Greater Thermophilisation in Communities of Juveniles Than Adults in Temperate European Forests

ABSTRACT Aim Warming of the climate system is increasing the dominance of warm‐adapted species, a process referred to as thermophilisation. In forests, adult trees in the forest canopy experience warmer daytime summer conditions than juveniles in the understorey, but they are less susceptible to warming. These differences can result in differing thermophilisation rates between adults and juveniles. Here, we quantify for the first time the long‐term thermophilisation of tree species in temperate European forests, comparing adults with juveniles. Location Europe. Methods We calculated the thermophilisation of adults and juveniles of forest tree species using layer‐specific climatic data, species cover and occurrence data from 2202 resurveyed vegetation plots recorded twice between 1933 and 2017 and located across 12 European countries. We inferred species' thermal profiles from species distribution maps matched to gridded open‐air temperature (proxy for above‐canopy macroclimate) for adults and to below‐canopy microclimate temperature for juveniles. Results The thermophilisation rate in the juvenile layer was seven times higher than in the adult tree layer. The thermophilisation rates of both adults and juveniles were primarily driven by gains in relatively warm‐adapted species and a concurrent, but less strong, decrease in cold‐adapted species. Conclusion The compositional change in favour of more warm‐adapted species was mainly driven by gains in warm‐adapted species. The magnitude of these responses and the influencing variables were different in the community of adults and juveniles. These results underpin the importance of separately quantifying the responses of individuals throughout their life cycle to improve our ability to understand the impacts of environmental change on forest biodiversity and composition and apply targeted management actions.