Factors affecting the bird diversity of planted and semi-natural oak forests in Ireland

Capsule: In Ireland, which has relatively low diversity of bird species, commercially mature plantation oak forests (aged 72-151 years old) have similar bird diversity to typical semi-natural oak woodlands, and higher bird diversity than over-browsed semi-natural oak woodland, with bird diversity related to habitat complexity. Aims: To investigate whether oak plantations can support comparable bird assemblages to semi-natural oak woodlands, and to assess if high levels of ungulate browsing and grazing impact on the quality of semi-natural oak woodland habitat for birds. Methods: Bird and vegetation surveys were conducted in commercially mature oak plantations (n = 4), semi-natural oak woodlands (n = 10) and intensively browsed semi-natural oak woodland (n = 4). Species richness, total bird density, warbler density and density of parids were compared between oak forest types. Variation in bird communities between sites was investigated using ordination, and relationships between bird and vegetation metrics were assessed using general linear models. Results: Bird diversity in plantation oak and the semi-natural forests subject to low levels of ungulate browsing, was similar, with no difference in species richness, total bird density or density of warbler and density of parids. However, browsed semi-natural oak woods had lower species richness than either of the other two study site types, and lower density of warblers than oak plantations. These observed differences in bird communities appear to be a result of browsing mediated differences in habitat complexity between the forest types. Conclusions: Plantation forests of native tree species may support comparable bird communities to semi-natural woodlands in areas that lack forest specialists. Bird diversity in woodlands subject to high levels of browsing and grazing is likely to be limited, unless ungulate populations and their access to these woodlands are managed to promote the development of a more complex understorey.