CONSPICUOUS ALGAL COLONIZATION OF THE ASH FROM MOUNT ST. HELENS1

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Abstract
Conspicuous growth of green and yellow‐green algae developed in arid steppe communities in eastern Washington on the silt‐sized ash from the 18 May 1980 eruption of Mount St. Helens. In a veneer on the ash, cell numbers per gram dry ash of these aerial immigrants equaled or exceeded the highest reported values for terrestrial chlorophytes (108 cells · g−1 dry soil). Such pronounced growth was restricted largely to three microsites (under the canopy of Artemisia tridentata, along the runways of the vole, Microtus montanus, and surrounding emergent agarics), apparently in response to water‐holding properties of the ash, localized nutrient input and possible reduction of predation by nematodes. Highest algal counts were in Microtus runways where daily water and urea‐N input per adult animal may be 8 mL and 23 mg, respectively. Cyanophytes, commonly considered early colonizers of volcanic ash, were extremely rare. The role of blue‐green algae in succession on volcanic materials may be more restricted geographically than previously recognized.