Seedling establishment patterns on the Pumice Plain, Mount St. Helens, Washington



Abstract. We examined the factors that control seedling establishment on barren substrates on the pyroclastic flows from Mount St. Helens. From June to September in 1993, we monitored seedling and microhabitat changes in 240 20 cm × 20 cm quadrats on the Pumice Plain. Seedlings emerged in 104 quadrats (43.3 %). The most abundant species were Anaphalis margaritacea, Hypochaeris radicata, Lupinus lepidus and Epilobium angustifolium. Measured site characteristics included topography, particle size distribution, ground surface movements, soil water content, organic matter, pH, and presence or absence of dead lupines. Quadrats with seedlings had higher cover of dead lupines, higher amount of rock and gravel substrate, and a greater cover of rills. More seedlings emerged where eroded material accumulated. Compared to coarse-textured surfaces, silt surfaces had higher organic matter, held more water, and showed higher pH. However, seedlings became established more frequently on coarse-textured surfaces. In greenhouse experiments, a higher percentage of Hypochaeris seeds germinated on silt than on sand or gravel. The germination of Anaphalis and Epilobium did not differ with soil texture, but was higher at higher moisture levels. Seedling colonization is more dependent on ground surface microtopography, particle size, and ground movement than on the chemical status of these volcanic deposits.