INFLUENCE OF MOUNT ST-HELENS ASH ON LITTER DECOMPOSITION .2. 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 (Pseudotsuga menziesii (Mirb .) Franco) needles was studied in a field experiment at the Cedar Rive r watershed near Seattle, Wash. Ash that fell on Yakima, Wash., was tr ansported to the site and placed on 5 X 5 m plots in a 50-year-old Dou glas-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 as h surface, buried under ash, and in an untreated control, (ii) the eff ect of ash depth on decomposition rates; and (iii) the influence of as h on forest floor temperature and moisture. Yakima ash had considerabl e influence on forest floor temperature and moisture and Douglas-fir n eedle decomposition rates. Needles buried under 5 cm of ash had increa sed 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 as h had a similar rate of decomposition to control needles (k = 0.349/ye ar), perhaps owing to ash compaction. Those on the ash surface had dec reased rates of decomposition relative to buried needles. These result s are similar to results found in the ash fall zone near Mount St. Hel ens, where needles located on top of ash decomposed slower than buried needles. Increased decomposition of tephra-affected forest floors, re lative to unaffected areas, may have facilitated plant regrowth by inc reasing the availability of limiting nutrients. Ash created a more fav orable temperature environment for decomposition beneath the ash with the forest floor under ash being cooler in summer and warmer in the co oler months. Forest floor moisture was reduced under the ash but did n ot appear to be limiting to decomposition.