Short-term effects of controlled burning and green-tree retention on beetle (Coleoptera) assemblages in managed boreal forests

Timber harvesting, and especially clear-cutting followed by soil scarification, has altered boreal forest ecosystems in important ways. For example, loss of decaying wood in many parts of boreal forests has reduced habitats for saproxylic species. Several methods have recently been introduced, however, to reduce the negative impacts of forestry on the natural biota. Use of controlled burning and green-tree retention in restoration and management of forests is growing rapidly, especially in Fennoscandia, but the effects of these measures on forest biota remain hypothetical. We performed a large-scale field experiment to study impacts of burning and green-tree retention on beetles in the Scots pine dominated forests of Eastern Finland. The factorial study design included a total of 24 sites (3-5 ha each), with three tree retention levels (0, 10 and 50 m3/ha) and uncut controls. Twelve of the 24 sites were burned. Each treatment combination was replicated three times. Beetles were sampled with window traps before and after the treatments in 2000 and 2001 to study short-term impacts of treatments on beetle assemblages. The data consisted of 34,175 beetle individuals, representing 740 species. The assemblages were greatly affected by the treatments, especially on the burned sites, while on the unburned sites higher tree retention level maintained assemblages closer to the pre-treatment structure. In general, the number of species increased with harvesting and burning. Burning had a significant effect on number of red-listed species, whereas harvesting intensity affected saproxylic, non-saproxylic and red-listed species. The numbers of individuals of red-listed and rare saproxylic species increased more on the burned than on the unburned sites. The results show that living conditions of many declined forest species, in particular saproxylics, can be enhanced in commercial forests by increasing the number of retained trees and by controlled burning. At least close to source areas not only strictly pyrophilous species but also many other rare and threatened species seem to be able to locate new resources quite effectively.