Postdoctoral Fellowship: EAR-PF: How Does Critical Zone Water Storage Impact Forest Drought Stress Across Complex Terrain?

Climate change is causing more frequent hot droughts, which can lead to extreme water stress for tall trees in forest ecosystems. The ways that trees deal with water stress under extreme drought are not well understood. To address this challenge, this research will address fundamental questions about how water stored below ground affects tree growth and physiological water stress. Trees can adapt their physical structure to deal with chronic water stress, but it is unclear how subsurface water storage dynamics do or do not lead to compensatory responses in tree physiology. For example, will a tree faced with intermittent drought make the same physiological adjustments as those faced with perennial drought? Questions like these are particularly relevant in places where water is already a limiting factor for tree growth and could become more limiting in the future. This study will take place in Colorado, USA, where snowpack in montane forests is declining due to climate warming. By understanding tree water stress across complex mountainous terrain, the results of this study will be broadly relevant for scientists and forest managers seeking to improve forest drought resilience and ensure sustainable management of water resources. The role of deep “critical zone” water storage in mitigating forest water stress during extreme drought may be confounded by the way trees adapt structurally and physiologically to subsurface hydrological conditions. For example, water stored in deep, highly weathered bedrock may mitigate seasonal water stress in trees. However, trees with access to deep water storage may be vulnerable to hydraulic failure during prolonged drought if they have maladapted physiological traits. This project employs field-based investigations using seismic surveys and electrical resistivity tomography to gather data on belowground structure and water storage along hillslope transects in the Boulder Creek Watershed in Colorado, USA, part of the Dynamic Water Critical Zone Cluster Network. Information on subsurface water storage dynamics will be combined with tree ecophysiological measurements to reveal how critical zone water storage influences forest drought vulnerability and transpiration fluxes in semi-arid environments. Ultimately, this research seeks to improve mechanistic understanding of how climatic conditions and subsurface structure regulate water fluxes across complex mountainous terrain. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.