ECOLOGICAL CONSTRAINTS TO SEEDLING ESTABLISHMENT ON THE PUMICE PLAINS, MOUNT-ST-HELENS, WASHINGTON

For both Epilobium angustifolium and Anaphalis margaritacea, the dominant species in primary succession on the Pumice Plains, density of the seed rain far exceeds the density of colonists. A. margaritacea established under a wider range of conditions than did E. augustifolium. Colonization by these species, and hence rate of primary succession, is limited by availability of safe sites for germination and establishment and not by seed dispersal. Safe sites are defined primarily by the level of subsurface moisture and secondarily by microtopography and seed size, and have a greater effect on seedling emergence than on seedling survival. -from Authors