Climate change threatens carbon storage in Europe's urban trees

Urban trees contribute substantially to numerous ecosystem services. Here we quantify the threat to carbon stored by urban trees from increased heat and drought arising from climate change. We use data from tree inventories in 22 European cities, spread across five Koppen-Geiger climatic zones, that record similar to 1.2 million trees from 188 species. We calculate species' climatic niches using global tree distribution data and estimate speciesspecific thermal and hydraulic safety gaps and margins for each city in 2050 and 2070 using the RCP 8.5 emissions scenario. This scenario provides the best match for emissions to at least 2050 under current and stated policy plans, and highly plausible emission levels to 2100. We then assess the proportion of current carbon storage at risk from changes in temperature (associated with thermal stress) and precipitation changes (associated with hydraulic stress). By 2070 a substantial amount of the current carbon storage in urban trees is projected to be threatened by climatic stress. Average values (depending on the precise methods used for calculating climatic niches) are: 99.96 % - 99.98 % in the cold semi-arid climate zone; 82.97 % - 92.61 % in the humid subtropical zone, 69.72 % - 72.00 % in the warm Mediterranean zone, 44.18 % - 55.06 % in the humid continental zone and 29.60 % - 43.22 % in the temperate oceanic zone - although within each climatic zone risks are lower in some cities. In each climatic zone the vast majority of this threat is associated with thermal stress, with precipitation changes projected to be a comparatively minor threat. Our analyses highlight individual species which are particularly vulnerable to future climatic conditions, and more resilient species that if rapidly planted on mass could improve resilience of urban tree stocks to climate change. Our findings inform the development of climate-ready urban forestry and planning strategies that will facilitate long term carbon storage capacity of Europe's urban forests, and emphasise the urgency of doing so.