Understory vegetation in young Douglas-fir forests: does thinning help restore old-growth composition?

We compared the understory herb and shrub communities of pre-commercially thinned and unthinned young- and old-growth Douglas-fir (Pseudotsuga menziesii) stands. Sites were located in the Cascade Mountains of western Oregon. Young stands were approximately 40 years old at the time of sampling; thinning occurred 20 years earlier. Although tree spacing was wider in thinned than in unthinned stands, faster tree growth in thinned stands meant that thinning had no long-term effect on basal area of P. menziesii. The very large, widely spaced P. menziesii individuals in old-growth stands contributed similar basal area as the smaller trees in young stands; however, old stands had much higher basal area of shade-tolerant conifers. Although low shrubs responded positively to thinning, the difference in cover between thinned and unthinned stands was too small to be statistically significant. Ordination of sample plots in terms of their understory shrub and herb species composition identified a strong gradient in community composition from young to old-growth stands. Understory species associated with old growth included both gap specialists and those that may be dependent on the litter depth and mycorrhizal fungi of old-growth forest floors. Although both thinned and unthinned young stand composition were quite distinct from that of old-growth stands, thinned stands were more similar to old growth than were unthinned stands. Thinned stands had higher frequency of late-seral herbs than did unthinned stands. Forest generalist and release species showed mixed responses to thinning. We conclude that pre-commercial thinning may be a useful management toot to encourage old growth associated understory herbs without precipitating dominance of the understory by low shrubs and weedy species. Although thinning did accelerate tree growth, changes in the understory appear to have been precipitated by the transient increase in resource levels following thinning rather than by any long-term changes in stand structure. (C) 2004 Elsevier B.V. All rights reserved.