John C. Stella Michael Bliss Singer Adam M. Lambert Steven L. Voelker John E. Drake Jonathan M. Friedman Lissa Pelletier Li Kui Dar A. Roberts Jared Williams 2024 <p><span>Drought-induced groundwater decline and warming associated with climate change are primary threats to dryland riparian woodlands. We used the extreme 2012–2019 drought in southern California as a natural experiment to assess how differences in water-use strategies and groundwater dependence may influence the drought susceptibility of dryland riparian tree species with overlapping distributions. We analyzed tree-ring stable carbon and oxygen isotopes collected from two cottonwood species (</span><i>Populus trichocarpa</i><span>&nbsp;and&nbsp;</span><i>P</i><span>.&nbsp;</span><i>fremontii</i><span>) along the semi-arid Santa Clara River. We also modeled tree source water δ</span>¹⁸<span>O composition to compare with observed source water δ</span>¹⁸<span>O within the floodplain to infer patterns of groundwater reliance. Our results suggest that both species functioned as facultative phreatophytes that used shallow soil moisture when available but ultimately relied on groundwater to maintain physiological function during drought. We also observed apparent species differences in water-use strategies and groundwater dependence related to their regional distributions.&nbsp;</span><i>P</i><span>.&nbsp;</span><i>fremontii</i><span>&nbsp;was constrained to more arid river segments and ostensibly used a greater proportion of groundwater to satisfy higher evaporative demand.&nbsp;</span><i>P</i><span>.&nbsp;</span><i>fremontii</i><span>&nbsp;maintained ∆</span>¹³<span>C at pre-drought levels up until the peak of the drought, when trees experienced a precipitous decline in ∆</span>¹³<span>C. This response pattern suggests that trees prioritized maintaining photosynthetic processes over hydraulic safety, until a critical point. In contrast,&nbsp;</span><i>P</i><span>.&nbsp;</span><i>trichocarpa</i><span>&nbsp;showed a more gradual and sustained reduction in ∆</span>¹³<span>C, indicating that drought conditions induced stomatal closure and higher water use efficiency. This strategy may confer drought avoidance for&nbsp;</span><i>P</i><span>.&nbsp;</span><i>trichocarpa</i><span>&nbsp;while increasing its susceptibility to anticipated climate warming.</span></p> application/pdf 10.1029/2023WR035928 en American Geophysical Union Seasonal and species-level water-use strategies and groundwater dependence in dryland riparian woodlands during extreme drought article