Riparian forest composition affects stream organic matter processing dynamics

Cross-ecosystem energy flows link streams and riparian forests. Forest harvesting alters the composition of riparian tree species, which can affect the structure and function of stream ecosystems through changes in terrestrial resource subsidies. We examined how variation in the ratio of deciduous to coniferous forest composition may affect stream invertebrate and microbial consumers and subsequent leaf litter breakdown rates of red alder (deciduous tree) and western hemlock (conifer) in 10 small streams. Breakdown rates of alder litter were faster in streams containing a greater proportion of deciduous than coniferous canopy; whereas breakdown rates of hemlock litter were independent of canopy composition. When invertebrates were excluded using fine mesh to isolate microbe-specific processing dynamics, breakdown rates of both species were an order of magnitude less and did not vary with canopy composition. Benthic invertebrates, and not microbes, appear to explain variation in organic matter processing dynamics attributed to forest canopy composition. Dominant invertebrates vary among streams with different forest canopies, whereas invertebrate community diversity is linked to litter type. Analyses of microbial communities may reveal trophic variation in responses of stream food webs to terrestrial subsidies. Our findings provide further evidence linking terrestrial energy resources with stream consumers and ecosystem function.