Quantifying the 2022-2023 altitudinal range-wide tree mass-mortality event and probability of forest loss in Central Chile

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The Mediterranean-type ecosystems of central Chile are highly biodiverse and rich in species that occur nowhere else, furthermore, they represent the entire Mediterranean biome of South America. However, Chile's Mediterranean-type ecosystems are threatened due to habitat loss and degradation due to conversion for agriculture, grazing and urbanisation, habitat fragmentation and forest fires. The escalating impacts of the climate crisis now represent a critical threat to their survival. The Chilean summer of 2022-2023 has seen sudden, severe and extensive forest mortality in central Chile. The region is experiencing a period of prolonged drought dubbed the 'Mega Drought' (MD), where precipitation has been at least 25% lower than usual since 2010. Two exceptionally dry years occurred in 2019 and 2021, with precipitation some 80% lower than average, followed by a 50% reduction in 2022. By the end of summer 2023 widespread tree death and forest ecosystem collapse was apparent. Our understanding of where drought impacts should be felt first across the geographic distribution of a species suggests that we should see these impacts concentrated in the already hotter and drier parts of species ranges. However, in the Mediterranean climate region of Central Chile we are now seeing forests dying right across their natural distribution in mountain regions - with even those occurring in cooler locations higher in the mountains succumbing to drought-driven death. Consequently, forest mortality is witnessed across the higher elevation forests typified by the tree 'Roble de Santiago" (Santiago Oak) as well as those lower forests typified by the peumo tree (the Chilean acorn) which is usually much more drought resistant. Neither of these dominant forest- forming trees has long-lived seeds. Consequently, there is a serious risk that as the adults die on such large scale, there will be very little potential for trees to regenerate. The risk is that forest will rapidly be replaced by shrubland ecosystems which are smaller in stature, store much less carbon, are highly flammable and with very different associated biodiversity. This exceptionally widespread forest mass mortality event in Chile presents an unprecedented opportunity to help us understand the pattern, process and implications of forest ecosystem collapse. Such an opportunity is highly rare and exceptionally valuable to help us better understand the risks to our forests at the global scale. In this project, we will conduct a detailed survey of the size and distribution of dead trees, any tree regeneration that we find from seeds and shoots and similar data from shrubs. We'll also survey the seed bank to discover which species are most likely to regenerate from seed. We'll use temperature and moisture sensors throughout the forest to understand small-scale variation in the climate that the trees are experiencing and link this to regeneration and the occasions where we find tree survival. As well as the plot-level data, we will access a detailed digital landscape model and survey the tree canopy using a drone mounted camera and unite these images with the field survey data so we can understand stand and canopy structure from above and below. This drone-based data will also enable us to scale up to remote sensing data from satellites so that we can understand the mortality extent and impacts at much larger spatial scales. In combination, the data will enable us to understand the extent and impacts of mortality on the forest itself, the potential for forest regeneration and the balance between tree and shrub survival and regeneration across the landscape. It will help us to understand where and why forests are dying - and what vegetation will remain after the trees die, enabling us to better plan for the impacts of climate change and to quantify what consequences forest loss will have for local and global models of carbon uptake and storage by trees.