EFFECTS OF THINNING ON STRUCTURAL DEVELOPMENT IN 40-YEAR-OLD TO 100-YEAR-OLD DOUGLAS-FIR STANDS IN WESTERN OREGON

We studied the composition and structure of the understory in thinned and unthinned Douglas-fir/western hemlock stands on 32 sites in wester n Oregon. These stands had regenerated naturally after timber was harv ested between 1880 and 1940; they were thinned between 1969 and 1984. Commercially thinned stands had 8-60% of their volume removed 10-24 ye ars before the study. Undisturbed old-growth Douglas-fir stands were p resent for comparison on 20 of these paired sites. Conifer regeneratio n density and frequency were consistently greater in thinned than unth inned stands. For example, average seedling density in thinned stands (1433/ha) was significantly (p less than or equal to 0.001) greater th an in unthinned stands (233/ha), but very similar to that in old-growt h stands (1010/ha). Seedling density and frequency were strongly relat ed to the volume removed and to stand density index (and other measure s of overstory density) just after thinning. In thinned stands, the de nsity of small trees (intermediate crown class overstory trees and adv anced regeneration) was 159/ha, significantly (p less than or equal to 0.001) greater than in unthinned stands (90/ha), but not significantl y different from that of old-growth (204/ha). The live crown ratio of these trees in thinned stands (66%) was greater than in unthinned (44% ) and old-growth (48%) stands. Cover and stem density of shrubs was va riable in all three stand types. There was significantly less tall shr ub cover in unthinned stands than in either thinned or old-growth stan ds, which did not differ. Thinned stands had the most low shrub cover. Salal and bracken fern cover was greater in thinned stands than in th e other stand types, but there was no difference in sword fern and Ore gongrape cover. Leaf area index in thinned stands (6.6) was not signif icantly different from that in unthinned (6.8) and old-growth stands ( 7.1); however, there was more leaf area in shrubs in the thinned stand s. Thinning young Douglas-fir stands will hasten the development of mu ltistory stands by recruitment of conifer regeneration in the understo ry as well as by enabling the survival of small overstory trees and gr owth of advanced understory regeneration. Thinning will also help deve lop the shrub layer by increasing tall shrub stem density and cover of some low shrubs. (C) 1998 Elsevier Science B.V. All rights reserved.