Tropical forests are approaching critical temperature thresholds

The critical temperature beyond which photosynthetic machinery in tropical trees begins to fail averages ~46.7°C (Tcrit) 1. However, it remains unclear whether leaf temperatures experienced by tropical vegetation approach this threshold or soon will under climate change. We found that pantropical canopy temperatures independently triangulated from individual leaf thermocouples, pyrgeometers, and remote sensing (ECOSTRESS) have midday-peak temperatures of ~34°C during dry periods, with a long high-temperature tail that can exceed 40°C. Leaf thermocouple data from multiple sites across the tropics suggest that even within pixels of moderate temperatures, upper-canopy leaves exceed Tcrit 0.01% of the time. Further, upper-canopy leaf warming experiments (+2, 3, and 4°C in Brazil, Puerto Rico, and Australia) increased leaf temperatures non-linearly with peak leaf temperatures exceeding Tcrit 1.3% of the time (11% >43.5°C, 0.3% >49.9°C). Using an empirical model incorporating these dynamics (validated with warming experiment data), we found that tropical forests can withstand up to a 3.9 ± 0.5 °C increase in air temperatures before a potential collapse in metabolic function, but the remaining uncertainty in our understanding of Tcrit could reduce this to 2.6 ± 0.6°C. The 4.0°C estimate is within the “worst case scenario” (RCP-8.5) of climate change predictions2 for tropical forests and therefore it is still within our power to decide (e.g., by not taking the RCP 8.5 route) the fate of these critical realms of carbon, water, and biodiversity 3,4.

热带树木光合机制的临界失效温度（T_crit）平均约为46.7℃[1]。然而目前仍不清楚，热带植被所经历的叶片温度是否已接近该阈值，或是在气候变化背景下即将逼近该临界值。我们通过三种独立手段——单个叶片热电偶、辐射热计以及遥感（ECOSTRESS）——对泛热带冠层温度进行三角测量，结果显示旱季时段的午间峰值温度约为34℃，同时存在一条可超过40℃的高温长尾分布。来自热带多个站点的叶片热电偶数据表明，即便在温度适中的遥感像元内，冠层上部叶片仍有0.01%的时间会超过临界失效温度。进一步的冠层上部叶片增温实验（在巴西、波多黎各与澳大利亚分别开展2℃、3℃与4℃的增温处理）显示，叶片温度呈非线性升高，峰值温度超过临界失效温度的时长占比达1.3%（其中11%的峰值超过43.5℃，0.3%超过49.9℃）。基于整合了上述动态过程的经验模型（已通过增温实验数据验证），我们发现热带森林在代谢功能出现潜在崩溃前，可承受的气温升高上限为3.9±0.5℃；但目前对临界失效温度的认知尚存不确定性，该耐受上限可能下调至2.6±0.6℃。4.0℃的升温阈值处于热带森林气候变化预测的"最坏情景"（RCP-8.5）范畴内[2]，因此我们仍有能力决定这些关乎碳循环、水循环与生物多样性的关键生态系统的命运[3,4]——例如通过规避RCP 8.5路径。