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Primary succession trajectories on pumice at Mount St. Helens, Washington




Does vegetation become less variable over time? Do floristic trajectories converge during succession? Can either allogenic (external) or autogenic (internal) factors predict species patterns during succession?


Pumice Plain of Mount St. Helens (46.23449°N; 122.15929°W, 1230 m a.s.l.), which was sterilized in 1980 by a direct volcanic blast, then buried in pumice.


We monitored a grid of 200 100-m2 contiguous plots annually (1989–2010) and classified the 2010 data into five community types (CTs). We characterized plots using external (e.g. distance from relict sites) and internal (e.g. moisture) factors and clustered plots by position and habitat factors to assess trajectories. We used redundancy analysis (RDA) to assess relationships between vegetation and these variables over time. Mantel tests, similarity changes and detrended correspondence analysis (DCA) were used to evaluate successional trends.


Five CTs were weakly related to habitats at this scale. Over time, vegetation became more homogeneous and successional trajectories of plots clustered by position and habitat factors became more similar or developed in parallel. The succession rate slowed fitfully as persistent species became dominant, while Lupinus fluctuated. Mantel tests indicated that relationships between species and environment were maximized using only autogenic factors. RDA found that autogenic factors were stronger than allogenic ones, while explained variance did not increase after 1992 and factor correlations to species stabilized by 1996.


The strength of links between vegetation and environment were weak at this scale, yet explained variation stabilized early in succession. As substantial vegetative variation persisted and convergence was far from complete due to weak links to habitat factors, habitat variation and stochastic establishment should be considered by vegetation managers.