Sensitivity of Carbon and Water Fluxes to Droughts across the Conterminous United States

Abstract:

The debate over decreasing or increasing ecosystem productivity in response to drought has been lasting for a long time period. However, it remains uncertain yet why ecosystem productivity differently responds to precipitation decrease, and how drought would shape future ecosystem functions at a regional scale. The sensitivity of biomes to droughts is of great significance in determining the ecosystem structure and function, and influencing the carbon exchanges between land and atmosphere, which may in turn affect future climate change. Understanding the contrasting responses of productivity among regions and the underlying mechanism is critical for improving our knowledge of the drought resilience and tolerance of plants and accurately projecting drought consequences and developing appropriate adaption and mitigating strategies in the context of rapid global warming. By using an integrated approach combining in-situ measurements, remote-sensing, empirical upscaling products and process-based model simulation, here we investigated the response of ecosystem Gross Primary Productivity (GPP) to droughts at annual scale across the conterminous United States (CONUS). The results indicate that annual GPP is more responsive to the year-to-year variations of precipitation in the western US (arid sites/regions) than that in the eastern US (humid sites/regions), as a consequence of the stronger inter-annual variations in soil water content and physiological and structural drought-adaptive strategies of xeric biomes. Insensitivity of annual GPP in the mesic biomes implies different drought-adaptive strategies by keeping a high level of non-structural carbohydrate. Our simulation further suggests that over the 21st century, the more pervasive global-change-type droughts (drought is projected to occur under warmer temperature condition as climate change proceeds) are likely to reduce vegetation productivity, change the current pattern of biome structure and composition, and induce higher tree mortality in the southeastern US.