SEEDLING ESTABLISHMENT IN DIFFERENT MICROSITES ON MOUNT ST. HELENS, WASHINGTON, USA

On the volcanically devastated Pumice Plain of Mount St. Helens, plant species colonized microsites differentially. Peak colonization did no t occur in the same microsites as peak establishment and growth. In ad dition, observed microsite colonization patterns differed between year s. Two studies were conducted. The first assessed seedling establishme nt and growth from seeds sown at different microsites. The second asse ssed colonization into four microsite types that were constructed on t he Pumice Plain. Hypochaeris radicata was the most common species to s urvive when the same number of seeds of four species were planted; how ever, Anaphalis margaritacea was the most common colonizer of microsit es. Microsites with the largest biomass plants in the first study gene rally had the highest colonization in the second study. Sites that do not possess features to trap seeds, such as flats and ridges, are not opportune places for a plant to grow since there is little microclimat ic or substrate amelioration. Thus, Rat microsites had low biomass in the establishment experiment due to the lack of amelioration and conta ined few plants in the colonization experiment due to a lack of seed t rapping mechanisms. These results show that in the primary successiona l landscape of Mount St. Helens microsites are critical to revegetatio n dynamics. Changes in the pattern of microsite colonization between y ears emphasizes the dynamic nature of the landscape and the important influences of climate, substrate amelioration and seed rain to plant e stablishment.