EARLY PRIMARY SUCCESSION ON MOUNT ST-HELENS, WASHINGTON, USA

The north slope of Mount St. Helens was sampled with 141 circular 100- m(2) plots to describe vegetation and environmental patterns 13 yr aft er the 1980 eruption. At least 114 vascular plant taxa were encountere d. We recognized four habitat types: Refugia, Pumice barrens, Pyroclas tic surfaces and Drainages. A fifth category, Lupine patches, includes samples on primary surfaces that were rapidly colonized. Refugia prov ided small enclaves where underground portions of several species surv ived the eruption. They retained an inconsistent array of forest under story species and contained 86 species (mean = 20.8 per plot). Refugia are dominated by woody species such as Penstemon cardwellii, Rubus sp p., Ribes spp. and Alnus sinuata, with herbs such as Agrostis diegoens is, Luzula parviflora and Anaphalis margaritacea. Anaphalis represents a suite of species that invaded Refugia after the eruption. Diversity (N-2 and H' is significantly greater in Refugia than in any other hab itat. No plants survived on primary sites, which remain sparsely veget ated and dominated by readily dispersed taxa. Total richness ranges fr om 36 species (9.9 per plot) on pyroclastic surfaces, through 42 speci es (11.2 per plot) in drainages, to 66 (11.7 per plot) on Pumice barre ns. H' and N-2 of the three habitats do not differ significantly. Lupi ne-dominated vegetation occurs sporadi cally in Pyroclastic and draina ge habitats. Lupine patches are characterized by high Lupinus cover an d a suite of invaders. These sites have high cover and 52 species (12. 6 per plot). H' and N-2 scores were significantly lower than any other habitat due to strong lupine dominance. Canonical Correspondence Anal ysis showed that site history and slope contributed most to species co mposition. Geographic effects accounted for 10-25% of the explained sp ecies-environment relationship. Forest understory species have migrate d only short distances and have made negligible contributions to veget ation development. A few species common in Refugia, including Agrostis diegoensis and Carex mertensii, have invaded barren surfaces, but mos t have not. Refugia also have been invaded by open site species abunda nt on the Pumice Plain. The heterogeneity of plots within habitat type s and small statistical linkage of vegetation to environmental and spa tial factors suggests that stochastic events have played a leading rol e in early primary succession.