Comparison of tree cavity abundance and characteristics in managed and unmanaged Swedish boreal forest

Sammanfattning: Tree cavities are major components of the forest ecosystem worldwide as they are required substrate for a large number of species (including nesting and roosting birds, mammals, fungi, and insects). The present study aimed to provide new knowledge on cavity density and cavity tree characteristics in the Swedish boreal zone where only little information exists on this subject. Assessing the impact of forest management on cavity availability was another main goal of this study. My study was conducted in old forests around Umeå in the Swedish middle boreal zone, in 25 managed and 25 unmanaged stands. The mean cavity density in managed forest (1.2 ha-1) was significantly lower than in unmanaged forests (2.5 ha-1). Natural-decay cavities stood for only 5.5% of the cavities, while the remaining 94.5% were excavated. This result emphasizes the importance of primary cavity excavators, mainly woodpeckers (Picidae), for cavity supply in the Swedish boreal forest. Compared to their density in the forest, deciduous and dead trees were overrepresented as cavity trees. The different cavity densities observed between the two management categories could then at least partly be due to the fact that unmanaged forest presented a higher amount of dead trees and birch (Betula spp.). As recorded in previous studies, aspens (Populus tremula) were overrepresented as cavity trees. Although additional studies on nest occupancy and breeding success are necessary to further assess the impact of forestry on cavity-users, my study already highlights a negative effect of forest management on cavity availability. I recommend that management strategies should focus on increasing the amount of suitable excavation substrates, meaning standing dead trees and deciduous trees. Particular attention should be paid to the conservation and regeneration of aspen.