This project addresses a key gap in understanding how tropical forests respond to drought across scales, from organ to tree and forest ecosystem. It will drive extended impact in new monitoring capability using satellite data, in advanced land surface modelling, and in drought risk mitigation planning, by engaging related stakeholders through 'Science & Impact' workshops. We propose the powerful combination of a unique large-scale field experiment in Amazônia together with detailed ecophysiological and new tower-based radar measurements to deliver new insights into drought responses across scales, both during drought, and importantly, during post-drought recovery. Water availability plays a dominant role in the global carbon cycle, with a large influence from Amazônia. However, our ability to predict the effects of changing water availability is substantially constrained by limited understanding of the ecological processes occurring in response to drought, particularly in tropical forests. These responses occur across different scales, from leaf to tree to forest ecosystem, with very large impacts on the carbon cycle observed regionally and globally. Understanding drought responses of tropical forests has proved challenging for several reasons: a lack of ecophysiological analysis at the right scales; limited capacity to deliver continuous monitoring of mechanistically-informed water stress responses at large scale, eg using satellites; and limited understanding of the ecological processes comprising drought stress and its consequences. We ask: How does drought stress affect whole-tree function, and can critical processes such as transpiration and growth recover after drought in tropical forests? Does drought stress leave a long-term legacy by limiting growth potential and by increasing the risk of possible tree mortality from future drought? And critically, how do the effects of drought on tree function affect performance at the scale of many trees, ie, that of a tropical forest? Multi-scale measurements are needed to address these questions. A combination of focused ecophysiological measurement with new tower-based radar (microwave) observations has the potential to enable large advances in understanding, scaling from tree to forest and region. This project will combine the world's only long-term drought experiment at hectare scale in tropical forest, which we have run for the past twenty years, with new radar sensors. We will use tower-based radar measurements to detect changes in vegetation water content at the scale of the experiment. This will provide higher resolution detection and mechanistic insight than was previously possible using satellite radars, and allow us to connect radar and plant ecophysiological data. Our specific hypotheses address: the links between organ-, tree- and ecosystem-scale responses to drought, and after drought; how these data advance our understanding of forest function and the risk to function and survival; and how this understanding can be used to advance satellite monitoring of drought impacts, and its wider use. In summary, we have three main goals: i) To use our ecosystem-scale drought experiment in Amazônian forest to quantify and understand the effects of drought at multiple scales, using plant physiology and tower-based radar (microwave) measurements. ii) To understand post-drought legacy effects on forest resilience by using the control enabled by our experiment to halt the drought and monitor recovery processes, and the outcomes for growth and survival. iii) To use (i) and (ii) to advance large-scale satellite detection capability in tropical forests for improved biomass and drought-response monitoring. We will lead two 'Science and Impact' workshops to rapidly multiply outcomes of the work by helping to improve prediction of land-atmosphere interactions using vegetation models, and better early-warning capability for land-use planning.

The overall aim of this project is to determine and communicate the risk of significant change to the Amazon rainforest caused by anthropogenic disturbance and climate change. We will address a fundamental issue of our time, on the likelihood of Amazon rainforest dieback in the 21st century and identify regions that are most susceptible. We will combine this new knowledge with policies and scenarios developed by key stakeholders to co-design a Safe-Operating-Space for Amazonia. To address the iconic issue of Amazon dieback we will advance new ecological understanding of how forests grow, decline and recover following disturbance from climate extremes, forest fire and deforestation and their interaction in the context of 21st Century global warming. We will build novel datasets using a new forest plot network, drones and satellites to produce near-real-time maps of the risk to forests from climate, and track individual large-tree mortality across the basin. Together this information will be used in mathematical models to help estimate the risk of future forest dieback. We will join this work with models used to predict the effects of land use (forest conversion, degradation) on forest function, and the ecosystem services these forests provide to humanity. The outputs will enable us to deliver new information to policy makers regarding future options for land use, helping them to build optimal land use pathways that minimise the risks that may arise out of large-scale forest loss or dysfunction in Amazonia. The Amazon forest plays a vital role in the world's climate. In addition, by annually absorbing 5-10% of human-related CO2 emissions via vegetation growth, the region acts as a large brake on climate change. Climate extremes (eg drought), forest fires and deforestation reverse this process, causing net emissions to the atmosphere. If this were to happen on a large enough scale, via increased forest loss or increased rates of climate change - or their interaction - the resulting positive effect on global CO2 and climate change, would make the already-challenging Paris climate targets virtually impossible. In short, climate change, forest fires and deforestation have been identified as major intensifying and interacting threats to Amazonia. A substantive loss of Amazonian forest, also known as "Amazon dieback", would have huge negative consequences for human well-being, biodiversity, biogeochemical cycling, and regional and global climate. However, the level of global climate change combined with human disturbance that could trigger large-scale dieback is not known. Climate change is predicted to become more intense in the region alongside increases in human-driven deforestation and forest degradation (e.g fires, logging). Their impacts are poorly understood because of a lack of data, and because models cannot currently represent the key processes well enough. We have gathered leading UK and S American scientists in the fields of ecology, ecophysiology, Earth observation (using satellites) and the mathematical modelling of vegetation growth, land-use and climate as applied to Amazonia. We are uniquely positioned to make a step-change in understanding the combined effects of climate stress and human disturbance on Amazonia. Our measurements will build new knowledge about intact and disturbed forests, their stability and the physiology driving their stress responses. These knowledge advances will enable new modelling of forest-climate-land-use interactions which we will use to inform policymakers. We will engage with stakeholders from state to international levels to co-develop land-use scenarios that minimise risk in future climate and forest ecosystem services. Overall, we propose multiple large and integrated advances in empirical and modelling studies of the forests of Amazonia, and will build a science-policy dialogue that delivers significant impact locally, regionally and globally.