Understory vegetation in old and young Douglas-fir forests of western Oregon

We studied understory composition in thinned and unthinned Douglas-fir (Pse udotsuga menziesii (Mirb.) Franco)/western hemlock (Tsuga heterophylla (Raf .) Sarg.) stands on 28 sites in western Oregon. These stands had regenerate d naturally after timber harvest, 40-70 years before thinning. Commercial t hinning had occurred 10-24 years previously, with 8-60% of the volume remov ed from below with the intent to homogenize spacing among trees. Undisturbe d old-growth Douglas-fir stands were present for comparison on 18 of these sites. Total herbaceous cover was greater in thinned (25% cover) stands tha n in unthinned (13% cover) or old-growth (15% cover) stands. Species richne ss was also greater in thinned (137) than in unthinned (114) and old-growth (91) stands (P=0.05). Part of the increased richness was caused by the pre sence of exotic species in thinned stands, but there were also more native grass and nitrogen-fixing species in thinned stands than in unthinned or ol d-growth stands. Groups of species differed among stand-types. For example, the frequency of tall cordate-leaved species was greater in old-growth sta nds (P = 0.009), but their relative cover was different only between old-gr owth and unthinned stands (P = 0.08). Both the cover and frequency of grass es and sedges in thinned stands were greater than in unthinned or old-growt h stands (P less than or equal to 0.002), Ordination of shrub cover showed differences among old-growth and unthinned stands compared to thinned stand s, mainly because of the amount of Gaultheria shallon Pursh and Polystichum munitum (Kaulf.) Presl in heavily thinned stands. Ordination of herbaceous community data showed that there were much stronger differences among site s than among stand-types. The lack of difference among stand-types demonstr ates the resiliency of herbaceous communities to disturbance associated wit h past and current forest management. (C) 1998 Elsevier Science B.V. All ri ghts reserved.