Overstory Development in Douglas-Fir-Dominant Forests Thinned to Enhance Late-Seral Features

Douglas-fir (Pseudotsuga menziesii [Mirb.] Franco var. menziesii) and Douglas-fir/western hemlock (Tsuga heterophylla [Raf.] Sarg.) stands were thinned at age 50 to different levels of residual stocking, with and without gaps on medium or better (McDonald) and excellent (Blodgett) sites in western Oregon. Postthinning basal areas ranged from 75 to 140 ft^sup 2^/acre in 35- 80 trees/acre. The range of densities led to periodic Scribner and cubic volume growth peaking at ages 60 -90 with mean annual increment peaking after age 100 based on Organon model projections. Postthinning stand growth was greatest with the highest residual basal area; percent growth was highest in the lowest densities. Projections indicated that allocating 20% of plot area in 1/6 -1/4 acre gaps with stand basal areas comparable to uniform plots led to 1-10% reductions in growth. Reductions were not statistically significant. Diameter increment and average size exceeded normal yield tables at one site and were inversely related to basal area. At age 103, the difference in projected quadratic mean diameter across the densities was about 6 inches. By age 65, some tree diameters were already late-seral sizes. Crown cover increased rapidly during 15 years after thinning. Mortality has been negligible in McDonald, but windthrow has removed trees, especially hemlock at Blodgett.