A biogeography of Amazon forest photosynthetic drought response predicts basin-wide resilience and vulnerability

Amazonia contains the most extensive tropical forests on Earth, but Amazon carbon sinks of atmospheric CO₂ are declining, as deforestation and climate change-associated droughts threaten to push these forests past a tipping point towards forest collapse. Forests exhibit complex drought responses, indicating both resilience (photosynthetic “greening”) and vulnerability (browning and tree mortality), that are difficult to explain by climate variation alone. Here, we combine remotely-sensed photosynthetic indices with ground-measured tree demography to identify mechanisms underlying drought resilience/vulnerability in different intact forest ‘ecotopes’ (defined by water-table depth, soil fertility and texture, and vegetation characteristics). In higher-fertility southern Amazonia, drought response was structured by water-table depth, with resilient greening in shallow-water-table-forests (where greater water availability heightened response to excess sunlight), contrasting with vulnerability (“browning” and excess tree mortality) over deeper water tables. Notably, shallow-water-table-forest resilience weakened as drought lengthened. By contrast, lower-fertility northern Amazonia, with slower-growing but hardier trees (or alternatively, tall forests, with deep-rooted water access), supported more drought-resilient forests independent of water-table depth. This new functional biogeography of drought response provides a framework for conservation decisions and improved predictions of heterogeneous forest responses to future climate changes, warning that Amazonia’s most productive forests are also at greatest risk, and that longer/more frequent droughts are undermining multiple ecohydrological strategies and capacities for Amazon forest resilience.