SEEDLING ESTABLISHMENT PATTERNS ON THE PUMICE PLAIN, MOUNT-ST-HELENS,WASHINGTON

We examined the factors that control seedling establishment on barren substrates on the pyroclastic flows from Mount St. Helens. From June t o September in 1993, we monitored seedling and microhabitat changes in 240 20 cm x 30 cm quadrats on the Pumice Plain. Seedlings emerged in 104 quadrats (43.3 %). The most abundant species were Anaphalis margar itacea, Hypochaeris radicata, Lupinus lepidus and Epilobium angustifol ium. Measured site characteristics included topography, particle size distribution, ground surface movements, soil water content, organic ma tter, pH, and presence or absence of dead Iupines. Quadrats with seedl ings had higher cover of dead lupines, higher amount of rock and grave l substrate, and a greater cover of rills. More seedlings emerged wher e eroded material accumulated. Compared to coarse-textured surfaces, s ilt surfaces had higher organic matter, held more water, and showed hi gher pH. However, seedlings became established more frequently on coar se-textured surfaces. In greenhouse experiments, a higher percentage o f Hypochaeris seeds germinated on silt than on sand or gravel. The ger mination of Anaphalis and Epilobium did not differ with soil texture, but was higher at higher moisture levels. Seedling colonization is mor e dependent on ground surface microtopography, particle size, and grou nd movement than on the chemical status of these volcanic deposits.