Johanna Van Passel, Paulo N. Bernardino, Stef Lhermitte, Bianca F. Rius, Marina Hirota, Timo Conradi, Wanda de Keersmaecker, Koenraad Van Meerbeek, and Ben Somers

PNAS, May 20, 2024; 121 (22) e2316924121

Significance

The increasing frequency and intensity of droughts in the Amazon rainforest raises concerns about potential forest dieback. However, the precise role of drought occurrences in this phenomenon remains unclear. We used trends in temporal autocorrelation of satellite-derived indices of vegetation activity as a proxy for the critical slowing down response of the Amazon and differentiated between drought frequency, intensity, and duration to investigate their respective effects on the slowing down response. We found that this slower recovery to perturbations prevails in regions experiencing more frequent, intense, and longer droughts, albeit with regional variations. Most of the Amazon does not show critical slowing down, but the predicted increase in droughts could disrupt this balance, signifying the importance of understanding these dynamics.

Abstract

Dynamic ecosystems, such as the Amazon forest, are expected to show critical slowing down behavior, or slower recovery from recurrent small perturbations, as they approach an ecological threshold to a different ecosystem state. Drought occurrences are becoming more prevalent across the Amazon, with known negative effects on forest health and functioning, but their actual role in the critical slowing down patterns still remains elusive. In this study, we evaluate the effect of trends in extreme drought occurrences on temporal autocorrelation (TAC) patterns of satellite-derived indices of vegetation activity, an indicator of slowing down, between 2001 and 2019. Differentiating between extreme drought frequency, intensity, and duration, we investigate their respective effects on the slowing down response. Our results indicate that the intensity of extreme droughts is a more important driver of slowing down than their duration, although their impacts vary across the different Amazon regions. In addition, areas with more variable precipitation are already less ecologically stable and need fewer droughts to induce slowing down. We present findings indicating that most of the Amazon region does not show an increasing trend in TAC. However, the predicted increase in extreme drought intensity and frequency could potentially transition significant portions of this ecosystem into a state with altered functionality.

See https://www.pnas.org/doi/10.1073/pnas.2316924121