Will they come? Long-term response by forest birds to experimental thinning supports the “Field of Dreams” hypothesis

Abstract

Intensifying forest management practices via use of tree plantations represents a common response to rising global demand for wood and paper products. However, a large proportion of tree plantations, globally, exist in closed-canopy mid-seral stages, which are frequently associated with reduced biodiversity. In some regions, silvicultural techniques such as thinning have been used to accelerate the development of complex vertical structure and composition (e.g., shrubs, understory, midstory, large trees) in these plantations; these attributes are expected to constitute suitable habitat for species associated with particular forest structures. We report on the results of a 15-year randomized-block manipulative study based in Oregon, USA, designed to test the hypothesis that ‘if you build it they will come’ (i.e., the “Field of Dreams” hypothesis; the ‘creation’ of complex vertical structure in forests via thinning will result in colonization by bird species associated with these structures). We conducted point counts of forest birds 7 times each year in stands thinned at two different intensities and in unharvested controls. Responses to thinning by some bird species [e.g., hermit warbler (Setophaga occidentalis), Swainson’s thrush (Catharus ustulatus) and Wilson’s warbler (Cardellina pusilla)] changed from being negative in the short term (<6years), to being positive over the longer term. However, some species were sensitive to thinning disturbance even after 15years of recovery [e.g., Hutton’s vireo (Vireo huttoni)]. Disturbance-associated species initially increased following thinning; but over the longer-term, detections of several of these species [American robin (Turdus migratorius) and Townsend’s solitaire (Myadestes townsendi)] notably declined or ceased. Our findings indicate that results from short-term studies (⩽6years) cannot be used to predict the longer-term response of birds to forest management treatments, particularly for species of birds whose response to thinning over time was non-linear. Further, none of the management prescriptions tested were alone sufficient to maintain long-term occupancy for all of the species in the study. Implementing thinning at intervals across landscape scales to develop different seral stages and stand-structures, while also maintaining unthinned areas for species negatively impacted by thinning, will likely have the greatest positive impact on beta diversity of birds in managed plantation landscapes.