In the 1990s, many public land managers in the Pacific Northwest were interested in changing their management approaches in even-aged conifer stands. The idea was that through forest management, biodiversity could be increased and the development of structural characteristics associated with older stands could be accelerated. At that time, however, the long-term effectiveness of non-uniform stand treatments was unknown. To address this need, Forest Service scientists established several long-term studies.
Results are now available from 2 studies in Washington state. The first study evaluated stand conditions 18 years after gap creation and thinning in a young Douglas-fir plantation established on land cleared by the 1980 eruption of Mount St. Helen. The other study examined stand conditions 14 years after implementing variable-density thinning in 5 older conifer stands on the Olympic Peninsula.
Both studies demonstrate that gap creation effectively increased variability in tree crowns, particularly by promoting longer crowns near gaps; increasing tree growth rate; and increasing recruitment of other species into understory or mid-story strata. The effects of these thinnings will decline over time; thus, additional treatments will be needed to allow the treatments to continue to diverge from unthinned stands.
Early results from the Olympic Habitat Development Study were used to update the small-tree model of the Forest Vegetation Simulator, a commonly used tool for projecting forest growth. These latest findings from both studies are informing discussions and decisions by foresters, wildlife biologists, and ecologists who are interested in accelerating the development of less-uniform stand structures.