THERMOS:Thermal Safety Margins of Earth's Tropical Forests

地球上的热带森林提供了一系列生态系统服务，拥有全球50%以上的生物多样性，吸收了每年8-13%的人为二氧化碳排放量，在大陆尺度上回收降雨，并直接为数百万人提供生计。维持这些服务的生物和生态过程（例如光合作用和蒸腾作用）对气候非常敏感，因此热带森林未来的大规模功能取决于将其气候空间保持在安全的操作范围内。目前我们不知道热带森林的安全工作温度限制是多少，也不知道它们离温度功能的上限有多近。这主要有三个原因：1)不同的植物过程受制于不同的温度阈值——例如，有光合作用的最佳温度，也有光合作用装置开始失效的温度；但是，大量的数据缺口使我们无法了解这些极限在热带森林和物种之间是如何变化的，即使对于那些我们知道关键生理功能（如光合作用机制的破坏）的温度阈值的物种，我们通常也没有叶温记录，使我们能够衡量热带树木离这些阈值有多近。叶片温度和空气温度之间的区别是这里的关键——叶片温度是生理上有意义的温度测量，它可能与空气温度有很大的不同。3)我们不知道叶片水平的温度耐受性指标对整个植物的生长和死亡性能意味着什么。目前尚不清楚叶片性状是否能预测热致死亡的风险。温度可以直接影响植物的性能（例如通过降低光合速率），但也可以通过增加空气和叶片之间的蒸汽压差（叶片对空气的蒸汽压差）间接影响植物的性能。较高的VPD增加了植物的水分损失，因为大气对水的需求增加，但也导致气孔导度和碳同化率降低。最近的研究表明，在一些温带和热带地区观察到的树木死亡模式的增加可能是由VPD增加引起的。然而，迄今为止还没有研究试图分离直接温度效应与间接VPD效应在诱导热应激导致死亡中的作用。THERMOS将解决这些当前的瓶颈，为热带森林的热风险提供前所未有的大规模见解。为此，将使用一套不同的互补方法，包括：1)在四大洲的热带森林中广泛收集现场数据，以确定关键植物过程的高温阈值；2)基于无人机的热成像，以确定不同地点达到的最高叶片温度；3)新的极端加热温室实验，以测试叶片热性状预测死亡率的能力，并评估直接与间接VPD效应在驱动死亡率方面的重要性。4)遥感以确定热带地区森林的热“安全”程度；5)森林动态记录分析以评估温度升高和VPD在导致热带森林死亡率上升中的作用。

Earth's tropical forests provide an array of ecosystem services, housing over 50% of global biodiversity, taking up 8-13% of annual anthropogenic CO2 emissions, recycling rainfall at continental scales and directly providing livelihoods to millions of people. The biological and ecological processes that sustain these services (e.g. photosynthesis and transpiration) are strongly climate-sensitive, such that the future large-scale functioning of tropical forests depends on keeping their climate space within safe operating limits. Currently we do not know what the safe operating temperature limits for tropical forests are nor how close they are to upper limits of temperature function. There are three main reasons for this:1) different plant processes are subject to different temperature thresholds - e.g. there are optimal temperatures for photosynthesis and also temperatures at which the photosynthetic apparatus begin to break down, but large data gaps prevent us from understanding how these limits vary across tropical forests and species2) even for species where we do know the temperature thresholds for key physiological functions (e.g. breakdown of photosynthesis machinery), we usually do not have the leaf temperature records that allow us to gauge how close tropical trees are to these thresholds. The distinction between leaf and air temperature is key here - leaf temperatures are the physiologically meaningful measure of temperature and can be substantially different to air temperatures3) we do not know what leaf-level metrics of temperature tolerance mean for the performance of the whole plant in terms of growth and mortality. It is unclear whether leaf traits can predict risk of heat-induced mortality. Temperature can affect plant performance directly (e.g. by reducing photosynthetic rate) but also indirectly by increasing the vapour pressure difference between the air and leaves (leaf-to-air vapour pressure deficit). Higher VPD increases plant water losses due to greater atmospheric demand for water but also results in reduced stomatal conductance and carbon assimilation rates. Recent studies have suggested that increasing tree mortality patterns observed in some temperate and tropical zones may be driven by increasing VPD. However, no study to date has sought to isolate the role of direct temperature effects vs. indirect VPD effects in inducing heat stress-driven mortality.THERMOS will address each of these current bottlenecks to deliver unprecedented large-scale insights into the thermal risk of tropical forests. To do this, a diverse set of complementary methodologies will be used including: 1) extensive field data collection in tropical forests in four continents to determine the high temperature thresholds of key plant processes, 2) drone-based thermal imaging to determine maximum leaf temperatures reached in different sites, 3) new extreme heating greenhouse experiments to test the ability of leaf thermal traits to predict mortality and to evaluate the importance of direct vs. indirect VPD effects in driving mortality, 4) remote sensing to determine how thermally 'safe' forests are across the Tropics and 5) analysis of forest dynamics records to evaluate the role of increasing temperature and VPD in driving increased mortality across tropical forests.