Vegetation Patterns 25 Years after the Eruption

We conducted a vegetation survey of newly formed sites on Mount St. Helens in 2004 and compared it to a 1992 survey. We asked whether vegetative heterogeneity had become reduced and sought evidence from floristic classifications, links between species and environment, and changes in community structure. From species composition in 271 250-m plots we determined diversity and dominance. Six loosely defined community types (CTs) were recognized. Their habitats, based on geographic and surface features, differed primarily in their location, but also by surface characteristics. The CTs were separated by detrended correspondence analysis (DCA), and DCA axes were related to location, elevation, exposure and surface variables. Canonical correspondence analysis confirmed similar weak, but significant correlations. Comparisons to the 1992 survey (in four habitats) demonstrated a large increase in plant cover that accelerated only since 2001. Pioneer species such as Anaphalis and Chamerion declined while mosses increased significantly. Rank order correlations of each habitat were significant, but low. Vegetation structure developed from differential accumulations of species set against a large cover increase. In lupine colonies, dominance was reduced and diversity increased due to the increased abundance of many formerly uncommon species. In habitats found on pumice, pyroclastics and drainages, dominance increased, while diversity changed little. This pattern suggested an incipient development of competitive hierarchies and differential response to herbivory. Weak correlations between vegetation and the environment suggested that establishment initiated by stochastic processes had not been erased by deterministic factors. A vegetation mosaic weakly tied to environmental factors may produce different succession trajectories and produce alternative stable communities in similar habitats.