CHANGE IN THE ACID-BASE STATUS OF AN APPALACHIAN MOUNTAIN CATCHMENT FOLLOWING FOREST DEFOLIATION BY THE GYPSY-MOTH

Infestation by the gypsy moth (Lymantria dispar) can alter biogeochemi cal conditions in affected catchments. Stream-water concentration data obtained over the period of 1980-1993 for white Oak Run, a stream in Shenandoah National Park, Va., indicate that change in catchment acid- base status is associated with forest defoliation by the moth larva. S tream-water concentration changes following defoliation included incre asing concentrations of strong-acid anions, base-cations, and hydrogen ion, as well as decreasing concentrations of acid-neutralization capa city (ANC) and sulfate. The largest change was in the concentration of nitrate; annual discharge-weighted mean concentrations increased from predefoliation levels consistently less than 5 mu eq L(-1) to postdef oliation levels greater than 50 mu eq L(-1). An intensification of aci dification was indicated by record-high hydrogen ion concentrations an d record-low ANC concentrations. The long-term biogeochemical implicat ions of the infestation are uncertain due to the nonlinearity of the o bserved responses and unknown patterns of recovery and recurrence.