Dr. Charlie Crisafulli
Date of Award
Master of Environmental Management (MEM)
Environmental Science and Management
The landscape of Mount St Helens was dramatically transformed when it erupted on May 18, 1980, creating an outstanding opportunity for ecologists to study succession following a large, intense, natural disturbance. Within the expansive (600 km2) blast area there is varied ownership and a mix of management objectives for the post-eruption landscape. On lands administered by the U.S. Forest Service, there are portions of the blast area that have been designated as a National Volcanic Monument, where the primary management objective is to allow for natural succession to proceed substantially unimpeded. In other areas outside the Monument, salvage logging and replanting of tree seedlings occurred and multiple-use is the designated management objective. Even within the Monument, activities such as road, trail, and visitor center construction has occurred. Such activities stand to increase the potential plant invasibility of the area through increased vectors and dispersal corridors. This study looks at the spatial pattern of exotic plant species richness and abundance along a road that traverses a volcanic disturbance gradient consisting of two distinct zones: a tephra impacted forest and three different vegetation types in the blow-down zone, as well as along a pedestrian trail in the roadless Pyroclastic flow zone. Plant communities were sampled along 20 replicate transects (70 m) in each of five different disturbance/management/succession areas. Transects were positioned perpendicular to the road axis to assess how exotic plant species richness, abundance, and relative abundance varied with distance from the road. Linear regression was used to determine whether the road influenced exotic plant richness and abundance. In the Pumice Plain, where primary succession is occurring, minimal amounts of two wind-dispersed, exotic species, Hypochaeris radicata and Rumex acetosella, were observed. In the blowdown zone, where the eruption destroyed vegetation, exotic plants in the replanted and shrub-dominated areas significantly decreased moving away from the road, while exotic plants did not significantly decrease away from the road in the forb-dominated blowdown area. Exotic plant species were restricted to within five meters of the road in the intact late seral forest that was disturbed by the deposits of 12 - 15 cm of cool tephra consisting of silt to gravel-sized particles. My research demonstrates an overall low abundance of exotic plants at Mount St. Helens, that exotic plant species are more prevalent along the roadsides, but that natural forest and shrubs slow their spread to the interior landscape.
Karr, Lindsey, "The Spread of Exotic Plant Species at Mount St Helens: the Roles of a Road, Disturbance Type and Post-Disturbance Management" (2014). Master of Environmental Management Project Reports. 43.