Identifying old-growth forests in Northern Europe needs a shared scientific approach, according to a research group. These forests are among Europe's most valuable ecosystems, offering critical benefits such as preserving biodiversity, regulating the climate, protecting cultural heritage, and supporting people's well-being. However, they are also among the most threatened. It is estimated that only about 3% of Europe's forests qualify as primary or old-growth, and their area is shrinking quickly in Northern Europe. Old-growth forests are especially important for biodiversity due to their complex structure and the ecological processes that have developed over long periods. Features like dead trees, decaying wood, and old-growth trees provide essential habitats for many species, including fungi, lichens, mosses, insects, and birds that depend on cavities for nesting. In naturally developed boreal forests, younger trees can be a significant part of the canopy, even in areas with many old trees and old-growth characteristics. In Finland, the government's criteria for old-growth forests focus on the average age of the dominant tree species and the volume of deadwood per hectare. However, Mikko Mönkkönen, a professor of applied ecology at the University of Jyväskylä, notes that these criteria alone are not very effective for identifying old-growth forests. Deadwood volume can vary significantly between forest types and regions, depending on the forest's disturbance history, making it difficult to set precise thresholds. The key to identifying old-growth forests, he says, is to consider the forest as a whole and assess a range of characteristics. The EU has set requirements for member states to identify and protect old-growth forests. The EU Biodiversity Strategy for 2030 and the EU Forest Strategy for 2030 ask member states to map and strictly protect all remaining primary and old-growth forests. The Renewable Energy Directive (RED III) also prohibits logging in such forests. While the EU Restoration Regulation does not explicitly mention old-growth forests, it prohibits the deterioration of habitat types in good condition as defined by the Habitats Directive. The research group's proposed criteria help assess the condition of forest habitats and align with the European Commission's guidelines, which define an old-growth forest as meeting three main indicators and at least two of four complementary indicators. The researchers recommend a minimum area of 0.5 hectares (1.2 acres) for assessing a forest stand, a threshold used internationally. The three main indicators are native tree species, deadwood, and old trees. Complementary indicators include natural regeneration, structural diversity, habitat trees, and species typical of old-growth forests. The researchers emphasize the importance of applying the precautionary principle to protect valuable forests. Overly strict or unscientific criteria could leave many old-growth forests unprotected, leading to biodiversity loss and higher future restoration costs. The researchers highlight that identifying old-growth forests involves some uncertainty due to natural variation between forests, limited field measurements, and differing results from various indicators. They recommend careful assessment of borderline cases rather than excluding them automatically. Kimmo Syrjänen, project manager at the Finnish Environment Institute, notes that even small old-growth forests can be vital for threatened species, especially in areas like Northern Finland and the Sámi Homeland. A transparent, scientifically grounded approach is essential to avoid biodiversity decline, societal conflict, and legal issues. The indicators proposed by the research group provide consistency and clarity in identifying old-growth forests in Northern Europe. They also offer a solid basis for the timber industry to assess the sustainability of wood products. The policy recommendations have been published in Wisdom Letters, and the researchers' article appears in Ambio.