Induced Spatial Heterogeneity in Forest Canopies: Responses of Small Mammals

Abstract

We hypothesized that creating a mosaic of interspersed patches of different densities of canopy trees in a second-growth Douglas-fir (Pseudotsuga menziesii) forest would accelerate development of biocomplexity (diversity in ecosystem structure, composition, and processes) by promoting spatial heterogeneity in understory, midstory, and canopy, compared to typical managed forests. In turn, increased spatial heterogeneity was expected to promote variety in fine-scale plant associations, foliage height diversity, and abundance of small mammals. Three years following treatment, understory species richness and herb cover were greater with variable-density thinning than without. Midstory and canopy species did not have time to develop significant differences between treatments. Variable-density thinning resulted in larger populations of deer mice (Peromyscus maniculatus), a species associated with understory shrubs; creeping voles (Microtus oregoni), a species associated with herbaceous vegetation, and vagrant shrews (Sorex vagrans), a species usually associated with openings but common in old growth. No forest-floor small-mammal species, including those associated with old-growth forest, declined in abundance following variabie-density thinning. Annual variation in population size was not related to treatment. Variable-density thinning may accelerate the development of biocomplexity in second-growth forest by promoting spatial heterogeneity and compositional diversity in the plant community, increasing diversity and abundance of small mammals, and similarly affecting other vertebrate communities. When combined with long rotations, legacy retention, and management for snags and coarse woody debris, variable-density thinning has broad applicability to enhance biodiversity in managed Douglas-fir forests across the Pacific Northwest.