From guidelines to practice: Operational criteria for identifying old-growth forests in northern Europe.

Primary and old-growth forests are globally valued for their biodiversity, ecosystem services, and cultural significance. The EU biodiversity strategy and EU forest strategy for 2030 require strict protection of remaining primary and old-growth forests, yet they cover only about 3% of EU forest area and remain highly threatened. The European Commission's guidelines define old-growth forests using three main indicators-native tree species, deadwood, and large/old trees-supported by four complementary indicators. Implementing these indicators for boreal and hemiboreal old-growth forests in Europe currently lack science-based operational criteria that meet EU legal standards. We provide recommendations for implementing European Commission's indicators with science-based criteria to minimize misclassification and ensure cost-effective conservation. Key criteria include native species dominance, ≥ 5% deadwood of the total wood volume, and ≥ 20 large/old trees per hectare. Additional guidance is offered for regeneration patterns, structural complexity, microhabitats, and indicator species, emphasizing that all indicators should be applied collectively.