Riparian Forest Composition Influences Multi-Trophic Stream Biodiversity and Ecosystem Functioning
Cross-ecosystem energy flows link streams and riparian forests. Forest harvesting alters the composition of riparian tree species, which can affect the structure and functioning of stream ecosystems through changes in terrestrial resource subsidies. We examined how variation in riparian forest composition (coniferous, mixed, deciduous) affects stream invertebrate and microbial consumers and subsequent leaf litter breakdown rates of red alder (Alnus rubra) and western hemlock (Tsuga heterophylla) in small coastal rainforest streams of southwestern British Columbia. 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 forest 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 forest composition. Stream consumer trophic levels responded differently to forest composition and litter quality. Benthic invertebrate diversity was not affected by forest composition and was greater for alder than hemlock litter. For microbial communities, bacterial diversity was highest for hemlock, and fungal diversity was highest in mixed-canopy streams. Microbial respiration was lower in coniferous streams, but alder and hemlock litter quality (carbon: nitrogen) did not vary with forest composition. Percent variation in litter breakdown rates was attributed to forest composition in the presence of both invertebrates and microbes, but multi-trophic stream biodiversity only partially explained these patterns. We provide further evidence linking riparian and stream ecosystems through exchange and processing of detrital resources. Our findings suggest that biodiversity among stream consumer trophic levels responds uniquely to riparian forest composition both in terms of structure and functioning.