Impacts of drought on ecosystem carbon uptake

Abstract

Climate extremes threaten the land carbon sink and it is important to understand their impact in a changing climate. An elevated incidence of drought has been observed recently in some regions (e.g. Europe), and projections indicate an increased prevalence with climate warming. This is concerning because ecosystem carbon uptake currently mitigates increases in atmospheric CO2 concentrations. Although the link between drought and reduced carbon uptake is well established, important questions remain regarding the impact of recurrent droughts, the strength of seasonal and regional compensation effects, land-atmosphere feedbacks that can exacerbate heatwaves, and forest management strategies in a changing climate. Here we present an overview on the current knowledge of drought impacts on ecosystem carbon uptake and related feedbacks on energy and water fluxes. These results are based on a recent perspective1 and a global synthesis of ecosystem flux measurements combined with terrestrial biosphere models (TBMs)2. Reduced carbon uptake during drought originates from stress-related declines in photosynthesis. Respiration from plants and soil is also reduced due to limitations in soil moisture, but this is typically to a lower extent than photosynthesis. These relative differences result in reduced net carbon uptake or even net losses. For forests, the combined stress of intense drought over prolonged periods (or recurrent events) leads to increased crown and eventually tree mortality. During severe drought, enhancing (i.e. positive) land-atmosphere feedbacks often further exacerbate extremely dry and hot conditions: as water transpired by plants and evaporated from soils is reduced, evaporative cooling becomes less efficient and more of the available energy heats the air. While ecosystem carbon uptake is typically reduced during severe summer drought1, there is also evidence for increased photosynthesis (i.e. gross primary productivity, GPP) during meteorological drought (i.e. precipitation deficit) in energy-limited ecosystems, particularly during spring2. Comparing ecosystem observations of GPP from eddy covariance (EC) flux towers across the Northern Hemisphere with TBM outputs across the water-energy limitation spectrum, we found a consistent increase in GPP from EC during spring drought in energy-limited ecosystems. Half of spring GPP sensitivity to precipitation was predicted solely from the wetness index (an indicator for aridity) , with weaker relationships in summer and fall. Our results suggest GPP increases during spring drought for 55% of vegetated Northern Hemisphere lands (>30° N). Comparing theses sensitivities with the output from TBMs indicated that the TBMs were insufficiently sensitive to spring precipitation deficits. Reduced carbon uptake during drought might be no longer exceptional in a warming climate, revealing the vulnerability of the land carbon sink to such climate extremes – particularly for forests. Comparing ecosystem EC observations for GPP with TBMs indicates a need for TBMs to better account for the varying effects of meteorological drought on carbon cycling in mid- and high-latitude ecosystems.