Plant Responses in Forests of the Tephra-Fall Zone

Tephra fall is the most widespread disturbance resulting from volcanic activity (del Moral and Grishin 1999), including the 1980 eruption of Mount St. Helens (Sarna-Wojcicki et al. 1981). Tephra is rock debris ejected from a volcano that is transported through the air some distance from the vent that produced it. Fine-textured tephra (less than 2 mm in diameter) is referred to as volcanic ash. Tephra may be transported far from a volcano and affect vegetation over thousands of square kilometers, well beyond the influence of other types of volcanic ejecta. Individual tephra deposits from volcanoes in the Cascade Range have been traced east into the Great Plains, and others cover much of the Pacific Northwest (Shipley and Sarna-Wojcicki 1983). Mount St. Helens has been the most frequent source of tephra in the Cascades for 40,000 years, producing dozens of tephra layers equal to or larger than the 1980 eruption, three experienced by trees alive in 1980 (1480, 1800, and 1980; Mullineaux 1996). The likely extent and magnitude of past volcanic eruptions are apparent in Cascade Range soils near or downwind from major volcanoes, soils that are largely formed from tephra (Franklin and Dyrness 1973), and in the large amounts of tephra in soils far east of the Cascade Range (Smith et al. 1968).