Functional traits explain growth response to successive hotter droughts across a wide set of common and future tree species in Europe

In many regions worldwide, forests increasingly suffer from droughts. The 'hotter drought' in Europe in 2018, and the consecutive drought years 2019 and 2020 caused large-scale growth declines and forest dieback. We investigated whether tree growth responses to the 2018-2020 drought can be explained by tree functional traits related to drought tolerance, growth and resource acquisition. We assessed the growth response, that is, growth during drought compared to pre-drought conditions of 71 planted tree species, using branch shoot increments. We used gap-filled trait data related to drought tolerance (P50, stomatal density, conductivity), resource acquisition (SLA, LNC, C:N, A_max) and wood density from the TRY database to explain growth responses, while accounting for differences in growth programmes (spring vs. full-season growing species). We found significantly reduced growth during the 2018 drought across all species. Legacy effects further reduced growth in 2019 and 2020. Gymnosperms showed decreasing growth with increasing P50 and acquisitiveness, such as high SLA, LNC, and A_max. Similar results were found for angiosperms, however, with a less clear pattern. Four distinct response types emerged: 'Sufferer', 'Late sufferer', 'Recoverer' and 'Resister', with gymnosperms predominately appearing as 'Sufferer' and 'Late sufferer'. 'Late sufferers' tended to be spring growing species. This study provides evidence for significant growth reductions and legacy effects in response to consecutive hotter droughts, which can be explained by functional traits across a wide range of tree species when accounting for fundamental growth programmes. We conclude that high drought tolerance bolsters growth reductions, while acquisitive species suffer more from drought.