Loss of ash trees in riparian forests from emerald ash borer infestations has implications for aquatic invertebrate leaf-litter consumers

Abstract

Rapid loss of ash (Fraxinus spp.) trees in riparian forests from an invasive insect, the emerald ash borer (EAB; Agrilus planipennis Fairmaire, 1888), could pose risk of altering organic matter inputs to water bodies that underpin many aquatic ecosystem processes. We measured the composition of riparian forests and their leaf-litter contributions to headwater streams and determined the relative palatability of ash leaves and leaves of three other common riparian trees to aquatic invertebrate leaf-litter consumers (the stonefly (Pteronarcys sp.) and the cranefly (Tipula sp.)) in laboratory microcosms and whole invertebrate communities in forest streams. Ash trees contributed, on average, 24% to riparian tree density and 20% to total litterfall. Among the four common streamside trees accounting for 65% of total litterfall, ash was the first or second most preferred food source for consumers. Leaf packs without ash decomposed at slower rates than packs containing 25%–100% ash leaves. Preferential feeding on ash leaves infers a high-quality food source selected by consumers, and this concurred with comparatively high N content and low C–N ratio of ash leaves. Aquatic invertebrate communities on leaf packs in streams differed among leaf mixtures with or without ash, although community dissimilarity was low. The loss of ash in riparian forests represents an EAB-induced reduction in a high-quality resource subsidy to organic matter consumers in streams. We discuss how this has implications for risk predictions and management response strategies.