Effects of stand density reduction on structural development in western Oregon Douglas-fir forests : a reconstruction study

Graduation date: 1997This research examined thinning effects on stand structure and species composition in\ud 50- to 120-year-old Douglas-fir forests. Thirty-two paired stands (thinned and unthinned)\ud were measured throughout western Oregon, as were 20 old-growth stands for comparison. Thinnings occurred 10 to 24 years previously and ranged in intensity from 8 to 60% volume\ud removal. Overstory and intermediate tree characteristics, conifer and hardwood regeneration,\ud and shrub/herbaceous species composition and cover were recorded. From this, I assessed\ud whether thinning young stands promoted vegetation structure and composition associated\ud with old-growth stands.\ud Overstory trees in thinned stands had diameters, live crown ratios, crown radii,\ud and radial growth rates greater than those in unthinned stands, and equal to or\ud approaching those found in old-growth stands. Stand volume production was neither\ud affected by earlier clearcut harvesting nor by thinning. Intermediate trees in thinned\ud stands, like old-growth stands, were typically young saplings with radial growth rates and\ud live crown ratios greater than those in unthinned stands (typically suppressed members of\ud the original cohort). Living intermediate structure (multi-storied canopies) was common\ud to thinned and old-growth stands. Given such overstory and intermediate tree responses,\ud canopy leaf area in stands thinned >20 years previously was greater than in unthinned\ud stands.\ud Conifer regeneration density, frequency and growth rates were greater in thinned\ud stands than in old-growth and unthinned stands. Densities and frequencies of tall shrubs\ud were similar in thinned and old-growth stands, and greater than unthinned stands. Low\ud shrub and herbaceous cover were 33% and 25% greater, respectively, in thinned than other\ud stand types. Herbaceous species frequency and richness were similarly stimulated. Low\ud shrub composition was altered by thinning (unthinned and old-growth stands did not\ud ordinate differently). However, herbaceous species composition was similar across all\ud stand types and showed more variability among sites than among stand types.\ud Thinning appeared to create old-growth-type structure by stimulating overstory and\ud intermediate tree crowns and growth rates, tall shrub densities and conifer regeneration.\ud Though low shrub and herbaceous species cover and frequency were greatest in thinned\ud stands, composition was not affected by thinning, nor by clearcut harvesting 50 to 120 years\ud ago or other differences in stand origin