California Drought Assessment

Drought is a global issue that is exacerbated by climate change and increasing anthropogenic water demands. The recent occurrence of drought in California provides an important opportunity to examine drought response across ecosystem classes (forests, shrublands, grasslands, and wetlands), which is essential to understand how climate influences ecosystem structure and function. We quantified ecosystem resilience to drought by comparing changes in satellite derived estimates of water use efficiency (WUE = net primary productivity (NPP)/ evapotranspiration (ET)) under baseline and drought conditions (Î WUE = WUE2014 â baseline WUE). With this method, areas with increasing WUE under drought conditions are considered more resilient than systems with declining WUE. Changes in water availability had a significant effect on carbon (C) dynamics in California ecosystems. Baseline WUE varied across California (0.08 to 3.85 g C mm-1 H2O) and WUE generally increased under extreme drought conditions in 2014 (p< 0.001; R2 = 0.83). Strong correlations between Î WUE, precipitation and LAI indicate possible physiological response to drought. For instance, ecosystems with a lower average leaf area index (LAI, i.e. grasslands) maintained greater C uptake rates when water was limiting by increasing WUE and/or elevating carbon uptake efficiency (CUE = NPP/ LAI). We also found that systems with a baseline WUE â ¤ 0.4 exhibited lower WUE under drought conditions, suggesting that a low baseline WUE and Î WUE is indicative of low drought resilience. Drought severity, precipitation and WUE were identified as important drivers of shifts in ecosystem classes over the study period. These findings have important implications for understanding climate change effects on primary productivity and C sequestration across ecosystems and how this may influence ecosystem resilience in the future.