Effects of undergrowth clearing on the bird communities of the Northwestern Mediterranean Coppice Holm oak forests

Abstract

Undergrowth clearing is a widespread forest management technique used in many Mediterranean regions to reduce dense vegetation in order to prevent fire or to facilitate other forest exploitation activities. Here, we analyze the effects of undergrowth clearing on biodiversity by focusing on the variations in bird diversity in Holm oak forests in Catalonia (north-east Iberian Peninsula) under different forest management regimes: (1) coppice Holm oak forests where the undergrowth layer has been completely cleared, (2) partially cleared forests and (3) cleared and tree thinned forests and finally, (4) undisturbed forests. The synchronic comparison approach was used in conjunction with a before–after control impact (BACI) experiment in which the effects of undergrowth clearing were explicitly measured. Complete undergrowth clearing resulted in the almost complete disappearance of three Warbler species (Subalpine Warbler, Sardinian Warbler and Garden Warbler). Partial clearing also led to a marked reduction in the numbers of these three species, but the presence of Sardinian and Garden Warblers was maintained in the treated forests. Complete undergrowth clearing accompanied by tree thinning also led to a decrease in amount of undergrowth species and involved additional negative effects for species such as Wren, Robin, Blackbird and Blackcap. Only one species, the Nightjar, appeared to benefit from undergrowth clearing, while many others increased their numbers only when clearing was applied together with tree thinning: Turtle dove, Mistle thrush and Cirl bunting. Undergrowth clearing brought about a significant simplification in the vertical structure of the forest, which probably reduced foraging opportunities and breeding resources for most undergrowth species. These effects became more pronounced when tree thinning was applied together with undergrowth clearing. To reconcile forest management and bird diversity, undergrowth clearing should be applied selectively to ensure that a certain number of undergrowth patches, preferably corresponding to different shrub species, are not affected by management treatments.