Density management and biodiversity in young Douglas-fir forests: challenges of managing across scales

摘要

Concerns about the dominance of dense, young plantations in the Pacific Northwest (USA) have spurred interest in the effects of density management on various aspects of biodiversity. We review results from numerous large-scale management experiments (LSMEs) in Oregon and Washington in terms of the early vegetation responses to thinning. Most studies were installed in young Douglas-fir (Pseudotsuga menziesii) stands. In particular, we focus on the contributions of variable treatments, such as gap creation, unthinned patches and variable density thinning to within-stand variability. Density management resulted in changes in overstorey and understorey vegetation, but results were similar and independent of thinning intensity. Even light thinning operations initially reduced understorey vegetation likely due to harvesting damage. The vegetation recovered quickly, but thinning resulted in a compositional shift towards invading, early seral species. Spatially variable treatments provide for variable canopy structure and understorey vegetation and results from the LSMEs suggest incorporating these into thinning prescriptions may be beneficial where biodiversity is of concern. The need for spatially variable treatments is highlighted by the hierarchical habitat-selection framework of various wildlife species. This framework provides a basis for matching spatial variability in forest structure and vegetation to the spatial scales that wildlife species perceive and respond to. Silvicultural prescriptions that increase within-stand variability can provide important habitat features across multiple scales and enhance habitat quality beyond that provided by stand-level prescriptions..