Comparison of breeding bird assemblages in conifer plantations managed by continuous cover forestry and clearfelling

Abstract

Continuous cover systems are increasingly advocated for stand management but the implications for biodiversity in European forests, and specifically in plantations of non-native trees, are poorly understood. Timed point counts were used to quantify differences in species richness and abundance of breeding birds supported by conifer plantations (with major Sitka spruce components) under two contrasting management systems in upland Britain: continuous cover forestry (CCF) and clearfelling with replanting (CFR). Each CCF study area was paired with a comparable CFR study area. Sample points within CCF areas were divided into areas with extensive regenerating understorey and areas with none; sample points within CFR study areas were placed within young thicket and pre-thicket stands (trees<10 years old) and older stands (15–30 years old). Poisson GLMMs were used to identify differences in bird species richness and abundance between the four treatments testing the predictions: (a) CCF can support an enhanced assemblage of forest birds relative to CFR (including mature CFR); and (b) CFR can support a broader range of open habitat and shrubland species relative to CCF (including those with a regenerating understorey). Ranking forest types in descending order of species richness gave: CCF with shrub understorey>CCF without shrubs>young pre-thicket CFR>mature CFR. Many ‘mature forest birds’ were more abundant, or recorded only, within CCF (e.g. blackcap, wood warbler, redstart and hawfinch). A small number of species associated with young-growth (‘shrubland’ and ‘shrub-layer’ species) were most abundant in pre-thicket CFR but a CCF understorey supported some species at densities approaching those found in pre-thicket CFR. Simulations of the effect of increasing the proportion of plantation under CCF indicated for example that a plantation managed exclusively as CCF could support as few as 53% of the willow warblers as one managed exclusively as CFR. A plantation managed exclusively as CCF could support as few as 70% of the lesser redpolls as one managed as CFR, but could support twice as many blackcaps. CCF could be of greater conservation value to many forest birds than CFR. However, CCF may not support such high densities of some species (e.g. dunnock, willow warbler and lesser redpoll) as those found in young growth stage CFR. Forest management that includes some young growth areas alongside CCF could prove to be a strategy that maximises the capacity of a forested landscape to support a greater diversity of bird species.