CANOPY-SURFACE CYCLING OF CHLORIDE IN A NORTHEASTERN HARDWOOD FOREST

This study reports biogeochemical cycling inferences made from thirty years of precipitation and stream water monitoring in Hopkins Memorial Forest, located in the Taconic Range of western Massachusetts and adjoining Vermont and New York. Net throughfall flux of biologically active ions (e.g. ammonium) shows expected seasonal trends, but our data identify a dramatic October spike in nearly every species measured, regardless of its biological relevance. The most dramatic example of this October increase is chloride – commonly used as a hydrologic tracer – which tracks within error of sodium every other month of the year. In October, chloride increases by 700% relative to other leaves-on months. This increase is not reported by other long-term ecological research stations around New England. However, we do not observe a corresponding October increase in total chloride flux through the watersheds. Such a discrepancy between net throughfall flux and total flux suggests a biologically-mediated cycling of chloride within the watershed. Combining seasonal trends of watershed and net throughfall flux suggests that chloride is mobilized in the fall, transported to the ground by precipitation, and washed out of the soil in the spring. Once summer begins, chloride is taken up by growth and retained in organic matter until October, at which point it is mobilized again. Another species that markedly increases in October is ANC, though we do not observe a corresponding decrease in the throughfall pH. As canopy-precipitation interaction accounts for half of all acid buffering that occurs in Hopkins Memorial Forest, this seasonal difference between the pH and ANC trends points to buffering occurring due to hydronium uptake by leaves, not neutralization with dry deposition. Overall, the long term monitoring suggests several important internal biogeochemical cycles that help elucidate the processes occurring better than those identified by total watershed flux measurements do.