Bird abundance is highly dynamic across succession in early seral tree plantations

Abstract

Tree plantations are integral for meeting society’s demand for wood products and are increasing in total area worldwide. It is therefore critical to understand how plantations affect native biodiversity. Our aim was to examine the degree to which plantations affect biodiversity by 1) quantifying songbird abundance across a gradient in forest stand age (stand initiation through canopy closure), 2) estimating the duration of early seral habitat, and 3) testing if forest structural and compositional elements prolong habitat availability. This approach is equivalent to the modeling of wood ‘yield curves’ common in forest management but applied to a biodiversity indicator.We used a chronosequence sampling design to survey songbirds and vegetation in 283 plantations aged 0–30 years in the Oregon Coast Range, USA. Stands were randomly selected within age strata and surveys were temporally replicated in order to use N-mixture models to estimate abundance after accounting for imperfect detection. The Coast Range is considered one of the most productive forest regions in the world and plantations occur on over 50% of the forested land base. Our chronosequence encompasses the assumed age of canopy closure, which we anticipated to be the point at which early seral biodiversity declines.We detected 5074 birds of 71 species during the 2018 and 2019 breeding seasons. Canopy closure occurred approximately 12 years following clearcut harvest and replanting. We found that bird abundance changed dynamically during this short early seral stage. Twenty species peaked in abundance either very early in stand development or with the approximate timing of canopy closure, and then subsequently declined to low levels by the end of the 30-year chronosequence. The estimated abundance of 3 species increased following canopy closure. We also found, contrary to our hypothesis, that the amount of broadleaf cover increased habitat longevity for only one species – Wilson’s warbler.To our knowledge, our study provides the first quantitative estimates for how bird species abundances change throughout the entire early seral stage in tree plantations in western North America – information that can be used to assess tradeoffs between timber production and biodiversity. We found that the duration of early seral habitat in plantations is short and generally cannot be ameliorated by managing for higher levels of broadleaf cover. This finding has important implications for early seral species in the rapidly shifting mosaic of tree plantation landscapes.