Response of stream chemistry to forest insect defoliation on the Allegheny high plateau, Pennsylvania.

摘要

I examined the response of headwater streams to severe insect defoliation in two forested watersheds at the Tionesta Scenic and Research Natural Areas, Allegheny National Forest (ANF), Pennsylvania. Stream nitrate concentrations increased within two months after defoliation by the elm spanworm (Ennomos subsignarius Hubner). Elevated nitrate, hydrogen ion, calcium, and magnesium concentrations after defoliation were consistent with results from forest cutting studies, in which elevated nitrification acidifies soils and promotes base cation losses. However, other changes in stream chemistry suggested the role of mechanisms other than nitrification. For example, elevated potassium concentrations during and after defoliation suggests that this cation was leached from insect frass and leaf fragments in and near stream channels. Inverse relationships between nitrate and dissolved organic carbon (DOC) and between sulfate and DOC in the month after defoliation suggest that labile DOC from frass and leaf fragments stimulated denitrification and sulfate reduction in anoxic riparian soils. Post-defoliation stream exports of nitrate-nitrogen from these Pennsylvania watersheds exceeded exports from defoliated watersheds in the southern Appalachians.I found no long-lasting (Insect frass, leaf fragments, and insect carcasses decomposed rapidly and released large quantities of dissolved nutrients (e.g., nitrogen and phosphorus) in aquatic microcosms. Nevertheless, stream ecosystems may retain temporarily the nutrients released by these forms of detritus: I calculated that frass, greenfall, and drowned caterpillars falling into a Tionesta stream would have released up to twenty times more nutrients than were exported by the stream during and immediately after the elm spanworm outbreak.Finally, I found evidence that soil hydrology influences stream chemistry in the ANF. In watersheds in which stream base cation (calcium and magnesium) concentrations were directly related to acid anion (sulfate and nitrate) concentrations, the percentage of soils with perched water tables was greater than in watersheds in which stream base cation concentrations were indirectly related to acid anion concentrations.