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.

Land regions are warming rapidly. While in a warming world at extra-tropical latitudes vegetation adapted to higher temperatures may move in from lower latitudes this is not possible in the tropics. Thus, the limits of plant functioning will determine the nature and composition of future vegetation. The most temperature sensitive component of photosynthesis is photosystem II. Here we report the thermal safety margin (difference between photosystem II thermotolerance (T₅₀) and maximum leaf temperature) during the beginning of the dry season for four tree species co-occurring across the forest-savanna transition zone in Brazil, a region which has warmed particularly rapidly over the recent decades. The species selected are evergreen in forests but deciduous in savannas. We find that thermotolerance declines with growth temperature >40 °C for individuals in the savannas. Current maximum leaf temperatures exceed T₅₀ in some species and will exceed T₅₀ in a 2.5 °C warmer world in most species evaluated. Despite plasticity in leaf thermal traits to increase leaf cooling in hotter environments, the results show this is not sufficient to maintain a safe thermal safety margin in hotter savannas. Overall, the results suggest that tropical forests may become increasingly deciduous and savanna-like in the future.