Mechanisms of Early Primary Succession in Subalpine Habitats on Mount St. Helens

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In the 6 yr following the 1980 eruption of Mount St. Helens in Washington state, vascular plant invasion of barren substrates in subalpine habitats has been limited despite the proximity of seed sources from resprouted vegetation. From 1983—1985 we counted 1st—yr seedlings and estimated percent cover of adults in grids of permanent plots located across the ecotone between residual vegetation and barren substrate created in 1980. We found that (1) seedling recruitment declined from 1983 to 1985, apparently in response to drought; (2) most seedlings occurred within 3 m of a conspecific adult; and (3) plots with an intermediate vegetation cover (5—72%) contained a significantly higher fraction of seedlings than did unvegetated plots. Thus, dispersal is limited in many species and nurse plants may play a key role in trapping seeds and promoting seedling establishment. We sowed 16 000 viable seeds from 22 subalpine species into 264 plots in barren substrates at two sites on the volcano. The design was a complete factorial, with species, site, and fertilizer as treatments. Survivorship through 1985 varied from 0 to 12%, with Sitanion jubatum, Stipa occidentalis, Polygonum newberryi, Eriogonum pyrolifolium, and Spraguea umbellata attaining the highest values. Survivorship was correlated with seed mass, and was higher at the site where the pre—eruption surface was exposed. Fertilizer increased the size of most seedlings but had only a marginal effect on survivorship. Species with high environmental tolerance generally dispersed short distances, whereas species that dispersed farther generally had low tolerances and apparently require site amelioration prior to establishment. The path of early succession depends upon the spatial position and dispersal abilities of species in the seed pool, and may not reflect environmental gradients.