Dual modeling approach reveals: Need for assisted migration to mitigate loss of forest productivity in Europe

Tree species selection is a critical forest management decision, raising fundamental questions: which species will survive and thrive under future climates, where, and to what extent? To address these questions, we applied a dual-model approach that combines 23 site index models with species distribution models. Drawing on a pan-European dataset of six million individual trees, the models project top height (site index) and climatic suitability (species distribution) for a reference period (1981-2010) and two future periods (2041-2070 and 2071-2100) under RCP4.5 and RCP8.5 scenarios. By integrating site index predictions with low-risk species distribution classifications, we restricted growth projections to climatically suitable ranges derived from Species distribution models (SDMs). Our analysis examined four key aspects: (1) overall changes in forest productivity, (2) regional gains and losses for each species across five major European regions, (3) the most productive species by region, and (4) species with the greatest productivity gains ("winners"). Results reveal contrasting trends: productivity is projected to increase in northern Europe but decline in southern and central regions. Beneficiary species include Douglas-fir, Norway spruce, and European larch in the north, and downy oak and black locust in the south. These findings point to potential management-relevant patterns, suggesting that non-native species may become increasingly suitable in northern regions, while southern forests may require a broader portfolio of adaptation and mitigation strategies to cope with projected productivity declines.