Most of the planet's diversity is concentrated in the tropics, which includes many regions undergoing rapid climate change. Yet, while climate‐induced biodiversity changes are widely documented elsewhere, few studies have addressed this issue for lowland tropical ecosystems. Here we investigate whether the floristic and functional composition of intact lowland Amazonian forests have been changing by evaluating records from 106 long‐term inventory plots spanning 30 years. We analyse three traits that have been hypothesized to respond to different environmental drivers (increase in moisture stress and atmospheric CO₂ concentrations): maximum tree size, biogeographic water‐deficit affiliation and wood density. Tree communities have become increasingly dominated by large‐statured taxa, but to date there has been no detectable change in mean wood density or water deficit affiliation at the community level, despite most forest plots having experienced an intensification of the dry season. However, among newly recruited trees, dry‐affiliated genera have become more abundant, while the mortality of wet‐affiliated genera has increased in those plots where the dry season has intensified most. Thus, a slow shift to a more dry‐affiliated Amazonia is underway, with changes in compositional dynamics (recruits and mortality) consistent with climate‐change drivers, but yet to significantly impact whole‐community composition. The Amazon observational record suggests that the increase in atmospheric CO₂ is driving a shift within tree communities to large‐statured species and that climate changes to date will impact forest composition, but long generation times of tropical trees mean that biodiversity change is lagging behind climate change.

Thermal sensitivity of tropical trees: A key uncertainty in climate change models is the thermal sensitivity of tropical forests and how this value might influence carbon fluxes. Sullivan et al. measured carbon stocks and fluxes in permanent forest plots distributed globally. This synthesis of plot networks across climatic and biogeographic gradients shows that forest thermal sensitivity is dominated by high daytime temperatures. This extreme condition depresses growth rates and shortens the time that carbon resides in the ecosystem by killing trees under hot, dry conditions. The effect of temperature is worse above 32°C, and a greater magnitude of climate change thus risks greater loss of tropical forest carbon stocks. Nevertheless, forest carbon stocks are likely to remain higher under moderate climate change if they are protected from direct impacts such as clearance, logging, or fires. Science , this issue p. 869

The tropical forest carbon sink is known to be drought sensitive, but it is unclear which forests are the most vulnerable to extreme events. Forests with hotter and drier baseline conditions may be protected by prior adaptation, or more vulnerable because they operate closer to physiological limits. Here we report that forests in drier South American climates experienced the greatest impacts of the 2015–2016 El Niño, indicating greater vulnerability to extreme temperatures and drought. The long-term, ground-measured tree-by-tree responses of 123 forest plots across tropical South America show that the biomass carbon sink ceased during the event with carbon balance becoming indistinguishable from zero (−0.02 ± 0.37 Mg C ha⁻¹ per year). However, intact tropical South American forests overall were no more sensitive to the extreme 2015–2016 El Niño than to previous less intense events, remaining a key defence against climate change as long as they are protected.

