Evaluating the ecosystem-friendliness and suitability of potential future autochthonous tree species for ecosystem-based restoration

Introduction With Regulation (EU) 2024/1991 on nature restoration (NRR), the European Union has created a binding framework to restore damaged ecosystems to good condition and halt the loss of biodiversity. A central objective of the NRR is the restoration of forest ecosystems, with the proportion of forests dominated by native tree species being defined as an indicator of success. However, the definition of a “native tree species” according to the NRR - as a species that occurs in its natural, former, or potential range which could be achieved without direct human introduction - raises practical and scientific questions of interpretation. Methods Against this background, this study develops and applies a multi-criteria evaluation framework to assess the ecological suitability of tree species that could potentially reach Germany through natural migration. The framework integrates three core dimensions: A. biogeographic proximity, B. phylogenetic proximity to the autochthonous flora, and C. ecosystem friendliness. Four main criteria were established for the latter, based on the species’ contributions to the functionality and adaptive capacity of forest ecosystems: 1. habitat function, 2. soil ecological effects, 3. microclimatic regulation and 4. promotion of resistance and adaptive resilience. These components were combined into a Tree Suitability Index (TSI) used to classify species according to their potential compatibility with German forest ecosystems. Results A total of 100 tree species were evaluated, including 17 archaeophytes and 83 neophytes from adjacent biogeographic regions, which could potentially migrate to Germany (for which the new term “metecophytes” is proposed). Several showed high potential for integration into Central European forest ecosystems, most notably Fagus orientalis (TSI 11), the only species classified as very well suited, together with well-suited species such as Fraxinus angustifolia , Quercus frainetto, and Tilia tomentosa . Sensitivity analyses indicated that soil-related processes, microbial interactions, and canopy-mediated microclimate regulation were the most influential determinants of ecosystem friendliness. Discussion The proposed framework provides a transparent and adaptable tool for evaluating potential future tree species under climate change and contributes to evidence-based discussions on the future species composition of forests.