Changes in stream chemistry associated with insect defoliation in a Pennsylvania hemlock-hardwoods forest

Disturbances to forested watersheds often lead to elevated concentrations and fluxes of solutes in stream water. We examined the solute chemistry of streams draining mostly old-growth hemlock-hardwoods forest in two adjacent watersheds on the Allegheny High Plateau in northwestern Pennsylvania before, during, and after a single season of defoliation of hardwoods by elm spanworm (Ennomos subsignarius Hübner) larvae. Concentrations of potassium, ammonium, and dissolved organic carbon increased during the defoliation or in the following month. During this same time, concentrations of sulfate, nitrate, calcium, and magnesium decreased. However, by the end of the summer, nitrate concentrations exceeded pre-defoliation concentrations (<0.4 mg NO3-N/L) and then peaked during the following summer (1.4 mg NO3-N/L). Nitrate concentrations declined to pre-defoliation concentrations by the end of the second summer following defoliation. Calcium and magnesium concentrations, which generally correlated most strongly with sulfate concentrations, increased in conjunction with elevated nitrate concentrations after defoliation. Increases in nitrate, calcium, and magnesium in stream water were consistent with biogeochemical changes following clearcutting and insect defoliation at other locations in eastern North America. Therefore, we hypothesize that elevated nitrification generated nitrate ions which in turn leached calcium and magnesium from soils into the streams. The short-term changes that occurred during and immediately after defoliation in our study suggest that defoliation may influence stream chemistry in other ways, as well (e.g., through the leaching of nutrients and labile carbon from insect frass and greenfall).