Drought resistance across California ecosystems: evaluating changes in carbon dynamics using satellite imagery

Abstract

Abstract. 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 resistance 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 = WUE 2014 − baseline WUE). With this method, areas with increasing WUE under drought conditions are considered more resilient than systems with declining WUE. Baseline WUE varied across California (0.08 to 3.85 g C/mm H 2 O) and WUE generally increased under severe drought conditions in 2014. Strong correlations between ΔWUE, precipitation, and leaf area index (LAI) indicate that ecosystems with a lower average 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. 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.

Monitoring Changes in Water Use Efficiency to Understand Drought Induced Tree Mortality

“…Evaluating shifts in water use efficiency (WUE) under non-drought and drought conditions provides a good approximation of ecosystem resistance to drought [8]. Defined here as net primary productivity (GPP; g C m −2 ) per amount of water lost (evapotranspiration: ET; mm m −2 ), WUE links the biological (i.e., photosynthesis and transpiration) and physical (i.e., evaporation) processes that control carbon and water dynamics.…”

“…An important measure of ecosystem functionality [9], variation in WUE connotes ecosystem resistance by signaling severe changes in resource availability that lead to shifts in ecosystem function. Here, resistance is the capacity to absorb a disturbance (i.e., drought) and maintain the same function (i.e., productivity) and sensitivity (i.e., WUE) [8,10].…”

“…Studies have shown that WUE increases with aridity [8,[11][12][13][14][15][16][17] and that water limited ecosystems tend to have higher WUE [11,12,18]. Across all systems, if a drought becomes severe enough, a breakdown in ecosystem resistance can lead to a reduction in WUE [8,14,15,17,19] and ecosystem type conversions [8,17].…”

“…Across all systems, if a drought becomes severe enough, a breakdown in ecosystem resistance can lead to a reduction in WUE [8,14,15,17,19] and ecosystem type conversions [8,17]. A recent study evaluated drought resistance across California ecosystems by quantifying deviations in WUE in 2014 (WUE Severe Drought ) from WUE under normal climate conditions (WUE BASELINE ) [8].…”

“…The frequency of drought years is also likely to be an important determinant of tree mortality. Studies have shown that a low WUE BASELINE is indicative of low drought resilience [8] and that extended droughts often lead to enhanced tree mortality rates [4]. In California forests, more trees die in years with below normal precipitation, and increases in tree mortality are often evident only for multi-year droughts (2-5 years) [4].…”