Background and aims
Management can influence woodland bird communities through effects on stand structure, tree species and tree age and a number of studies have demonstrated (generally negative) relationships between the density of trees and the diversity and abundance of birds. However, tree density in these studies is often confounded with other variables such as grazing, tree age, foliage height diversity and adjacent habitats. Planting densities within commercial conifer plantations in Britain tend to be relatively high, in order to suppress competitive ground vegetation through a quick closure of the canopy and to reduce the proportion of ‘juvenile’ wood or branches within the final timber crop. Thinning, to remove a proportion of the growing trees in late thicket and older stands, is undertaken in some plantations to produce trees of wider girth and/r maximize timber production. There has been some speculation that such restructuring of plantations can provide benefits to birds but there has been a notable absence of studies which have attempted to specifically address this. This study assessed the influence of thinning on the diversity and abundance of breeding birds in first-rotation conifer plantations at 20 study sites within the Moray Firth Catchment of northern Scotland.
Thinned sites showed no significant differences in the number of species recorded when compared to paired sites in which no thinning had taken place. Similarly no notable effects of thinning on population densities of the more commonly recorded bird species were demonstrated. It was concluded that the levels of thinning as carried out routinely in upland Scotland are insufficient to permit the development of ground vegetation and shrub layers that are likely to enhance woodland bird communities.
Calladine, J., Humphreys, E.M., Strachan, F. & Jardine, D.C. 2009. Forestry thinning incommercial conifer plantations has little effect on bird species richness and breeding abundance. Bird Study 56:137-141. (http://www.tandfonline.com/doi/abs/10.1080/00063650802681698)
BTO Scotland, Highland Birchwoods and Forestry Commission Scotland
Northern Periphery Programme
Staff contact: John Calladine