Identifying old-growth forests in Northern Europe requires common scientific criteria

Old-growth forests are invaluable natural ecosystems in Europe, crucial for biodiversity, climate regulation, cultural heritage, and well-being. However, only about 3% of Europe's forests qualify as primary or old-growth forests, and their numbers continue to decline rapidly in Northern Europe. These forests are characterized by their structural diversity and the ecological processes that have evolved over long periods.

Dead and old trees, deadwood, provide habitats for a wide range of species, including fungi, lichens, mosses, deadwood-dependent insects, and cavity nesters.

Finland's government sets mandatory criteria for old-growth forests, focusing on the average age of the dominant tree stand and the total volume of deadwood per hectare. However, these criteria alone are not sufficient for identifying old-growth forests due to the variation in deadwood volume and disturbance history across forest types and geographical zones.

The EU Biodiversity Strategy for 2030, the EU Forest Strategy for 2030, and the Renewable Energy Directive (RED III) necessitate the definition, mapping, and protection of remaining primary and old-growth forests. The EU Restoration Regulation also prohibits the deterioration of good condition forest habitats, though it does not explicitly mention old-growth and primary forests.

Researchers propose a set of scientifically grounded indicators to assess the condition of forest habitats and identify old-growth forests. These indicators include native tree species, deadwood, and old trees as main indicators, and natural regeneration, structural diversity, habitat trees, and indicator species as complementary indicators.

A minimum area of 0.5 hectares (1.2 acres) is suggested for assessing a forest stand, a globally used threshold. The researchers recommend applying the precautionary principle, considering borderline cases carefully and assessing their ecological value. Even small old-growth forests can be crucial for threatened species, particularly in Southern Finland where the remaining sites are small and fragmented.