The tropics are warming and the frequency of extreme heat events, often accompanied by drought, is increasing across most of the tropical forest biome. It is currently unclear what the effects of increasing heat on tropical forests will be. This key question is the focus of the current IOF proposal, based on three integrated strands of NERC research in Amazonia, which we lead and propose to pilot in India. The approach consists firstly of a targeted real-time observation program at a forest site at the Southern border of the Amazon humid forests, as part of the NERC BIO-RED consortium, co-led by Gloor and Phillips. Real-time observations of forest performance rely heavily on cameras overlooking the canopies, which measure canopy temperatures, measures of productivity performance and stress, and phenology. To characterize the climate forcing, we measure continuously climate and soil humidity. In order to understand observed patterns of tree performance responses to heat extremes, we measure separately traits of the site's dominant tree species related to tree hydraulics, as well as productivity. Secondly, as part of another ongoing NERC grant (TREMOR, led by DG) we are measuring tree hydraulic properties of dominant trees at 10 sites distributed across the Amazon. Knowledge of these characteristics across wide areas permits us to generalize mechanistic results measured with the in-situ monitoring approach. Finally, in both ongoing and past grants OP has developed a tropical forest plot-monitoring network in Amazonia, Africa, and Borneo (~1000 1-ha plots now), capable of tracking longer-term shifts in forest biomass, productivity, and composition. We propose here to work with leading Indian scientists to apply these approaches in this critical region. In large parts of tropical India heat waves have increased considerably in recent years with peak temperatures reaching up to 50C. Model projections suggest that up to 45% of Indian forests may be at risk of shifting to non-forest vegetation states, yet there are only very limited data to evaluate these projections. India lacks both a comprehensive observational system as at our Amazon site, and has relatively few permanent plots, and those that do exist are mostly not integrated into international forest monitoring networks. To address these challenges we have formed new connections with key experts in India covering the areas of forest ecology, eco-physiology, and climatology. Between them our new Indian collaborators are strongly linked to national and international forest conservation efforts, and lead most available forest plots. The scientific focus of this proposal, the extensive, biodiverse and potentially climate-sensitive evergreen forests in Western Ghats, is where the team's interests coincide geographically. We propose to jointly install a canopy-overlooking continuous forest heat and drought-response monitoring site, as in S Amazonia, close to existing plots in the Western Ghats. Together we plan a site-level traits campaign of dominant species and local integration of plot- and canopy-observation monitoring. We further propose to harmonize protocols of plot censuses and to include Indian plot data in the pan-tropical forest census database to support larger scale geographical analyses and syntheses. I-FOR will also aim to support mutual exchange of skills, focused in three steps. The first, in the Western Ghats, is a workshop dedicated to student and young scientist education in field skills and protocols. Secondly, we plan several visits of Indian colleagues to Leeds to support joint analyses and post-project planning. The final workshop, to be hold at one of the Indian scientist's home institution, will include wider participation to discuss implications of the results and to take practical steps toward ensuring these activities become long-term efforts.

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.

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.