Vegetation changes in blown-down and scorched forests 10-26 years after the eruption of Mount St. Helens, Washington, USA

We examine patterns of vegetative change in blown-down and scorched forests in the blast zone of Mount St. Helens (USA), 10-26 years after the eruption. We compare trends in community attributes in four post-eruption environments, or site types, defined by severity of disturbance, presence/absence of a protective snowpack at the time of eruption, and seral state (previously clearcut vs. mature/old forests). Permanent plots established in 1980 at 16 sites were sampled at 5- to 6-year intervals between 1989 and 2005. Data on species presence and abundance were used to characterize changes in total plant cover, life-form spectra, species diversity, species turnover, and community composition. Due to the magnitude and heterogeneity of disturbance, vegetation re-establishment was gradual and highly variable among sites. Total plant cover averaged 36-70% after 26 years. Early-seral forbs were dominant except in snow-protected sites, where surviving shrubs were most common. Tree regeneration remained sparse after 26 years (< 6% cover in all but two sites). Species richness increased in all site types, reflecting greater species gain than loss, although rates of gain declined with time. Species heterogeneity, integrating the number and abundance of taxa, did not increase. Successional trajectories were distinct, but parallel among sites, reflecting legacies of pre-eruption composition, variation in disturbance severity, and differences in composition of early-seral colonists. Slow re-colonization by forest herbs and trees likely reflects seed limitations and abiotic stress rather than competition from early-seral species. Succession following this major eruption is both slow and contingent on pre-conditions, nuances of the disturbance, and species' life histories.