Early Primary Succession on a Barren Volcanic Plain at Mount St. Helens, Washington

Abstract

The invasion pattern on a barren plain on the eastern flank of Mount St. Helens, Washington, devastated by the 1980 volcanic eruption was monitored between 1988 and 1992. All vascular plants on a grid consisting of 400 contiguous 100-square-meter quadrats were recorded with a cover score. The substrate was initially homogeneous, but significant heterogeneity had developed by 1988. Vascular plant species richness increased from 24 in 1988 to 41 in 1992. Mean species richness per quadrat increased from 0.44 to 5.71, mean cover increased from 0.04% to 0.51%, and mean diversity index (H') increased from 0.08 to 1.56. A variance/mean test of species richness pattern showed that invasion occurred sporadically since plots tended to have either several or no species. By 1992, mean species richness was more evenly distributed. Most seedlings continue to result from long-distance dispersal, but reproductive colonies of species are developing. Seedling distributions are controlled by microsites. Eleven common species strongly and similarly preferred safe-sites created by small rocks, undulations, or rills. However, many apparent safe-sites are empty, suggesting that seeds are scarce and that even the most favorable microsites are marginal. The niches of these species seem to overlap broadly. The Plains of Abraham is in the earliest stage of primary succession. The detailed invasion pattern permitted us to distinguish species still dependent on immigration from those now locally established. Pronounced microsite preferences emphasize that physical amelioration (e.g., nutrient input and erosion) must occur before further succession can commence. We have observed the early stages of succession where an inhospitable site is gradually and heterogeneously changed into a habitat where safe-sites do not limit succession, but where stochastic factors remain important.