Timber harvesting residue treatment. Part II. Understory vegetation response

Foresters must have knowledge of understory vegetation responses to harvest ing rehabilitation operations to manage competition effects on tree seedlin gs, protect species diversity, and reduce invasive weeds. Understory vegeta tion response to six post-harvest slash treatments, ranging from a high-int ensity burn to mechanically chopped slash and untreated slash left in place , was documented for four eastern Washington mixed forest sites. We use spe cies abundance graphs to show the relative effects of the six slash-treatme nt disturbances on vegetation dominance and diversity. Harvest substantiall y reduced understory cover, particularly among forbs. Shrub species persist ed following harvest and in some treatments increased in cover. Slash treat ments increased the abundance of weedy species that are not normally presen t in these forests. Treatments such as broadcast burning and pile/burn show ed greater dominance by invader species. Overall, harvesting reduced specie s diversity but the response among slash treatments varied. After three gro wing seasons, species cover, richness, and diversity had no clear effect on seedling growth in slash-treatment plots. (C) 2000 Elsevier Science B.V. A ll rights reserved.