Drought decreased annual cumulative carbon uptake in Southwest China mainly through its influence on phenology rather than physiology

Abstract

Drought poses a significant threat to the sustainability of ecosystems, particularly in regions like Southwest China, where it has been observed to substantially reduce the annual cumulative carbon uptake (ACCU). Understanding the mechanisms driving this reduction is therefore crucial for effective ecosystem management. To address this issue, this study decomposed ACCU into three indicators, i.e., start of the growing season (SOS), end of the growing season (EOS), and peak gross primary productivity (GPPmax), through the Statistical Model of Integrated Phenology and Physiology (SMIPP), and explored the response of ACCU to drought. The results revealed that phenology (SOS and EOS) and physiology (GPPmax) components collectively captured 74% of the variation in ACCU, confirming the applicability of the SMIPP model in Southwest China. Among the three indicators, GPPmax contributes to ACCU more than the two phenological indicators. We also found that, from 2001 to 2020, drought decreased ACCU by 65.58 Tg C through delying SOS, by 21.47 Tg C through advancing EOS, but increased ACCU by 11.15 Tg C through increasing GPPmax, resulting in a net loss of 75.90 Tg C in ACCU. Our findings highlight the predominant negative impact of drought on ACCU, achieved primarily through alterations in phenology. Overall, this study quantified the respective contributions of phenology and physiology in the response of ACCU to drought, and would inform the prediction of ACCU of ecosystems under future drought in Southwest China.