ABSTRACT:

Quantifying fine-scale controls on tree-stand drought response remains challenging due to the interacting effects of landscape, moisture availability, and vegetation. We investigated drought resistance—the ability of a forest to continue transpiring during drought—and resilience—the ability to rebound post-drought—in a 0.5 km2 subcatchment of the Southern Sierra Critical Zone Observatory (King’s River Experimental Watershed), during and after the 2012-2016 California drought. Using a multi-scale approach, we integrated catchment-wide remote sensing data (LiDAR and Normalized Difference Vegetation Index, NDVI) with tree-scale in situ ecohydrological (sapflow and soil moisture), meteorological (air temperature and vapor pressure deficit), and geophysical (electrical resistivity) data from six stations. We fitted generalized additive models (GAMs), which capture nonlinear relationships, using eight remote-sensing-derived predictors—elevation, slope, aspect, distance to stream, topographic wetness index (TWI), snow depth, canopy height, and baseline NDVI. The intercorrelated variables of elevation, slope, distance to stream, TWI, and baseline NDVI were the strongest predictors of resistance and resilience. Notably, baseline NDVI had approximately the opposite effects on resistance and resilience, highlighting the need to distinguish between drought impacts during versus after drought events. The GAMs explained 51% of the variance in resistance and 39% in resilience, indicating that additional covariates are needed, potentially at the finer, plant-scale. Our in-situ data from the valley bottom indicated the presence of hydrologic refugia—areas that retain higher soil moisture than surrounding terrain—which helped explain the added drought resistance observed there. By combining our ecohydrology and geophysical data, we tracked when trees sourced water from sources other than their root zone (such as internal tree water stores) and identified changes to the active rooting zone extent on the sub-daily scale. During seasonal water deficits, trees accessed deeper rather than broader water stores and increasingly relied on internal reserves until depletion. Stations where internal stores were exhausted more rapidly exhibited lower drought resistance. Altogether, this work emphasizes that accurate predictions of forest drought response—especially in complex montane watersheds—require attention to multi-scale processes.