Forest stands were studied to determine if old-growth forest structure could be mimicked in younger stands via overstory manipulation. Cover and species composition of understory plants were systematically sampled in sixteen thinned second-growth stands and sixteen adjacent unthinned second-growth Douglas fir (Pseudotsuga menziesii (Mirabel) Franco.) stands. The stands were thinned twenty-four to thirteen years ago. These were compared to seven nearby old-growth stands. Thinned and unthinned stands had matching elevations, aspect, and soils, yet differed primarily in management treatment. Leaf area indices were determined for these stands. Thinned stands differed from the old-growth and unthinned stands in having significantly higher cover values and species numbers, apparently resulting from increased light to the forest floor and a greater variety of microhabitats created by thinning. Young unthinned and old-growth stands were comparable in terms of cover and richness, but differed in species composition. Diversity indices showed no difference in species diversity between the three types of stands. Ordination of the species/sample data using Detrended Correspondence Analysis showed that understory species composition of the young unthinned and thinned stands was nearly identical. Species composition of old-growth stands differed from thinned and unthinned stands. The ordination indicated that age of the stands, structure of the canopy layers and climate were major determining factors in the species composition of the understory plant communities. Management manipulation of the second growth stands did not yield stands with understory vegetation communities that mimicked those of old-growth stands. The conclusions of this study were: 1) Shrub cover increased with thinning as compared to unthinned and old-growth stands. 2) Thinning increased the species richness of the stands, without increasing the number of exotics. 3) Diversity was not altered by thinning. Old-growth, thinned and unthinned stands did not differ in diversity values. 4) Patterns of community composition in thinned stands were more similar to unthinned equivalent stands than to nearby old-growth. / Graduation date: 1996