Timber harvesting residue treatment. Part II. Understory vegetation response

Abstract

Foresters must have knowledge of understory vegetation responses to harvesting rehabilitation operations to manage competition effects on tree seedlings, protect species diversity, and reduce invasive weeds. Understory vegetation response to six post-harvest slash treatments, ranging from a high-intensity burn to mechanically chopped slash and untreated slash left in place, was documented for four eastern Washington mixed forest sites. We use species abundance graphs to show the relative effects of the six slash-treatment disturbances on vegetation dominance and diversity. Harvest substantially reduced understory cover, particularly among forbs. Shrub species persisted following harvest and in some treatments increased in cover. Slash treatments increased the abundance of weedy species that are not normally present in these forests. Treatments such as broadcast burning and pile/burn showed greater dominance by invader species. Overall, harvesting reduced species diversity but the response among slash treatments varied. After three growing seasons, species cover, richness, and diversity had no clear effect on seedling growth in slash-treatment plots.