominant trees were measured in δ­ 13C CELL. Throughout our study, we observed that codominant ponderosa pine tree-ring growth responded to pulses of warm-season precipitation more readily than dominant trees. If southern Nevada continues to trend toward a drier and warmer future, deep soil water recharge will be less reliable and drought stress will threaten dominant trees in old-growth forests.

Bark beetle outbreaks have impacted over 58 million acres of coniferous forest in the Western US since 2000, an area slightly larger than the state of Utah. Most of these beetle-impacted forests are in semi-arid, snow-dominated headwater catchments that generate a disproportionate fraction of water supplies. Limited previous studies have shown severe beetle-kill can cause mixed increases and decreases in streamflow. This study is the first to empirically explore changes in streamflow following a recent spruce beetle outbreak in southwest Colorado using a paired catchment approach. The period following beetle kill (2014-2019) was 0.95° C warmer and 5.8 cm/year drier than the 21-year period prior to the disturbance’s peak (1993-2013). There was no change in streamflow in the control basins after beetle kill. In contrast, post-beetle kill had 34% higher peak flows on average and consistent predictions of >14% increases in streamflow in wetter basins and >20% in drier basins. Our results suggest that higher streamflows are primarily driven by 44% higher runoff efficiencies during the snowmelt period. The increased flows due to beetle kill are occurring at a time when control catchments have unchanged runoff efficiencies. These findings are the first to clearly show streamflow increases following extensive spruce beetle kill in watersheds that contribute water to millions of downstream residents. Moreover, our findings contrast with evidence of unchanged or decreased streamflow following mountain pine beetle kill in nearby parts of Colorado, highlighting the need for better post-disturbance hydrologic predictions in these important montane forests.