INTERACTION OF DOUGLAS-FIR WITH RED ALDER AND SALAL FOLIAGE LITTER DURING DECOMPOSITION

Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) foliage litter was incubated in the laboratory in 100/0, 75/25, 50/50, 25/75, and 0/100% mixtures with foliage litters from red alder (Alnus rubra Bong.), a nitrogen-fixing tree species, and salal (Gaultheria shallon Pursh) an ericaceous shrub. Mass loss and mineral nitrogen content were measured at eight dates during die 32-week incubation. The occurrence of interaction between litters, observed as deviation from a linear trend across mixtures at each date, was determined by polynomial contrasts. Douglas-fir - red alder mixtures decomposed faster than predicted by mass loss from pure litters throughout the study, apparently because of nitrogen mineralized by the red alder component. Douglas-fir-salal mixtures decomposed more slowly than predicted between 7 and 25 weeks. The study demonstrates the potential for litters to interact with positive or negative effects on decomposition rates and emphasizes the need to examine such interactions if litter decomposition in the field is to be understood.