Bridging Single-Tree Processes and Regional-Scale Patterns to Explain Vegetation Drought Resistance and Resilience in a Sierra Nevada Headwater Catchment

Droughts have been extensively studied at small to large scales, yet limited work has integrated the mountain-range and tree-level perspectives to explain tree drought response at the intra-catchment scale where management decisions are made. 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 Landsat-derived normalized difference vegetation index (NDVI) over a 0.5 km2 catchment of the Southern Sierra Critical Zone Observatory. 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. Topography and baseline greenness were the strongest predictors and exhibited opposite effects on resistance versus resilience, underscoring the need to distinguish their drivers. Aspect and snow depth were significant for resilience only, further highlighting that resistance and resilience are governed using partially distinct processes. Slope and elevation effects contradicted regional-scale patterns, whereas canopy height effects were consistent across scales. Remote sensing revealed spatial patterns of drought response, while in situ ecohydrological, meteorological, and geophysical (electrical resistivity) data from six stations offered process-based insights into the conditions underlying them: valley-bottom hydrologic refugia, inferred reliance on internal sapwood water stores, and consistently low atmospheric demand were all associated with locations that experienced greater drought resistance. This work demonstrates that forest vulnerability emerges from coupled, scale-dependent interactions among hydrology, vegetation structure, and topography.