To provide an inter-continental overview of the floristics and biogeography of drought-adapted tropical vegetation formations, we compiled a dataset of inventory plots in South America (n=93), Africa (n=84), and Asia (n=92) from savannas (subject to fire), seasonally dry tropical forests (not generally subject to fire), and moist forests (no fire). We analysed floristic similarity across vegetation formations within and between continents. Our dataset strongly suggests that different formations tend to be strongly clustered floristically by continent, and that among continents, superficially similar vegetation formations (e.g. savannas) are floristically highly dissimilar. Neotropical moist forest, savanna and seasonally dry tropical forest are floristically distinct, but elsewhere there is no clear floristic division of savanna and seasonally dry tropical forest, though moist and dry formations are separate. We suggest that because of their propensity to burn, many formations termed “dry forest” in Africa and Asia are best considered as savannas. The floristic differentiation of similar vegetation formations from different continents suggests that cross-continental generalisations of the ecology, biology and conservation of savannas and seasonally dry tropical forests may be difficult. Spanish Con el objetivo de facilitar una visión intercontinental de la florística y la biogeografía de las formaciones vegetales tropicales adaptadas a la sequía, hemos recopilado un base de datos de parcelas de inventario forestal en América del Sur (n=93), África (n=84) y Asia (n=92) consistentes en sabanas (susceptibles a incendios forestales), bosques secos tropicales (generalmente no susceptibles a incendios forestales), y bosques húmedos (sin incendios forestales). Hemos analizado la similitud florística intracontinental e intercontinental entre formaciones de vegetación. Los resultados del análisis de nuestra base de datos sugieren que las diferentes formaciones tienden a estar fuertemente agrupadas florísticamente por continente, pero que, entre continentes, las formaciones vegetales que superficialmente parecen similares (p. ej. sabanas) son florísticamente muy diferentes. En el Neotrópico, el bosque húmedo, la sabana y el bosque seco son florísticamente distintos, y en otras regiones no existe una clara división florística entre la sabana y el bosque seco tropical, aunque si existe una separación entre las formaciones húmedas y las secas. Sugerimos que, debido a su propensión a los incendios forestales, sería mejor considerar como sabanas muchas formaciones clasificadas en África y Asia como “bosque seco”. La diferenciación florística de formaciones vegetales similares en continentes diferentes sugiere que las generalizaciones intercontinentales sobre la ecología, la biología y la conservación de las sabanas y los bosques secos tropicales pueden ser difíciles. French Pour donner une vue inter-continentale de la floristique et de la biogéographie des formations végétales tropicales adaptées à la sécheresse, nous avons compilé une base de données de parcelles d'inventaire en Amérique du Sud (n=93), Afrique (n=84) et Asie (n=92) des savanes (exposées au feu), des forêts tropicales saisonnières sèches (généralement non soumises au feu), et des forêts humides (sans risque de feu). Nous avons analysé la similitude floristique dans les formations végétales dans et entre des continents. Nos résultats suggèrent fortement que différentes formations ont tendance à être fortement groupées floristiquement par continent, et qu'entre les continents, les formations végétales superficiellement similaires (par exemple les savanes) sont floristiquement très dissemblables. Dans les Néotropics, les forêts tropicales humides, les savanes, et les forêts tropical saisonnières seches sont floristiquement distinctes, mais ailleurs il n'y a pas de division floristique claire entre la savane et la forêt tropicale saisonnière sèche, bien que les formations humides et sèches soient séparées. Nous suggérons qu'en raison de leur propension à brûler, des nombreuses formations appelées “forêt sèche” en Afrique et en Asie seraient mieux considérées comme des savanes. La différenciation floristique des formations végétales similaires de différents continents suggère que les généralisations transcontinentales de l'écologie, de la biologie et de la conservation des savanes et des forêts tropicales saisonnières sèches peut être difficile.

Tropical forests hold 30% of Earth’s terrestrial carbon and at least 60% of its terrestrial biodiversity, but forest loss and degradation are jeopardizing these ecosystems. Although the regrowth of secondary forests has the potential to offset some of the losses of carbon and biodiversity, it remains unclear if secondary regeneration will be affected by climate changes such as higher temperatures and more frequent extreme droughts. We used a data set of 10 repeated forest inventories spanning two decades (1999–2017) to investigate carbon and tree species recovery and how climate and landscape context influence carbon dynamics in an older secondary forest located in one of the oldest post‐Columbian agricultural frontiers in the Brazilian Amazon. Carbon accumulation averaged 1.08 Mg·ha⁻¹·yr⁻¹, and species richness was effectively constant over the studied period. Moreover, we provide evidence that secondary forests are vulnerable to drought stress: Carbon balance and growth rates were lower in drier periods. This contrasts with drought responses in primary forests, where changes in carbon dynamics are driven by increased stem mortality. These results highlight an important climate change–vegetation feedback, whereby the increasing dry‐season lengths being observed across parts of Amazonia may reduce the effectiveness of secondary forests in sequestering carbon and mitigating climate change. In addition, the current rate of forest regrowth in this region was low compared with previous pan‐tropical and Amazonian assessments—our secondary forests reached just 41.1% of the average carbon and 56% of the tree diversity in the nearest primary forests—suggesting that these areas are unlikely to return to their original levels on politically meaningful time scales.