Family imprint reveals basin-wide patterns of Amazon forest embolism resistance

The Amazon region is home to the largest and most diverse tropical forest in the world, and plays an important role in planetary biogeochemical cycles. Recent findings have documented substantial changes in non-flooded Amazonian forests (terra-firme), including floristic and functional composition 1-3 , structure, and dynamics 4,5 , potentially associated with ongoing changes in climate and atmospheric composition. In recent decades, the Amazon has been subjected to a sequence of large-scale drought events (1998, 2005, 2010, 2015-16, 2023 and 2024) 6-12 , as well as a continued warming of 0.6°-0.7 °C since 1950 13 , exposing plants to higher water stress due to increased vapour pressure deficit (VPD). Climate model projections also suggest that the severity of drought effects on forests will continue to increase and that temperatures will likely rise to levels without historical analogues 14,15 . Together, these climatic changes are expected to exacerbate water stress in Amazon rainforests.

Xylem embolism resistance is a key structural trait that determines the ability of plants to tolerate water stress. This is because water stress is associated with increasingly negative xylem water potentials, which may result in the formation of water vapour/air bubbles (emboli) in the xylem and compromise water transport to the canopy 16 . Typically, embolism resistance is quantified as the xylem water potential at which a tree's hydraulic conductivity declines to 50% of its maximum value 17 (Ψ 50 ), with a more negative Ψ 50 implying higher embolism resistance. In Amazonia, embolism resistance has been shown to explain transpiration and canopy conductance responses to extreme drought 18,19 , as well as patterns of species distributions 20,21 and differential mortality patterns under imposed drought 22 .

In recent years, advances have been made in understanding how embolism resistance varies locally 18,19,22-29 and along basin-wide pre-