Contrasting variations of ecosystem gross primary productivity during flash droughts caused by competing water demand and supply

Abstract

Flash drought events (FDEs) are projected to increase frequently in a warming world, significantly impacting ecosystem productivity and the global carbon cycle. The development of FDEs, induced by anomalies in different environmental variables, may cause different responses to the ecosystem’s gross primary productivity (GPP). However, the GPP variations and underlying mechanisms during the FDEs have rarely been quantified. This study collected long-term (>10 years) high-quality flux observations from the FLUXNET 2015 dataset to investigate GPP variations and their driving mechanisms during FDEs. Results showed that all vegetation types have two contrasting GPP variations during FDEs. One variation is a decreasing then increasing standardized GPP anomaly (V-shape response). The other shows an increase followed by decreasing standardized GPP anomaly (inverted V-shape response). The V-shape GPP response to FDEs was induced by increased soil water content deficit at the onset stage of FDEs. In contrast, the inverted V-shape GPP response to FDEs was induced by increased net radiation at the onset of FDEs. Such results indicated competing moisture supply and atmospheric moisture demand at the onset of FDEs, controlling the two contrasting ecosystem’s carbon responses with its development. Moreover, the contribution of water use efficiency to the magnitude of the V-shape GPP response (64.5 ± 22.4%) is greater than that to the inverted V-shape GPP response (47.6 ± 18.7%). This study identified the two contrasting types of GPP variations during FDEs and their driving mechanisms across multiple ecosystem types which can improve our ability to predict the future effects of more frequent FDEs on ecosystem productivity.