What is the biodiversity of northern forests made of?

Jari Kouki

Faculty of Forest Sciences, University of Joensuu
PO Box 111, FIN-80101 Joensuu, Finland

The fauna and flora of arctic and boreal areas are typically impoverished as compared with more southern ecoregions. Most of the organism groups show the well-documented general pattern of decreasing species richness towards the north. However, some of the groups show opposite trends. For example, number of aphids and sawflies increase from the tropical regions to the boreal zone. The generally low number of species implies that ecosystem functional properties may be dependent on smaller number of species in the north. This suggests also that even small reduction in the number of species in boreal and arctic areas may have potentially larger impact on ecosystems in the north than in the south.

The composition of northern biota has changed a lot during the past decades, especially in the forest ecosystems. Many of the observed changes are assumed to be related to human activities. For example, ca. 700 forest-dwelling species are currently considered as threatened, and forestry operations are the main reason of threat for 40 % of all the threatened species in Finland. Most of these threatened species are classified as old-growth specialists. Old-growth forests have inevitably declined in Fennoscandia during the 20th century.

The species composition and diversity in boreal forests are primarily determined by four factors: vegetational zone, forest site type, successional stage, and human-caused structural modifications. These factors form the basis for the forest biodiversity. The interplay between forest disturbance, succession and species' occurrences is, however, poorly understood. New evidence points to the direction that the importance of younger forests in preserving forest biodiversity may have been underestimated as compared with old-growth areas. In fact, naturally originated early successional phases are very uncommon currently, and definitely more uncommon than old-growth areas. The structural properties between managed and natural early phases differ dramatically. Many threatened species seem to survive or even prefer younger stages if only the natural properties in these habitats remain.

This finding has far-reaching consequences: it urges on maintenance of natural disturbance in protected areas, and, even more importantly, emphasizes the underexploited potentials that managed forests may have in the protection of boreal forest biodiversity. For the new developing forestry practices this result adds essential elements.