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 species 2) 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 temperatures 3) 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.

The Southern Amazon faces the greatest climatic threat of all Amazon regions. This region is drier and warmer than 'core' areas of the Amazon and has been subject to the most pronounced drying and warming trends. It is also the region of the Amazon where increases in tree mortality have been most marked and where atmospheric measurements suggest forests are no longer acting as a carbon sink but as a net source of carbon to the atmosphere. Given that Southern Amazon is at the front line of the Amazon's battle against climate change, it is essential that we better understand how resistant its forest species are to climate stress. In Lethal Psi, we will construct a new 1-hectare drought experiment to better understand the physiological survival limits of southern Amazon trees. It has become increasingly clear that the process of hydraulic failure plays an important role in drought-induced tree mortality. Water is transported from the soils to the canopy under tension. As drought ensues and the soil dries, the tension in the xylem vessels that transport water intensifies and this can lead to the formation of air bubbles (embolism) in xylem vessels, disrupting water transport to the canopy and ultimately resulting in tree death. While this process is understood in general terms, one critical current knowledge gap is that we don't know the thresholds in embolism formation that result in the death of tropical trees. This lack of understanding of the physiological thresholds that result in death constitutes a key uncertainty for accurately modelling tree mortality under climate change. Determining the hydraulic thresholds of tree death is not an easy task and requires monitoring tree hydraulic status up to the point of death. In Lethal Psi, we track key indicators of hydraulic function (e.g. leaf water potentials and sap flux) from the beginning of our imposed drought all the way to the death of the tree to quantify how loss of xylem conductance translates into mortality risk. While other drought experiments have been set up in Amazonia, these did not monitor embolism status before and during the mortality process and were thus unable to provide insights into physiological thresholds of survival. Up to now, drought experiments have only been set up northeastern Amazonia, where annual rainfall is almost twice that of our study site and where changes in climate have been much less pronounced than in southern Amazonia. Given their ecotonal nature and the rapid climate change experienced in southern Amazonia, we expect that trees in this region are much closer to their climatic limits and will experience much more accentuated mortality under imposed drought than observed in northeastern experiments. Ultimately, we plan to use the newly acquired field data to develop improved mortality functions that we will apply more broadly across southern Amazonia to better predict drought mortality risk of this critically important region. This will be done by updating a unique trait-based model specifically developed to simulate Amazon forests and their responses to environmental change.

SUMMARY The Amazon is the most important biome of South America, harbouring extraordinarily high levels of biodiversity and providing important ecosystems services. This biome is particularly notable for evolving independently from fire and in a moist, warm climate. In recent decades, altered fire regimes and an increasingly hotter and drier climate has pushed this key biome towards ecological thresholds that will likely lead to major losses in biodiversity and ecosystem services. Similarly, the ecotonal forests at the Amazon-Cerrado transition are unique ecosystems in terms of form and function, but they may be the first to suffer large-scale tree mortality and species loss due to the combined effects of increased anthropogenic disturbance, altered fire regimes and a drier climate. Vulnerability of fire and droughts are closely intertwined in Amazonian and transitional forests because fires in this region only occur when there is water stress and a human ignition source. Thus, drought increases vulnerability to fire, but we do not yet understand the magnitude and spatial variation of these vulnerabilities. Once a forest burns there is immediate tree mortality, but recent evidence also shows a significant time-lagged mortality that can last for decades, becoming an important carbon source. However, the mechanistic processes that lead to time-lagged tree mortality in this myriad of forest ecosystems encompassing the Amazon biome and the Amazon-Cerrado transition are still poorly understood. We also lack knowledge on how these processes might vary spatially across the biome and its transition. A better understanding of the mechanisms that lead to tree mortality after fires and droughts is needed to design future policies that emphasise nature-based solutions including restoration and natural regeneration. This proposal presents a multi-level approach that aims at deciphering the mechanisms that underly vulnerability to fire and time-lagged post-fire mortality across the tropical forests in Amazon and Amazon-Cerrado transition. To achieve this aim, we will quantify fire vulnerability at three different scales and link them through an upscaling approach. First, we will identify the ecological mechanisms, reflected through functional traits, that explain why individuals and species die after fires occur. For this, we will focus on poorly understood traits that can be related to fire and/or hydraulic functioning. Second, at the community scale, we will examine how vegetation structure, community traits and microclimate affect the probability to burn, through an intensive characterisation of different vegetation types with multispectral and light detection and ranging (LIDAR) imagery. Third, we will use our our unique ground-dataset on functional traits, vegetation structure and moisture dynamics, and the latest state-of-art remotely sensed information on structure and water stress to predict the vulnerability of the Amazon forests and Amazon-Cerrado transitional forests. This information will be directly applicable for the detection of sensitive hotspots (areas particularly vulnerable to fire) through satellite products. We will deliver quantifiable early-warning metrics of ecosystem vulnerability to fire that can be mapped and incorporated into fire management policies. This is a revised version of a NERC proposal that was rejected with a score of 7 by the NERC Panel in July 2020, and we have carefully addressed the Panel's comments. Specifically, we have clarified the methodology and we have reformulated the hypotheses, so they address vulnerability to fire and not drought fire-interactions.

