Background/Question/Methods
Factors that define ecosystem health and recovery, including plant growth, diversity, and ongoing disturbances, have been explored on the debris avalanche deposit at Mount St. Helens (MSH) since the 1980 eruption and may help to identify indicators that drive patterns in a post-disturbance landscape. These disturbances included natural events such as the reestablishment and ongoing meandering of the Toutle River, shallow landslides, lahars, and erosion, as well as land-management activities such as aerial seeding of plant species with the intent to reduce erosion. The most recent disturbance is the outbreak of elk hoof disease concentrated around MSH. This 40-year study of vegetation reestablishment on the debris avalanche deposit uses information from 62 permanent plots to understand changes in vegetation and begins to relate them to dynamics of elk herds in the region. Long-term measurements in these plots include plant reestablishment, growth, and biomass for areas undergoing natural succession and those where grass and legume seeds were distributed in 1981. This study addresses how ecological conditions have changed over time following the 1980 eruption; what do alterations in key indicators suggest for ecological resilience to disturbance; and what is the relationship between ecological conditions and the occurrence of elk hoof disease.
Results/Conclusions
Many changes have occurred on the debris avalanche deposit since the eruption. Very few plants survived the eruption, and primary succession was initiated by seeds dispersed primarily by wind onto the newly emplaced material. During the 40 years post-eruption, understory plant cover rose to almost 60%; trees were established; canopy closure occurred in many places; and now the early successional alders are dying. The number of species continues to increase, and aerial distribution of largely non-native seeds had a pronounced effect on vegetation communities. Environmental factors are likely to be affecting initiation and propagation of the elk hoof disease. The effects of natural disturbances and human activities range in scale both in their cause and impacts. Examining the scale of these disturbances is a useful way to synthesize major patterns and make predictions that aid in management. Key observations regarding ecological reestablishment include: biotic and abiotic legacies from the disturbance influence patterns and rates of ecosystem development; ecological response is rapid in some systems but is spatially and temporally heterogeneous; secondary disturbances, human activities, and ongoing geophysical processes influence ecological response; random events are important; ecological succession is complex; resource managers should plan for disturbances, and persistence is essential.
