Bridging single-tree processes and landscape-scale patterns to explain vegetation drought resistance and resilience in a headwater catchment

Quantifying controls on tree-stand drought response remains challenging due to the interacting effects of landscape, moisture availability, and vegetation. Here, we investigated tree response to drought in terms of resistance—the ability of a forest to continue transpiring during drought—and resilience—the ability to rebound post-drought. We estimated resistance and resilience using remotely sensed normalized difference vegetation index (NDVI) over a 0.5 km2 sub-catchment of the Southern Sierra Critical Zone Observatory in California, USA. At the catchment-wide scale, we fitted generalized additive models with eight remotely sensed predictors to explain 51% of the variance in resistance and 59% in resilience. Elevation, slope, distance to stream, topographic wetness index, and baseline greenness were the strongest predictors and exhibited opposite effects on resistance versus resilience, underscoring the need to distinguish the drivers of resistance and resilience. We further explored these results through ecological process analysis at the tree scale using in-situ ecohydrological (sapflow and soil moisture), meteorological (air temperature and vapor pressure deficit, VPD), and geophysical (electrical resistivity) data from six stations selected based on differing drought responses. The data revealed valley-bottom hydrologic refugia, internal tree water stores, and consistent sapflow-VPD coupling are all associated with higher drought resistance. Together, our sub-catchment work identifies spatial patterns in drought resistance and resilience while our tree-level analysis reveals underlying mechanisms of drought response, demonstrating that forest vulnerability emerges from coupled, scale-dependent interactions among hydrology, vegetation structure, and topography.