Press release 7.9.2006
According to a study conducted at the Finnish Forest Research Institute (Metla) retention trees saved in regeneration cutting clearly enhance species diversity. However, currently the average number of retention trees is too low to guarantee the preservation of threatened and near-threatened species and many species with more specific requirements occurring in old forests. The significance of the valuable habitats listed in forest law for preserving biodiversity varies with the habitat type and the species group. More species occur in valuable habitats than in average commercial forests.
Live and dead trees saved in regeneration cutting host many species that are missing from stumps, cutting residues, under-growth trees and other substrates left in clearcutting areas. In the studied regeneration areas the majority of threatened polyporous fungi grew on large-diameter logs left behind in cutting. The probability of incidence of threatened polyporous fungi increased clearly on research sites where the amount of dead wood was at least about 10 m 3 per hectare. In the long run, preserving species on regeneration sites is possible provided the mortality of the living retention trees produces a sufficient amount of suitable substrates. The retention trees may later also provide sources for dispersal of species.
Seven habitats of special importance are defined in forest law. Brookside forest habitats comprise roughly one third of the privately owned forest law sites. On average, biodiversity in brookside forests is greater than in ordinary commercial forests. At the moment, the brookside forests do not host more threatened species than ordinary mature or regeneration-aged commercial forests. The structural features of a stand are more important factors affecting species richness and the incidence of threatened species than the location of the stand on a brookside or elsewhere. As the forest-law habitats are excluded from forest management the stand structure becomes more favourable for the threatened species, but on the other hand, persistence of small populations on small sites is uncertain in the long run. Furthermore, the study showed clear regional variation in the abundance of threatened species: the lowest number of species was observed in south-western Finland and the highest number in the mid-boreal zone of North-Karelia in eastern Finland.
In a Metla project “The effects of biodiversity-oriented forest management on species diversity – a monitoring system” an inventory of polyporous fungi, epiphytic lichen and beetle species was taken during the period 2003-2005 on retention-tree sites and in forests on brooksides, herb-rich groves, and rocky outcrops. The study is part of the MOSSE - Biodiversity and Monitoring Programme. The results were published in the METSO Forest Biodiversity Programme for Southern Finland research report.