Bird biodiversity in relation to forest composition in Sweden

Abstract: The large development of timber activities in Sweden has induced important changes in forest composition. Today they are less old, deciduous trees and snags in Swedish forests. Such changes threaten the survival of certain bird species in Sweden. Today, bird biodiversity restoration plans try to re-establish, in harvested forests, the structural straits that will foster high bird diversity. The aims of this study were: 1) to identify in the literature which forest composition traits are essential for forest birds in Sweden; 2) to find the environmental variables that explain the occurrence and abundance of forest bird species in Sweden, using regression analyses conducted on data from a national monitoring program. Those analyses were conducted for a group of species, considered as being indicator of forests with high quality habitats for birds. Past studies have identified several main drivers of bird diversity, notably the presence of old standing trees, of decaying timber and the amount of deciduous trees. Lack of information on such variables limited the power of our analysis. However, regression models revealed the importance of forest edges, which can compensate the lack of deciduous trees in harvested forests. Thus in a biodiversity perspective the scope of concern should be broadened to the areas close to forests. The results of the analyses have also shown the importance of mixed forest for bird biodiversity, since such type of forest increases the nesting and foraging opportunities. The lack of clear links between the distribution of forest indicator species and forest composition traits in the data analysed also questions the practical relevance of indicator species as they are currently defined.