Species composition and diversity during secondary succession of coniferous forests in the western Cascade mountains of Oregon

Species diversity and community composition were studied at 23 sites onsimilar western hemlock/Douglas-fir forest habitats, in undisturbed old-growth standsand stands at 2, 5, 10, 15, 20, 30, and 40 years after clearcutting, broadcast burning, andplanting with Douglas-fir. Vegetation was sampled with three 5 x 60 m transects at eachsite. Invading herbs, then invading and residual shrubs, and finally conifers dominatedthrough the first 30 years. Late seral species, which account for 99% of cover in old-growth stands, are nearly eliminated immediately following disturbance, but account foralmost 40% of vegetative cover after 5 years, 66% after 10 years, 83% after 20 years, and97% at 40 years. After an initial drop following disturbance, species diversity trendsweakly upward with heterogeneity peaking at 15 years and richness at 20 years. Thisinitially high diversity (higher than that of old-growth stands) is short-lived. After thetree canopy closes. species diversity declines reaching its lowest values at 40 years.Only two species were eradicated after disturbance, both mycotrophs. Pacific North-west old-growth forests are relatively poor in species, but moderately high in heteroge-neity values. FOR. So. 34(4):960-979.
ADDITIONAL KEY WORDS. Old-growth forests, disturbance, Pacific Northwest.