Influence of Mount St. Helens ash on litter decomposition. II. Experimental studies with Douglas-fir needles

The influence of air-fall tephra (ash) from the 1980 Mount St. Helens eruptions on decomposition of Douglas-fir (Pseudotsugamenziesii (Mirb.) Franco) needles was studied in a field experiment at the Cedar River watershed near Seattle, Wash. Ash that fell on Yakima, Wash., was transported to the site and placed on 5 × 5 m plots in a 50-year-old Douglas-fir stand at depths of 5 and 20 cm. Objectives of the study were to determine: (i) decomposition rates of Douglas-fir needles on the ash surface, buried under ash, and in an untreated control; (ii) the effect of ash depth on decomposition rates; and (iii) the influence of ash on forest floor temperature and moisture. Yakima ash had considerable influence on forest floor temperature and moisture and Douglas-fir needle decomposition rates. Needles buried under 5 cm of ash had increased rates of decomposition after 2 years (k = 0.595/year) relative to control needles (k = 0.347/year), while those buried under 20 cm of ash had a similar rate of decomposition to control needles (k = 0.349/year), perhaps owing to ash compaction. Those on the ash surface had decreased rates of decomposition relative to buried needles. These results are similar to results found in the ash fall zone near Mount St. Helens, where needles located on top of ash decomposed slower than buried needles. Increased decomposition of tephra-affected forest floors, relative to unaffected areas, may have facilitated plant regrowth by increasing the availability of limiting nutrients. Ash created a more favorable temperature environment for decomposition beneath the ash with the forest floor under ash being cooler in summer and warmer in the cooler months. Forest floor moisture was reduced under the ash but did not appear to be limiting to decomposition.

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.

Long-term decomposition and nutrient dynamics in Pacific silver fir needles in western Washington

Long-term needle decomposition and nutrient dynamics (N, P, K, Ca, Mg, and Mn) were studied over a 6-year period in a Pacific silver fir (Abiesamabilis (Dougl.) Forb.) ecosystem in western Washington, U.S.A. Weight loss of needles was 41.2% after 14 months and 64.3% after 6 years. Decomposition constants (k values) declined with time of decomposition, but tended to stabilize afer4–6years. The mean residence time of needles was estimated to be 9 years. After 4 years decomposition weight loss paralleled lignin loss. Nitrogen was strongly immobilized in needles with 242% of original mass of N remaining after 4 years and 213% after 6 years. Net mineralization occurred when the C:N ratio fell below 20. Phosphorus appeared to be slightly immobilized in the 9- to 48-month period. Only 49% of the original P mass remained after 6 years. None of the other elements was immobilized during the 6-year period. The element mobility series was N < P < Mn < Ca < Mg < K. Two-year litter bag studies in subalpine coniferous ecosystems are not long enough to study litter decomposition rates and nutrient dynamics.