Olympic Habitat Development Study

Project Description

The OHDS was started in 1994 as a joint study between Region 6 (Olympic National Forest) and the Pacific Northwest Research Station (Olympia Forestry Sciences Laboratory).

The goal of OHDS is to test if active management in 40- to 70-year-old conifer stands can accelerate the development of stand structures and plant and animal communities associated with late-successional forests. Eight stands were selected to provide a range in stand conditions typical of those that would be managed in the future. These included three stands which had been planted and five natural stands, three of which previously had been thinned. Tree species composition was generally dominated by Douglas-fir or western hemlock, however, Sitka spruce, western redcedar, Pacific silver fir, and other conifers and hardwoods were also present. Pre-treatment monitoring included 3 years of small-mammal surveys (live trapping) as well as vegetation surveys and establishment of stem-mapped tree-growth plots, prism plots, and smaller scale surveys of amphibians and other ecosystem components.

The primary forest management treatment implemented was variable density thinning that included skips (no-thin blocks on 10 percent of the area), gaps (patches where all common trees were removed but less common conifers and hardwoods were retained on 15 percent of the area) and a matrix thinning of up to 30 percent of the basal area. This thinning with skips and gaps was the first formal trial of this type of variable density thinning in the region, and the “thinning with skips and gaps” terminology used throughout the region originated with this study. The gaps were kept fairly small (generally about 65 feet wide) to reduce the likelihood of windthrow, a particular concern in stands that previously had not been thinned (windthrow post thinning was surveyed on four stands, and the results were published).

This study is unique in providing long-term, large-scale areas (plots are a minimum of 16 acres each) which have been thinned using a variable-thinning prescription. Tree-growth data from this trial also has been used to test and update the small tree portion of the Forest Vegetation Simluator (FVS) tree-growth model in western Washington and Oregon.