Declining resilience of forest carbon sinks linked to increased atmospheric water deficit during droughts in the Northern Hemisphere

Abstract

Droughts have posed intense threats to the forest carbon sink (i.e., net ecosystem productivity, NEP), potentially elevating the risk of forest degradation and challenging the achievement of climatic and ecological goals. While global forest NEP endured, the resilience of NEP served as the ability of ecosystems to withstand and recover from perturbations and the underlying maintenance mechanisms during droughts remain unclear. Here, we explored the responses of NEP resilience, quantified by the lag-1 temporal autocorrelation coefficient (TAC) of two consecutive time series, to droughts based on 87 drought events across 45 forest sites with flux and meteorological observations in the Northern Hemisphere. Furthermore, an interpretable machine learning algorithm was utilized to disentangle the synergistic effects of environmental and biotic factors on TAC, achieving a mean coefficient of determination of 0.87 for drought events with significantly increasing TAC and 0.91 for other drought events. Here, we found that the increase in NEP resilience could alleviate the negative effects of droughts, in contrast to a 2.5 times increase in the probability of NEP decline events associated with decreased NEP resilience. However, NEP resilience declined with the rise of drought intensification. The reduced reference canopy conductance (Gcref) was the primary constraint on NEP resilience maintenance, contributing 48% to the total influence of biotic factors. In addition, high vapor pressure deficit (VPD) exacerbated the negative effects of soil moisture deficit, jointly leading to the decline in NEP resilience. Specifically, elevated VPD during droughts significantly reduced Gcref, indicating the vulnerability of tree hydraulic systems to compound stress. Overall, our study emphasizes the potential risks of the compound soil and atmospheric water deficit on forest NEP resilience and carbon sink across the Northern Hemisphere in the future.