Small mammals, songbirds, and ecological forestry: Responses to understory development following variable-density thinning

There is great potential for restoring biodiversity within the millions of acres of young, managed forests throughout the Pacific Northwest. Such forests tend to be simplified relative to older, unmanaged forests. Variable-density thinning (VDT) is increasingly being used as a tool to restore complexity and biodiversity in these forests, but this tool is relatively new and little experimental data exist to support specific prescriptions. The Forest Ecosystem Study, established in 1991, uses a suite of biotic indicators to evaluate the efficacy of VDT. Here we report small mammal and understory songbird community responses to forest development 8-12 years after VDT was applied to 8 of 16 second-growth Douglas-fir stands in western Washington. Our VDT prescription was designed to mimic small-scale disturbances that lead to biological diversity in unmanaged, mature forests in the region. Most species responded positively to understory development following thinning. Total abundance of small mammals was higher on VDT-treated stands than on control stands. Peromyscus keeni, for example, was found almost exclusively on VDT-treated stands. Variable-density thinning also increased songbird species richness and total abundance. Shrub-associated birds in particular showed strong positive responses. Some birds and mammals were less abundant on VDT stands than on control stands. However, our overall findings suggest that VDT can promote understory complexity — the first stage in developing overall complexity and providing for complete small mammal and forest songbird communities. VDT shows promise as part of a suite of silvicultural tools for restoring and maintaining healthy, resilient forests in the region.