Avian richness and abundance in temperate Danish forests: tree variables important to birds and their conservation

Many studies on avian diversity and forest structure have focused on finer scale forest variables such as foliage height diversity, foliage diversity, foliage density, vertical distribution of vegetation and horizontal vegetation density. From a conservation and forestry operational point of view it would be of great interest if tree variables influenced directly by forestry management decisions also had significant influence on avian richness and abundance. The species, age and size of a tree are examples of such tree variables. A great number of studies also have focused on avian diversity indices to reveal relationships with vegetation variables. However, it may be more appropriate for foresters and conservation officers to operate with richness and abundance measures directly, because indices complicate interpretations on the relative importance of the two variables (richness and abundance) constituting the index. Fourteen managed temperate forests in Denmark were investigated for avian species richness and abundance and related to measures on different tree variables influenced directly by forestry management decisions. A rapid assessment method of avian richness and abundance was employed. It consisted of point-counts of bird richness and abundance within 1 km(2) of forest. General linear models were tested by analyses of variance statistics to reveal the tree variables most important to avian richness and abundance. It was found that more old trees, more tree species and more tree size-classes correlated with more bird species and individuals. However, some variation in bird richness and abundance was also related to site quality and/or chance colonization. Moreover, it was shown that the guild of cavity-nesting birds correlated positively to age of tree stand. The potential number of bird species in Danish forest is similar to that in nearly pristine forest in Poland, and much larger than that recorded in any of the forests investigated. Together with the results above, this indicates a high potential for squeezing in more avian species in a higher quality forest from a biodiversity point of view. Modern Danish forestry affects tree variables influenced directly by forestry management decisions. Such tree variables have great influence on avian richness and abundance, but simple measures in forestry practices can be taken to enhance the conservation of bird richness and abundance.