Overstory and Understory Vegetation Dynamics in Response to Thinning in Coniferous Stands in Western Oregon

Thinning even-aged coniferous stands in the Pacific Northwest is aimed at accelerating development of late-successional features, while maintaining long-term forest productivity. We examined effects of thinning on overstory and understory vegetation 11 years after harvest in 40- to 60-year old forests dominated by Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) on three sites in western Oregon. Each site contained an unthinned control (238-1446 tpha) and three thinning treatments selected to enhance overstory structural diversity by decreasing densities, and enhance spatial variability within stands (high density = 120 tpha; moderate density = 80 tpha; variable thin with 120, 80 and 40 tpha with circular 0.2-ha gaps comprising 20% of the stand). Stand basal area and overstory cover was greater in controls than in thinning treatments in two of the three study sites, despite large increases in thinned stands. Seedling regeneration varied greatly among sites, but was generally more abundant in variable thins with 80 tpha than in high density thins and controls. Cover of forbs, grasses, low shrubs, hardwood, and understory conifers tended to be significantly greater in the variable density treatments than in controls. On the other hand, cover of moss and litter, bare soil and logs was generally greater in controls than in thinning treatments. Cover of grasses, ferns and low shrubs was greater in variable density treatments with 40 tpha than in all other treatments; while tall shrub cover was greater in high density thin treatments. The wide range of responses suggests that applying combinations of the thinning treatments may be beneficial to encourage a high diversity of understory conditions