Early primary succession on the volcano Mount St. Helens

focus on primary succession in habitats formed on May 18, 1980, to explore mechanisms that control establishment of plants on new landforms. The rate of early primary succession is generally believed to be slow (cf. Crocker & Major 1955; Shure & Ragsdale 1977; Houle & Phillips 1989), though rates can differ significantly. As conditions are ameliorated by weathering and nutrient accumulation, the rate of succession gradually accelerates. Recruitment patterns shift as populations establish and reproduce successfully. Community structure may change strongly once a few poorly dispersing species become established. Primary succession may take centuries to unfold, but events that shape the entire process occur within a decade. Well-defined successional stages of primary succession on volcanoes have been described (Beardsley & Cannon 1930; Smathers & Mueller-Dombois 1974), but discrete stages have not been demonstrated in harsh arctic environments (Bliss & Peterson 1992). Successional stages may relate primarily to habitat differences. Early succession in stressful volcanic situations frequently involves only the gradual accumulation of species and incremental increases in biomass (Tagawa et al. 1985; Fridriksson 1987). Species numbers may plateau for many years while biomass continues to increase. Several abiotic processes must occur on bare substrates before biotic succession mechanisms can be initiated. Until recently, most studies of primary succession have emphasized retrospective descriptions including historical records (Whittaker, Bush & Richards, 1989; van der Maarel et al. 1985; Rydin & Borgegård 1991) or transects in which space is substituted for time (Houle & Phillips 1989). Long-term studies using frequently recorded permanent plots are less common (cf. Roozen & Westhoff 1985). Here we explore succession mechanisms using repeated sampling on permanently marked grids, pattern descriptions and small-scale experiments. Early succession on lahars at lower elevations has been described by Halpern & Harmon (1983), Frenzen, Krasney & Rigney (1988) and Dale (1989), whereas del Moral (1983; del Moral & Wood 1986; del Moral & Wood 1988) emphasized the development of species Abstract. Primary succession on Mount St. Helens, Washington State, USA, was studied using long-term observational and experimental methods. Distance from potential colonists is a major factor that impedes early primary succession. Sites near undisturbed vegetation remain low in plant cover, but species richness is comparable to intact vegetation. Sites over 500 m from sources of potential colonists have as many species, but mean species richness is much lower than in undisturbed plots. Cover is barely measurable after 11 growing seasons. Highly vagrant species of Asteraceae and Epilobium dominate isolated sites. Sites contiguous to undisturbed communities are dominated by large-seeded species. For a new surface to offer suitable conditions to invading plants, weathering, erosion and nutrient inputs must first occur. The earliest colonists are usually confined to specific microsites that offer some physical protection and enhanced resources. Primary succession on Mount St. Helens has been very slow because most habitats are isolated and physically stressful. Well-dispersed species lack the ability to establish until physical processes ameliorate the site. Species capable of establishment lack suitable dispersal abilities. Subsequently, facilitation may occur, for example through symbiotic nitrogen fixation, but these effects are thus far of only local importance. Lupinus lepidus usually facilitates colonization of other species only after it dies, leaving behind enriched soil lacking any competitors. Experiments and fine-scale observations suggest that successional sequences on Mount St. Helens are not mechanistically necessary. Rather, they result from local circumstances, landscape effects and chance.