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 ( Keywords: Succession, Species diversity, Species turnover, Vegetation dynamics