Influence of Mount St. Helens ash on litter decomposition. I. Pacific silver fir needle decomposition in the ash-fall zone

The influence of air-fall tephra (ash) from the May 18 1980 eruption of Mount St. Helens on decomposition of Pacific silver fir (Abiesamabilis (Dougl.) Forbes) needles was studied at two sites along the ash-fall plume northeast of the mountain, Elk Pass and Chambers Lake (20 and 60 km from the crater, respectively). Ash depths beneath the canopy of the old-growth stands at Elk Pass and Chambers Lake were 18 and 5 cm, respectively. Objectives of the study were to determine: (i) the decomposition rates of needles on the ash surface, buried under ash, and in control plots with ash removed; (ii) the effect of site on decomposition rates; and (iii) the influence of ash on forest floor temperature, moisture, and oxygen levels. Ash had considerable influence on litter decomposition. After 3 years, needles buried under ash had faster decomposition rates at both sites (k = 0.34 and 0.29/year at Elk Pass and Chambers Lake, respectively) than needles on the ash surface or in cleared control plots. There was a trend for needles on the ash surface to have slower decomposition (k = 0.18–0.23/year) than needles on control plots (k = 0.22–0.28/year). Site had little influence on buried needle decomposition; rates at Elk Pass and Chambers Lake were not significantly different, despite differences in ash texture and depth. Ash apparently did not reduce oxygen levels enough to reduce decomposition, but instead increased decomposition by influencing substrate moisture and temperature. Fastest decomposition occurred under the ash where conditions were moist and cool; slowest decomposition occurred on the ash surface where conditions were drier and warmer.