Stand density management has the potential to improve forest structure, diversity and function. We examined the effects of thinning disturbance on understory vegetation cover and richness in seven 40- to 60-year old forest stands dominated by Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) in Oregon, USA. The objectives of the study were to 1) determine the understory vegetation response to thinning treatments that increase stand complexity, 2)  test whether understory communities were more variable within more structurally-complex treatments, and 3) compare the contribution of individual stand structural components (i.e., forest matrix, gaps, and leave island reserves) to the understory vegetation response. At each site, one unthinned control was preserved and three thinning treatments were implemented: low complexity (LC; 300 trees ha^-1), moderate complexity (MC; 200 trees ha^-1), and high complexity (HC, variable densities from 100 to 300 trees ha^-1). Circular gap openings and leave island reserves were established in all thinning treatments.

Results/Conclusions

Six years after harvest, richness of total herbs, forest herbs, early seral herbs and shrubs, and introduced species increased in all thinning treatments although early seral herbs and introduced species represented a small component of the understory (< 5% cover). Only cover of early seral herbs and shrubs increased in all thinning treatments. Forest shrub cover increased in the MC and HC treatments indicating a release following harvest. Across the seven sites we found 284 vascular plant species. Understory plant communities differed significantly among sites. The most prominent differences were among Cascades and Coast Range sites with the latter containing more species common of lower elevation, drier sites. After accounting for site-level differences, understory communities in thinned stands differed from those in control stands, where more shade-tolerant forest species were abundant. Community differences among thinned stands were less apparent. In thinned stands, harvest disturbance resulted in overall higher species richness because of the recruitment of early seral and forest herbs and both low and tall shrubs, greater cover of early seral herbs and shrubs, a release response of forest understory shrubs such as Gaultheria shallon and Rubus ursinus, and greater abundance of introduced species. Although gap openings and heavy-thin components represented a relatively small area in a given treatment, these components were important in the recruitment of early seral species and tall shrubs. Increased stand complexity by thinning appeared to increase herb and shrub richness six years after imposing the treatments.