Defoliation and Atmospheric Deposition Influences on Spring Baseflow Chemistry in 56 Pennsylvania Mixed Land-Use Watersheds

Abstract

Using samples of spring baseflow chemistry on 56Pennsylvania watersheds with predominantly forested to mixedland-uses and widely varying geology/physiography, weattempted to determine spatial patterns in stream chemistrydue to insect defoliation and atmospheric deposition. Landuse and land form relations to stream chemistry wereexamined as well. Defoliation effects on stream chemistrydue to repeated, and sometimes intense, insect defoliationover the past several years were seen as reduced streamnitrate concentrations in a watershed data set (n = 11) thatincluded 100% forested lands only. Basins in regions withhigher atmospheric sulfate deposition loads had higherstream concentrations of sulfate in 100% forested basins.Significant positive correlations of stream nitrogen andpotassium with agricultural land use indicated possiblecontamination of stream waters by excess fertilizers and/oranimal wastes. Weak positive correlations were also foundwith many of the stream chemistry parameters and percentageurban/barren land use. Ridge-top versus valley bottomwatersheds also showed differences in baseflow chemistry dueto changing surficial geology and/or land use. Overall, thestudy showed that agricultural, urban, geologic, andphysiographic influences on spring baseflow chemistry maskthe effects of insect defoliation and atmospheric depositionon mixed land-use basins (<100% forest). Regionaldifferences in atmospheric deposition on 100% forestedbasins were directly reflected in spring baseflow SO4concentrations. When restricted to 100% forested basinswith relatively uniform geology, insect defoliation appearedto reduce stream nitrogen concentrations in the long term.This is believed to be due in part to nitrogen bound invegetative growth and a dilution of nitrogen from increasedflows as a result of defoliation and tree mortality bringingabout reduced evapotranspiration.