The fate of Amazon rain forests under drought: collapse or stabilisation?

Drought-induced mortality is expected to cause substantial biomass loss in the Amazon Basin. However, responses by rain forest to prolonged drought remain largely unknown. Critically, how drought impacts individual trees over decades, whilst potential changes in forest structure alter competition for resources, remain unreported for any tropical forest globally. We demonstrate that an Amazonian rain forest subjected to more than two decades of drought at a throughfall-exclusion experiment reached long-term eco-hydrological stability. The stabilisation was largely driven by ecosystem-level structural changes that resulted in the remaining trees to no longer experiencing drought stress. The loss of the largest trees to drought-related mortality during the first 15 years of the experiment led to increasing water availability for the remaining trees, facilitating a stabilisation in biomass in the last seven years of the experiment. The elimination of water stress led to hydraulic variables commonly associated with physiological stress, such as leaf water potential, sap flow, and tissue water content to be equal to those in corresponding non-droughted control forest, indicating hydraulic homeostasis. This work reveals that significant resilience to persistent (multi-decadal) soil drought in tropical rain forest. The resilience emerges from structural feedbacks at ecosystem scale that prevent drought-induced collapse, whilst also resulting in a forest with reduced biomass and lower but positive net wood productivity.