The Spread of Exotic Plant Species at Mount St. Helens: The Roles of a Road, Disturbance Type, and Post-disturbance Management
Mount St. Helens and the surrounding landscape were dramatically altered during and after the 18 May 1980 eruption, creating large expanses of disturbed land that could be susceptible to exotic plant invasion. We studied spatial patterns of exotic plant richness and abundance along a road and pedestrian trail that traverse a volcanic disturbance gradient with varying management prescriptions. In blast areas, exotic plant richness and abundance increased near the road, demonstrating the role that both roads and canopy cover play in the dispersal and establishment of exotic species. However, the overall richness and abundance of exotic plants were relatively low, and the pyroclastic-flow zone vegetation was dominated by native plants.
KeywordsPlant communities Biological invasion Land management Volcanic disturbance Species richness Community composition Succession Invasive species
Field data collection assistance was provided by Jessica Camp, Sara Copp, Abi Groskopf, and Kate Rhodes. Bill Becker, John Bishop, Sara Copp, Tim Elder, Ed Guerrant, Richard Halse (and the Oregon State University Herbarium), Dominic Maize, Ashley Smithers, Duncan Thomas, and Anthony Wenke provided assistance with plant identifications. We thank Sara Copp, Mat Dorfman, Yangdong Pan, and Jill VanWinkle for statistical consultation. We also thank Robert “Rocky” Pankratz for providing US Forest Service data regarding planting, thinning, and bough harvesting. LLK thanks the Mount St. Helens Institute for providing living arrangements while in the field. The USFS Pacific Northwest Research Station provided funding for CMC. This work was supported by the National Science Foundation. LTREB Program DEB-0614538 to CMC.
In the case of the Mount St. Helens 1980 eruption, failure of the volcano’s north flank unroofed pressurized magma and superheated water. Rapid exsolution of magmatic gases and conversion of superheated water to steam produced a laterally directed blast, which formed a density current that flowed across rugged topography. The current contained fragmented rock debris as well as shattered forest material.
A rapid granular flow of an unsaturated or partly saturated mixture of volcanic rock particles (± ice) and water, initiated by the gravitational collapse and disintegration of part of a volcanic edifice. Debris avalanches differ from debris flows in that they are not water-saturated. Although debris avalanches commonly occur in association with eruptions, they can also occur during periods when a volcano is dormant.
An Indonesian term for a rapid granular flow of a fully saturated mixture of volcanic rock particles (± ice), water, and commonly woody debris. A lahar that has ≥50% solids by volume is termed a debris flow; one that has roughly 10–50% solids by volume is termed a hyperconcentrated flow. Flow type can evolve with time and distance along a flow path as sediment is entrained or deposited.
Rapid flow of a dense, dry mixture of hot (commonly >700 °C) solid particles, gases, and air that moves across the ground surface, often following landscape typography. Flows are generally gravity driven but may be accelerated initially by impulsive lateral forces of directed volcanic explosions. Flows typically move at high velocity (up to several hundred km/h).
A rain of volcanic particles to the ground following ejection into the atmosphere by an explosive eruption. Tephra is a collective term for particles of any size, shape, or composition ejected in an explosive eruption.
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