Wildfire severity mediates fluxes of plant material and terrestrial invertebrates to mountain streams

Wildfire effects upon riparian plant community structure, composition, and distribution may strongly influence the dynamic relationships between riparian vegetation and stream ecosystems. However, few studies have examined the influence of fire on these processes. To that end, we compared the quantity and composition of allochthonous inputs of plant material and terrestrial invertebrates among stream tributaries characterized by various degrees of burn severity 5 years post-fire in the Frank Church Wilderness of central Idaho, USA. The magnitude of inputs of coniferous leaf litter to unburned stream reaches was five times that of inputs to severely burned reaches. Deciduous leaf litter inputs to unburned reaches were 1.5 times, and inputs of terrestrial invertebrates were twice, the magnitude of inputs to severely burned reaches. NMS ordination and MRPP analysis indicated that the taxonomic composition of terrestrial invertebrate inputs to unburned stream reaches was significantly different than the composition of invertebrate inputs to either high-severity or low-severity reaches (A = 0.057, p = 0.040). Unburned and low-severity stream reaches received greater inputs of large-bodied invertebrates belonging to the orders Hymenoptera, Lepidoptera, Orthoptera, and Diptera. Taken as a whole, our results indicate that fire can significantly alter terrestrial–aquatic connectivity via alterations in riparian-to-stream inputs of leaf material and arthropods. Given these findings, wildfire severity might be expected to be a critical factor in shaping stream–riparian food webs in fire-prone areas.