No clear second choice to long-term sustainability: climate and land-use change impacts on tree distributions in Southeast Asia

Abstract Future biodiversity outcomes are expect to vary across Shared Socioeconomic Pathways (SSPs) of climate and land-use change. We modelled 1476 tree species distributions in Southeast Asia, classified them into 12 spatially associated groups, and projected distributional changes under four SSPs. The sustainable pathway, SSP1-2.6, led to the best outcomes for tree distributions. However, there was no clear second-best pathway for both climate and biodiversity. Distribution outcomes were generally worst for intermediate pathways of climate mitigation because they involved greater land-use change, while the second-best distribution outcome was under the least-sustainable climate pathway. Furthermore, the second-best pathway varied starkly across species groups, such that the second-best pathway for one group was also the worst for another. We demonstrate how halfway measures to mitigate climate change can lead to worse biodiversity outcomes, and that deviating from the sustainable pathway will involve trade-offs. We emphasise that policymakers must prioritise the most sustainable pathway.