EARLY PRIMARY SUCCESSION ON A BARREN VOLCANIC PLAIN AT MOUNT ST-HELENS, WASHINGTON

The invasion pattern on a barren plain on the eastern flank of Mount S t. Helens, Washington, devastated by the 1980 volcanic eruption was mo nitored between 1988 and 1992. All vascular plants on a grid consistin g of 400 contiguous 100-square-meter quadrats were recorded with a cov er score. The substrate was initially homogeneous, but significant het erogeneity had developed by 1988. Vascular plant species richness incr eased 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 varianc e/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 seedl ings continue to result from long-distance dispersal, but reproductive colonies of species are developing. Seedling distributions are contro lled by microsites. Eleven common species strongly and similarly prefe rred 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 patter n permitted us to distinguish species still dependent on immigration f rom those now locally established. Pronounced microsite preferences em phasize that physical amelioration (e.g., nutrient input and erosion) must occur before further succession can commence. We have observed th e early stages of succession where an inhospitable site is gradually a nd heterogeneously changed into a habitat where safe-sites do not limi t succession, but where stochastic factors remain important.