Amazon forest carbon sinks are surprisingly resilient, but vulnerable to increasing drought length

Amazon tropical forests, the most extensive on earth, are a major but now declining sink for atmospheric CO2, due to both direct human-caused deforestation and increasing mortality in intact forests from rising temperatures and more frequent droughts. However, specific mechanisms underlying rising mortality in intact forests (and associated C sink declines), and its heterogeneous spatial distribution remain unresolved. Using MODIS-based remotely sensed observations of forest responses to the exceptionally hot 2023/2024 El-Niño drought (whose duration was unprecedented in the satellite era), we tested whether a statistical model of the biogeography of remotely sensed photosynthetic responses to droughts (2005, 2010, 2015/16 and 2023/24)---including vulnerability and resilience of canopy greenness---could also explain the biogeography of forest carbon sink vulnerability and resilience across the basin. 

 

We found that the remote sensing-derived biogeography of canopy greenness also explained decadal C sink trends in ground-based forest plots, with the resilience of canopy greenness predicting which plots had sustainable C sinks and which had weakening C sinks over time due to increasing tree mortality. Factors predicting vulnerability to increased mortality (and declining C sequestration) included: deep water tables (where water resources are far from trees’ roots), shorter forests with shallow rooting depths (where tree access to water is limited), and especially, forests on fertile soils (which grow quickly with little investment in drought tolerance traits). Our biography of carbon sink resilience and vulnerability suggests that the distribution of ground-based monitoring plots are biased towards more vulnerable regions, and hence they over-estimate the rate of carbon sink decline. Adjusting for the distribution of biogeographic factors controlling carbon sink dynamics, we find the recent basin-wide carbon sink remains effectively stable. However, the exceptionally long 2023 drought guided identification of a critical drought-length threshold of 6 months, beyond which vulnerable forest regions expanded, suggesting that since longer droughts are becoming more common, C sinks may be destabilizing. This new approach, based on remotely derived forest sensitivity to climatic perturbations, identifies key drivers of forest demography and carbon dynamics, and reveals drought-length as a major contributor to the risk of forest tipping points and loss of carbon storage, with implications for resilience of Earth’s climate system.