Tree and understory responses to variable-density thinning in western Washington.

Abstract

The Olympic Habitat Development Study was initiated in 1994 to evaluate whether active management in 35- to 70-year-old stands could accelerate development of stand structures and plant and animal communities associated with late-successional forests. The study used a variable-density thinning prescription as the main tool to alter stand structure; the prescription entailed creating gaps and retaining uncut areas, and thinning the remaining forest matrix. We assessed tree damage (primarily windthrow) following thinning, 5-year tree growth, and 3-year vegetation development in control and thinned plots. Windthrow damage was minor in most plots, occurring primarily in stands with high height-to-diameter ratios and located in vulnerable topographic positions. Tree growth responded positively to thinning. In addition, tree growth differed spatially—trees near gaps or along skid trails had better-than-average growth whereas trees near uncut patches had poorer-than-average growth. Understory vegetation responded to thinning with increased percentage of cover and number of herbaceous species in thinned areas and in created gaps. Percentage of cover of mosses and liverworts was greatest in undisturbed areas. Early results indicate that the thinning is operationally feasible and demonstrate that the variable-density thinning increases spatial heterogeneity within the stands.

Citation

Harrington, Constance A.; Roberts, Scott D.; Brodie, Leslie C. 2005. Tree and understory responses to variable-density thinning in western Washington. Balancing Ecosystem Values Proceedings, Regional Experiments: 97-106