Wetland development in primary and secondary successional substrates fourteen years after the eruption of Mount St. Helens, Washington, USA
Titus, Jonathan H.
Titus, Priscilla J.
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The eruption of Mount St. Helens deposited new substrates upon which wetlands are now developing. Furthermore, the blast damaged vegetation in wetlands surrounding the deposition area. This study describes wetlands in the blast zone north of the crater as documented 14 years after the 1980 eruption with 220 100 m2 plots. Five wetland types were recognized in the field: moist areas, wet areas with shallow standing water, seasonal streams, permanent streams, and ponds or lakes. The variables, elevation, slope and aspect were determined; hot water springs were discriminated from cold water springs by a nominal variable, and substrate was characterized as stable or unstable. Four levels of disturbance intensity were recognized. Spatial variables, i.e., the position of wetlands on the landscape, were also used in analysis of the data. Using TWINSPAN, we organized the vegetation into 24 wetland plant communities. Based on canonical correspondence analysis, we determined that the measured environmental and spatial variables were unimportant in structuring primary successional vegetation. The lack of a relationship between the wetland types and environmental factors in the primary successional wetland communities suggests that stochastic events, such as seed dispersal, have played a leading role in early primary succession. Vegetation within secondary successional wetlands appears to be more closely related to the wetland types and environmental variables. Thus we conclude that environmental factors were more important in structuring wetland vegetation in areas that were less impacted by the eruption than in those areas where wetlands were newly created