Wildfires are becoming the new normal across Amazonia. Deforestation is transforming the region at a rate of around 10,000 square km/year (half the area of Wales), and now the area degraded annually -forest logged and burned but not cut down-is greater than the area deforested. Fire has historically been rare in Amazonia, meaning that the forests are not adapted to fire and the trees often die from fires - releasing carbon (C) back to the atmosphere and amplifying global climate change. Burning of tropical forests is already releasing more climate-warming carbon dioxide than fossil fuel burning in the whole of Europe. Trees in Amazonia contain around 7x more C than humans are releasing every year, and soils contain the same amount again, so it is vital to understand what is happening to this C and minimize emissions. As vegetation sheds its leaves, branches, and roots, or dies, some of the C released remains in the soil, and some is later decomposed and released back to the atmosphere. Carbon exists in the soil in many different forms, from new inputs from decomposing plant material to ancient C formed over millennia. Burning adds pyrogenic carbon (PyC) to the soil, a partially burnt form of C that is resistant to decomposition and could make the soil more fertile. Because soil C takes a long time to form, its conservation is particularly important. Despite the widespread increase in fire in Amazonia, there have been few measurements of soil C fractions and dynamics in burned areas - most have focussed on natural forests. Burned forests will have different composition, forest structure, and C dynamics. Understanding how different soil C fractions are formed and lost is crucial to understand how fire and climate change affect C storage. We propose to make major advances in understanding fire impacts, including the processes that affect the type and quantifies of soil C formed, and how C gains/losses vary over time, with soil type, and climate. We will combine new measurements with innovative modelling to inform land management strategies and C budgets. We have already collected data from across Amazonia in intact forests that have not recently burned. Crucially our project will collect a new, comprehensive dataset from human-modified forests, including logged, burned and abandoned land. We will use an approach known as a chronosequence, where we take samples at sites that were burnt at different times in the past, so we can see how the soil C has changed after e.g. 1 year, 2 years, or up to 20 years after a fire. This will then be used to develop a state-of-the-art land surface model, JULES, which forms part of the UK Earth System Model. At our sample sites, we will evaluate how different burn severities affect soil C, both in surface and deep soils, and how these change over time post-burning and with soil, climate, and land-use such as logging. At 3 focal sites, we will take detailed measurements of the decomposition rate of the C over 4 years, comparing measurements with different land-use, burn severity and wet vs dry seasons. Knowing what forms C takes after a fire and how fast it decomposes under different conditions will enable us to build these processes into the JULES model. We will model PyC globally for the first time and make projections of land C changes in Amazonia over the next ~40-60 years under different management practices. As well as transforming scientific understanding of post-fire soil C and its resilience to climate and management, our project will inform socio-environmental planning for sustainable resource use to conserve soil C. We will work with regional partners, fire managers, state and national policymakers to integrate our findings into decision-making to minimise negative fire impacts. Due to the Amazon Basin-scale of our work, these strategies are a crucial step to limit the risk of large-scale loss of soil C.

Forests are a critical component of the global carbon cycle because they take carbon dioxide out of the atmosphere through photosynthesis, and store the carbon in wood and soil. All living things in forests also produce carbon dioxide through respiration as an inevitable consequence of sustaining themselves and growing. At present, forests take in more carbon dioxide than they release, helping to reduce the amount of carbon dioxide present in the atmosphere, but this 'free gift' from forests is not guaranteed to continue at its current rate indefinitely under climate change. As well as the carbon cycle, forests are also crucial in the water cycle as trees pump water from the soil into the atmosphere. Leaves are the key part of the plant that regulates the exchange of gases (water, carbon dioxide) with the atmosphere. The pores in the leaf surface (stomata) are important for water loss and temperature control as well as the entry of carbon dioxide. Leaves exposed to direct sunlight can be more than ten degrees hotter than the air, even in temperate latitudes. Leaf temperature is important because many biological processes, including photosynthesis and respiration, are sensitive to temperature; very high temperatures can cause immediate and acute damage to leaves. Over the coming century, we expect carbon dioxide concentrations and air temperatures to continue to rise. When trees are grown in higher carbon dioxide concentrations, stomata close and limit water loss; this prevents the plant dehydrating but also reduces how much leaves can cool down. However, there is limited monitoring on forest canopy temperatures, and limiting understanding on how different species and forests in different climate zones are responding to climate change. This project will build a global network of researchers working to measure forest canopy temperatures using thermal infrared cameras, which will provide both greater understanding and also a crucial data resource for scientists in other disciplines to utilise. The network will ensure that the data collected by separate groups are comparable, and aid data processing and analysis by providing clear guidance and tools. This is will encourage other researchers to take up use of thermal infrared cameras, the analysis of which can be challenging. Our network will monitor canopy temperatures at fourteen sites in tropical and temperate forests and savannah, in UK, China, India, Australia, Brazil, Peru, Panama, USA, and Ghana. The sites in the UK and Peru will be newly established by this project. Ten sites already have established data collection, while the final two sites (Australia, Ghana) are in development. Having data collected using cameras will allow us to understand not only how forests in different locations are behaving, but also whether and how different species within sites respond. The long-term nature of the project means that seasonal variation will be included, and the forest response to extreme events such as heat waves and droughts will be quantified. Future work will establish in more detail how changes to canopy temperature link to changes in forest carbon and water cycling. Our work providing insight into the response of forest canopies to climate change will inform models produced to assess the impacts of greenhouse gas emissions on the planet, which are used to inform global climate change policies. Further, the current global emphasis on mitigating climate change through tree planting makes it crucial to assess how these trees will cope under future conditions.