Studies of primary succession on Mount St. Helens have mainly employed large scale, landscape-level approaches with coarse resolutions. However, the mechanisms of successional dynamics are individual- and species-level processes such as facilitation, inhibition, and tolerance. We conducted annual vegetation surveys from 1983 to 2007 in a 12x14m study plot to examine the role of these smaller scale processes in shaping the successional plant community on the Pumice Plain at Mount St. Helens. We described the succession process by calculating species richness, total cover, and cover evenness over time. We used agglomerative hierarchical clustering methods to examine the community types formed within our study plot and how they changed across the study period.
We observed 47 species over the 25-year study. Patterns in species richness, plant cover, and cover evenness suggested three periods of successional dynamics. From 1983-1989 all three metrics were low but increasing as species accumulated. Between 1990-1997 all three measures were high although total cover fluctuated across years. Finally, from 1998-2007 species richness continued to increase, cover evenness remained high, but total cover decreased to a moderate level. These patterns suggest there were periods of amelioration, fluctuation, and stabilization to a state in which there was high species richness but also high evenness. Results from cluster analysis supported this trend: both cluster diversity and the number of species defining community clusters increased over time. Only one community type, dominated by Lupinus lepidus, was present from 1983-1988. Subsequently Hypochaeris radicata, Anaphalis margaritacea, Hieracium albiflorum, bryophytes Ceratodon purpureus and Polytrichum juniperinum, and grasses Elymus glaucus and Festuca rubra colonized and formed six additional community types in varying combinations. Cluster and species turnover rates did not decline over time but were high in 2005 and 2007, potentially indicating an additional phase coinciding with species recruitment and development of new community types. Continued monitoring will inform the extent and direction of successional change.