Questions: Primary succession, measured by changes in species composition, is slow, usually forcing a chronosequence approach. A unique data set is used to explore spatial and temporal changes in vegetation structure after a 1980 volcanic eruption. On the basis of data from a transect of 20 permanent plots with an altitudinal range of 250 m sampled through 2005, two questions are asked: Do changes along the transect recapitulate succession? Do plots converge to similar composition over time?
Location: A ridge between 1218 and 1468 m on Mount St. Helens, Washington, USA.
Methods: Repeat sampling of plots for species cover along a 1-km transect. Floristic changes were characterized by techniques including DCA, clustering and similarity.
Results: Species richness and cover increased with time at rates that decreased with increasing elevation. The establishment of Lupinus lepidus accelerated the rate of succession and may control its trajectory. Diversity (H′) at first increased with richness, then declined as dominance hierarchies developed. Primary succession was characterized by overlapping phases of species assembly (richness), vegetation maturation (diversity peaks, cover expands) and inhibition (diversity declines). Each plot passed through several community classes, but by 2005, only four classes persisted. Succession trajectories (measured by DCA) became shorter with elevation. Similarity between groups of plots defined by their classification in 2005 did not increase with time. Similarity within plot groups converged slightly at the lower elevations. Despite similarities between temporal and spatial trends in composition, trajectories of higher plots do not recapitulate those of lower plots, apparently because Lupinus was not an early colonist. Any vegetation convergence has been limited to plots that are in close proximity.
Nomenclature: A list of species used in this study is at http://protist.biology.washington.edu/-delmoral/. Nomenclature follows the Integrated Taxonomic Information System: http://www.itis.usda.gov/advanced_search.html.