Widespread expansion of tree cover compromises European biodiversity capacity

Expanding tree cover is widely seen as a nature-based solution for climate mitigation and biodiversity conservation. However, widespread increases in tree cover can unintentionally reduce habitat heterogeneity or replace open ecosystems, thereby posing risks to biodiversity. Here, we developed a framework integrating measures of tree cover, connectivity, and heterogeneity, and quantified the dynamics in these measures in Europe from 2001 to 2021. We also explored potential drivers of tree dynamics, including land management, natural disturbance, climate, and human activities. We found that 23% of the European land area experienced widespread increases in tree cover and connectivity, accompanied by a decline in heterogeneity. Importantly, former cropland areas were linked to considerable tree-cover expansion and improved tree connectivity. Our findings indicate widespread densification and homogenization of vegetation, which will compromise the capacity of European ecosystems to support biodiversity. Restoring natural disturbance processes, including herbivory and fire, is essential to fully capitalize on the biodiversity potential of nature recovery in Europe.