Effects of growth stage and tree species composition on breeding bird assemblages of plantation forests

Capsule: Bird species assemblages are strongly dependent on growth stage and forest structure, but do not appear to be greatly affected by tree species composition. Aims: To investigate changes in bird species composition of plantation forests in relation to crop tree species and forest growth stage. Methods: We surveyed populations of birds in 44 plantation forests throughout Ireland in the summers of 2001 and 2002. Our study sites represented three combinations of tree species (pure Sitka Spruce Picea sitchensis, pure Ash Fraxinus exelsior, and Sitka Spruce/Ash mixed at inter-stand level) at five ages (4-8 years, 9-17 years, 23-29 years, 35-50years and 50+ years). We used ordinations and indicator species analyses to explain the influence of growth stage and tree species composition on the bird assemblages of these forests. Results: Bird species assemblage is dependent on growth stage. Changes in bird species composition over the commercial forest cycle are related to shrub layer and proximity to the forest edge. The bird assemblages of pre-canopy closure forests are typical of open habitats, some of the constituent bird species being of conservation concern within Ireland. As forests mature, these are replaced by commoner, more generalist species. We found no significant effect of tree species on bird species assemblage. Possible reasons for this include the relatively narrow range of tree species mixtures studied, and the paucity of forest specialist birds in Ireland. Conclusions: Forest management for bird diversity in Ireland should promote the shrub layer in and around forest plantations, and ensure that new afforestation does not take place in areas with diverse communities of open-habitat birds. Opportunities for Irish forest plantations to provide habitat for bird forest specialists may be limited.