Tropical forests face increasing climate risk^1,2, yet our ability to predict their response to climate change is limited by poor understanding of their resistance to water stress. Although xylem embolism resistance thresholds (for example, $$\varPsi $$Ψ ₅₀) and hydraulic safety margins (for example, HSM₅₀) are important predictors of drought-induced mortality risk^3–5, little is known about how these vary across Earth’s largest tropical forest. Here, we present a pan-Amazon, fully standardized hydraulic traits dataset and use it to assess regional variation in drought sensitivity and hydraulic trait ability to predict species distributions and long-term forest biomass accumulation. Parameters $$\varPsi $$Ψ ₅₀ and HSM₅₀ vary markedly across the Amazon and are related to average long-term rainfall characteristics. Both $$\varPsi $$Ψ ₅₀ and HSM₅₀ influence the biogeographical distribution of Amazon tree species. However, HSM₅₀ was the only significant predictor of observed decadal-scale changes in forest biomass. Old-growth forests with wide HSM₅₀ are gaining more biomass than are low HSM₅₀ forests. We propose that this may be associated with a growth–mortality trade-off whereby trees in forests consisting of fast-growing species take greater hydraulic risks and face greater mortality risk. Moreover, in regions of more pronounced climatic change, we find evidence that forests are losing biomass, suggesting that species in these regions may be operating beyond their hydraulic limits. Continued climate change is likely to further reduce HSM₅₀ in the Amazon^6,7, with strong implications for the Amazon carbon sink.

Tropical forests are experiencing unprecedented high‐temperature conditions due to climate change that could limit their photosynthetic functions. We studied the high‐temperature sensitivity of photosynthesis in a rainforest site in southern Amazonia, where some of the highest temperatures and most rapid warming in the Tropics have been recorded. The quantum yield (F_v/F_m) of photosystem II was measured in seven dominant tree species using leaf discs exposed to varying levels of heat stress. T₅₀ was calculated as the temperature at which F_v/F_m was half the maximum value. T₅ is defined as the breakpoint temperature, at which F_v/F_m decline was initiated. Leaf thermotolerance in the rapidly warming southern Amazonia was the highest recorded for forest tree species globally. T₅₀ and T₅ varied between species, with one mid‐storey species, Amaioua guianensis, exhibiting particularly high T₅₀ and T₅ values. While the T₅₀ values of the species sampled were several degrees above the maximum air temperatures experienced in southern Amazonia, the T₅ values of several species are now exceeded under present‐day maximum air temperatures.

The South American Cerrado, the largest savanna of the Americas and the world's most tree-biodiverse, is critically endangered, with just 8% protected and more than half deforested. However, the extent of its tree diversity and abundance remains poorly quantified. Using a unique biome-wide eco-floristic dataset with 222 one-hectare plots, we estimate the Cerrado has ~1605 tree species and has extreme hyperdominance, with fewer than 2% (30 species) accounting for half of all trees. A single family, Vochysiaceae, represents 17% of all trees, and the most abundant species, Qualea parviflora, accounts for 1 in 14 trees. In contrast, 63% of the species are rare, with fewer than 100 trees across all plots. Remote sensing and spatial modelling suggest the Cerrado has lost 24 billion trees since 1985, equivalent to three times the Earth's human population. We estimate up to 800 tree species may remain undetected in Cerrado ecosystems and could face extinction in a few decades due to deforestation. This hyperdominance parallels patterns in Amazonian forests and highlights risks both biomes face for species loss due to fragmentation, deforestation, and land-use change. Our findings highlight the Cerrado’s critical but undervalued role in global biodiversity, its vulnerabilities, and the urgent need for conservation to avoid irreversible species and biome loss.

The effects of altered fire regimes within open ecosystems are poorly understood and can have serious consequences on functioning and conservation across savanna ecoregions worldwide. In South American savannas like the Cerrado, there is a gap of knowledge relating to carbon cycling in the presence of fire, meaning the impacts of altered fire regimes on the carbon fluxes and budgets are virtually unknown. We thus investigate vegetation carbon flux dynamics within the Cerrado making use of an experimental fire and carbon monitoring research project at the Estação Ecológica da Serra das Araras, Brazil. We present a thorough carbon budget of woodland-type savannas (cerrado sensu stricto), and investigate how annual (every year), biennial (every two years), and triennial (every three years) controlled fire frequencies have influenced net primary productivity and respiration fluxes. Six years of experimental fire had noticeable effects on the vegetation structure and carbon dynamics, reducing woody cover and productivity in favour of grass-dominated carbon balances. Woody NPP increased by 35 % in the unburnt plots from 2017 to 2019 to 2021-2023, but decreased by 75 %, 33 % and 20 % in the triennial, biennial and annual fire frequencies. By 2023, burnt plots revealed around three times higher herbaceous NPP than unburnt plots. Fluxes corresponding to different ecosystem components (canopy, stems, roots, herbs) showed varying patterns of change across the gradient of experimental fire frequencies, indicating other fire regime properties distinctively affect each vegetation segment. Fire intensity and severity appear to be linked with patterns in woody stems and the herbaceous layer. Our results indicate periodically burnt cerrado sensu stricto vegetation experiences different carbon dynamics than unburnt vegetation. Burning is revealed as a strategy that can successfully limit woody encroachment and help conserve open ecosystem structure in the Cerrado. Our study highlights the importance of long-term monitoring efforts in investigating the effects of management interventions and environmental shifts on ecosystem functioning.