Regionalization of European Habitat Classification ( EUNIS ) for Effective Ecosystem Assessment

Aims We develop and apply a regionalization framework for the European Nature Information System (EUNIS) habitat classification. Our approach integrates a continental‐scale classification of broad habitat categories with a data‐driven grouping of lower hierarchical units based on regional habitat descriptions. We test the applicability of the regionalized classification for ecological assessment and conservation applications. Location Ibero‐Atlantic region (N Portugal, NW Spain, and SW France). Methods We first classified 21,935 vegetation plots into broad habitat types (EUNIS levels 1–2) using a preexisting expert system algorithm based on formal definitions. The classified plots were subjected to a series of TWINSPAN classifications to identify regional habitats at EUNIS Level 4, defined as ecological groups associated with specific climatic and soil conditions. We finally conducted a semi‐supervised classification to assign the remaining plots to one of the predefined habitats or to new ones. The resulting habitat types were described using indicator species, key environmental variables, and correspondence with habitats of community interest in Europe. They were also characterized with habitat‐based metrics of alpha‐, beta‐, and gamma‐diversity. Results The expert system classified 91% of plots at EUNIS Level 1, and 55% at both Levels 2 and 3. The semi‐supervised classification resulted in 99 regional habitats at EUNIS Level 4. From these, 36% were absent or described at lower levels in the official EUNIS habitat list, mainly corresponding to shrublands, wetlands, and forests. Of 64 habitats under legal protection, 56 were univocally linked to one specific code of Annex I of the Habitats Directive. All habitats were described by a high number of indicator species and differentiated by environmental factors. Habitat descriptions are available at https://iberoatlantica.eu/ . Conclusions This study demonstrates that a regionalization framework may overcome the limitations inherent in broad‐scale ecosystem classifications, suggesting that EUNIS Level 4 is a suitable resolution for characterizing regional habitats with an ecological basis. A regionalization approach like the one conducted here may support biodiversity and conservation by (i) adapting the floristic and ecological description of EUNIS habitats to regional properties, (ii) allowing a more precise definition of regional units for habitat mapping and ecosystem accounting, and (iii) providing meaningful correspondences with the Annex I habitats of the Habitats Directive.