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 effects across ecosystem classes (i.e., forests, shrublands, grasslands, and wetlands), which is essential to understand how climate influences ecosystem structure and function. We quantified ecosystem vulnerability to drought by comparing changes in satellite-derived estimates of water use efficiency (WUE = net primary productivity (NPP)/ evapotranspiration (ET)) under normal (i.e. baseline) and drought conditions (ΔWUE = WUE2014– baseline WUE). With this method, areas with increasing WUE under drought conditions are considered more resilient than systems with declining WUE.
We were able to define baseline conditions for all forests, shrublands, and grasslands within each ecoregion (n=1128 pixels). At these sites, baseline WUE varied (0.08 to 3.85 g C mm-1 H2O) and WUE generally increased under severe drought conditions in 2014 (84%). Only 18% of baseline sites exhibited a decline in WUE under drought conditions (i.e., vulnerable ecosystems), and <1% experienced no change in WUE (i.e., persistent ecosystems). Strong correlations between ΔWUE, precipitation and LAI indicate that ecosystems with a lower average leaf area index (LAI, i.e. grasslands) also had greater C uptake rates when water was limiting and higher rates of carbon uptake efficiency (CUE = NPP/ LAI) under drought conditions. We also found that systems with a baseline WUE ≤ 0.4 exhibited a decline in WUE under drought conditions, suggesting that a baseline WUE ≤ 0.4 might be indicative of low drought resistance. Drought severity, precipitation and WUE were identified as important drivers of shifts in ecosystem classes over the study period. Across California, > 50% of the stated was identified as vulnerable to drought and vulnerability was particularly high in forested areas (> 70%). Vulnerable forests in 2014 also had significantly higher tree mortality rates in 2015 (p<0.001). These findings have important implications for understanding climate change effects on primary productivity and C sequestration across ecosystems and how this may influence ecosystem resistance in the future.