Lichens, invertebrates and birds in spruce canopies: impacts of forestry.

I analysed the impacts of forestry on lichens, invertebrates and birds in boreal spruce (Picea abies) canopies in five paried (old-growth and managed) sites from central to northern Sweden. Paper I shows that old-growth compared to managed stands had 6 times more lichen mass per branch and 2 times more as a percentage of branch mass. Lichen mass showed strong positive relationship to mass, diameter and age of branches. Thus, forestry reduces lichen mass on spruce branches. Paper II shows that old-growth forests had greater invertebrate diversity and 5 times more invertebrates per branch. The number of invertebrates of the sizes important to foraging birds was higher in old forests. Forestry may thus reduce foraging habitat quality for passerine birds through a reduction in invertebrate abundance. Paper III shows that abundance of both birds and spruce seeds during winter varied strongly between years. In the seed-rich year of 1990 there was 3 times more birds in the old-growth and 6 times more in the managed stands than in the seed-poor year 1992. Several groups of birds (e.g. seed-eaters and southern taiga birds) were correlated with seed abundance, but the number of northern taiga birds was instead related to type of forest (old/managed). Old-growth forest may thus be important for northern taiga birds druing years with low availability of spruce seeds. Paper IV shows that old-growth forests had 3 times more spiders per branch than the managed forests. Diversity of spiders was higher in old-growth forest, but rarefaction plots indicated no effect of forestry except for reduced abudance in managed forests. In paper V the community structure and species range distributions of aboreal spiders on spruce were analysed from 12 sites in northern Europe. The proportion of web spinners increased with both latitude and altitude and decreased with mean annual temperature. Most arboreal spiders had a narrow geographic range, and there was a positive relationship between range size and loacal abundance. Geographic range size thus appears to be a useful tool to evaluate impacts (like forestry) on a larger (e.g. national) scale. Overall, spruce branches in old-growth forests had more lichen mass (I), invertebrates (II) and spiders (II and IV). Forestry was associated with a reduction in lichen abundance and decreased foraging habitat quality for passerines, especially for northern taiga birds (II-III). Habitat structure (branch size and epiphytic lichen mass) could explain the greater number of invertebrates and spiders in old spruce forests (II and IV). Therefore, the results suggest that epiphytic lichens should be considered an important structural component for the maintance of biodiversity. Furthermore, the reduction in epiphytic lichen abundance may have cascading effects on boreal forest food-webs.