Pattern and process in primary succession in high elevation habitats on Mount St. Helens

Abstract

Thesis (Ph. D.)--University of Washington, 1987The mechanistic basis of primary succession remains poorly known. The facilitation model, thought to describe primary succession, stresses the environmental modifications caused by early colonists as important in promoting the establishment of species intolerant of unmodified site conditions. This model has rarely been tested experimentally. I evaluate the facilitation model in subalpine mudflows created by the 1980 eruption of Mount St. Helens in Chapters 1 and 2. In Chapter 3, I discuss the colonization of pyroclastic flows. Chapter 4 integrates my findings.Chapter 1. Plant invasion of subalpine mudflows is limited despite the proximity of recovered vegetation. In sampling across the vegetation-mudflow ecotone, I found that (i) most seedlings occurred within 3 m of a conspecific adult, and (ii) intermediate plant cover promoted seedling establishment (nurse plant effects). Survivorship of planted seeds of 22 species in mudflows varied from 0 to 12% and was positively correlated with seed mass. Invasion of mudflows is limited because stress tolerant species are poor dispersers.Chapter 2. In a direct test of the facilitation model, I predicted that survivorship of stress intolerant species on mudflows would be increased if I experimentally mimicked the environmental effects of plant colonists. I altered both above- and below-ground factors in a factorial design. Both seedling emergence and survival were significantly greater in treated plots than in controls. Seven of 21 species survived only in treated plots. For these, the facilitation model is supported. For the remainder, facilitation is facultative (11 species) or not required.Chapter 3. Seed rain density into the pyroclastic zone averaged 350 seeds m\sp{-2} yr\sp{-1} over 3 yr, with Epilobium angustifolium and Anaphalis margaritacea the most abundant species. These, plus Lupinus lepidus, were also the most common colonizers (n = 32 species). Sixteen of 24 species in the seed rain were colonists. Colonists and the seed rain were floristically similar, with herbaceous species predominating. Mean species richness was only 1 per 100 m\sp2, but wet sites contained up to 12 species per 100 m\sp2. As yet, most colonization occurs in habitats of low stress and there is no evidence for direct facilitation