Induced spatial heterogeneity in forest canopies: Responses of small mammals

We hypothesized that creating a mosaic of interspersed patches of different densities of canopy trees in a second-growth Douglas-fir (Pseudotsuga menz iesii) forest would accelerate development of biocomplexity (diversity in e cosystem structure, composition, and processes) by promoting spatial hetero geneity in understory, midstory, and canopy, compared to typical managed fo rests. In turn, increased spatial heterogeneity was expected to promote var iety in fine-scale plant associations, foliage height diversity, and abunda nce of small mammals. Three years following treatment, understory species r ichness and herb cover were greater with variable-density thinning than wit hout. Midstory and canopy species did not have time to develop significant differences between treatments. Variable-density thinning resulted in large r populations of deer mice (Peromyscus maniculatus), a species associated w ith understory shrubs; creeping voles (Microtus oregoni), a species associa ted with herbaceous vegetation, and vagrant shrews (Sorex vagrans), a speci es usually associated with openings but common in old growth. No forest-flo or small-mammal species, including those associated with old-growth forest, declined in abundance following variable-density thinning. Annual variatio n in population size was not related to treatment. Variable-density thinnin g may accelerate the development of biocomplexity in second-growth forest b y promoting spatial heterogeneity and compositional diversity in the plant community, increasing diversity and abundance of small mammals, and similar ly affecting other vertebrate communities. When combined with long rotation s, legacy retention, and management for snags and coarse woody debris, vari able-density thinning has broad applicability to enhance biodiversity, in m anaged Douglas-fir forests across the Pacific Northwest.