Understanding the where, when, and why of forest mortality in Mediterranean Chile

The Mediterranean-type ecosystems of central Chile are highly biodiverse and rich in species that occur nowhere else and 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 has experienced 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. The drought broke over the 2023

southern hemisphere winter, enabling us to now make a clear assessment of mortality and regeneration patterns. 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 a 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.