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.

陆地地区正在迅速变暖。虽然在热带外纬度变暖的世界中，适应更高温度的植被可能会从低纬度地区移入，这在热带地区是不可能的。因此，植物功能的限制将决定未来植被的性质和组成。光合作用对温度最敏感的成分是光系统 II。在这里，我们报告了热安全裕度（光系统 II 耐热性之间的差异（T ₅₀) 和最高叶温) 在旱季开始时，四种树种在巴西的森林-稀树草原过渡区同时出现，该地区近几十年来变暖特别迅速。所选择的物种在森林中是常绿的，但在稀树草原中是落叶的。我们发现热带稀树草原中个体的耐热性随着生长温度> 40 C而下降。当前的最高叶片温度在某些物种中超过 T ₅₀并将超过 T ₅₀在大多数被评估的物种中，世界温度升高了 2.5 摄氏度。尽管叶片热性状具有可塑性以在较热的环境中增加叶片冷却，但结果表明这不足以在较热的稀树草原中保持安全的热安全裕度。总体而言，结果表明热带森林在未来可能会变得越来越落叶和稀树草原。