Headwater streams derive a majority of their energy from allochthonous inputs; alteration of these inputs may lead to changes in stream communities and ecological function. Common buckthorn (Rhamnus cathartica) is invasive over much of the northern United States and southern Canada, and has the potential to become an ecosystem dominant and alter stream communities. However, while much is known of the effects of buckthorn on terrestrial ecosystems, little is known of its effects on aquatic ecosystems. Using leaf collection nets, we estimated that leaf fall to the stream consisted predominantly of green ash (69.0%) and common buckthorn (24.2%). Green ash leaves fell from September through mid-October, reaching its peak in early October, whereas common buckthorn contributed leaves for an additional month until mid-November. We placed leaf packs of common buckthorn and two native species, American elm and green ash, in a headwater stream to determine differences in leaf decomposition rates. Common buckthorn leaves decomposed more rapidly than the native species, with processing coefficients of 6.9 (ash) and 5.3 (elm) times greater. After 21 days of incubation, buckthorn leaf packs had less than half the initial biomass remaining, whereas ash and elm did not reach this point within this 84-day study. These results suggest that buckthorn has the potential to alter stream food webs by changing the timing of leaf fall and the duration of available allochthonous energy sources. Changes to riparian forests are likely to be exacerbated as common buckthorn alters soils to inhibit the growth of other trees and as the emerald ash borer, an invasive beetle, adversely affects native ash forests.

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