1980 tephra from Mount St. Helens: Spatial and temporal variation beneath forest canopies

Summary Tephra and underlying litter and soil were sampled in 1980, 1982, and 1987 beneath subalpine forests where 4.5 and 15 cm of tephra fell during the 1980 eruption of Mount St. Helens, Washington State, USA. Coarse pumice had a higher initial pH and less total N, less exchangeable K, Ca, and Mg, and less extractable B and S than finer textured layers. Tephra pH and concentrations of cations and S decreased rapidly with time, especially during the first 2 years in the finer layers. Total N, Bray-Kurtz P, and organic C concentrations in the tephra increased with time. Changes within a site in total N, pH, organic C, P, Ca, Mg, and S from 1980 to 1987 exceeded the differences among sites at any one time. By 1987 a forest floor covered much of the tephra surface, and differences in Ca associated with site vegetation and seepage had developed in tephra layers of a similar depth. In 15 cm deep tephra at one site in 1987, the tephra crust beneath the forest canopy was thicker and had higher concentrations of coarse particles, organic C, total N, and cations than beneath forest openings. In concave microsites the crust was thicker, with higher pH, organic C, and total N but lower S than in adjacent convex microsites. Spatial and temporal chemical differences are sufficient to affect patterns of vegetation recovery.