INFLUENCE OF MOUNT ST-HELENS ASH ON LITTER DECOMPOSITION .1. PACIFIC SILVER FIR NEEDLE DECOMPOSITION IN THE ASH-FALL ZONE

The influence of air-fall tephra (ash) from the May 18 1980 eruption o f Mount St. Helens on decomposition of Pacific silver fir (Abies amabi lis (Dougl.) Forbes) needles was studied at two sites along the ash-fa ll plume northeast of the mountain, Elk Pass and Chambers Lake (20 and 60 km from the crater, respectively). Ash depths beneath the canopy o f 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 dec omposition rates of needles on the ash surface, buried under ash, and in control plots with ash removed; (ii) the effect of site on decompos ition rates; and (iii) the influence of ash on forest floor temperatur e, moisture, and oxygen levels. Ash had considerable influence on litt er decomposition. After 3 years, needles buried under ash had faster d ecomposition rates at both sites (k = 0.34 and 0.29/year at Elk Pass a nd Chambers Lake, respectively) than needles on the ash surface or in cleared control plots. There was a trend for needles on the ash surfac e to have slower decomposition (k = 0.18-0.23/year) than needles on co ntrol plots (k = 0.22-0.28/year). Site had little influence on buried needle decomposition; rates at Elk Pass and Chambers Lake were not sig nificantly different, despite differences in ash texture and depth. As h apparently did not reduce oxygen levels enough to reduce decompositi on, but instead increased decomposition by influencing substrate moist ure and temperature. Fastest decomposition occurred under the ash wher e conditions were moist and cool; slowest decomposition occurred on th e ash surface where conditions were drier and warmer.