Vegetation development on pumice at Mount St. Helens, USA

This study explores early vegetation development on pumice at Mount St. Helens. We monitored species annually in a grid of 200 contiguous 100-m(2) plots between 1989 and 1999. Of interest were how vegetation changed and if it became more homogeneous over time. Species richness and cover increased annually. Diversity (H') stabilized by 1996 and began to decline as long-lived stress-tolerant species such as Agrostis pallens, Carex mertensii and Penstemon cardwellii began to dominate. Protected sites had more species and higher cover than did exposed ones. Plots next to relict vegetation had more species and cover than did distant plots. The vegetation initially was dominated by species with good dispersal, but subsequently those with poor dispersal became dominant. We compared species expansion patterns to a model based on random colonization. The results implied that populations with poor dispersal derived from a few colonists that then produced seeds for local expansion. Detrended correspondence analysis showed a pronounced shift in species composition. This analysis also showed that species composition was becoming more homogeneous over time. However, significant heterogeneity remained and some plots are diverging from others. As yet, this vegetation is not developing towards a regional vegetation type. Rather, an unusual community with Agrostis spp., Carex spp., Penstemon cardwellii, Lupinus lepidus, Anaphalis margaritacea and Salix commutata has developed. The accumulation phase of primary succession is nearly complete. The next phase, in which vertical structure develops as Salix and conifers mature, has scarcely begun. It should be marked by the invasion of forest understory species and loss of subalpine meadow species. Assembly rules based on biotic interactions may then become evident.