Changes in stream chemistry and nutrient export following a partial harvest in the Catskill Mountains, New York, USA

摘要

Clearcut forest harvesting typically results in large changes in stream water chemistry in northeastern North America. The effects of partial forest harvests on stream chemistry have not received as much attention, even though partial cutting is a more common forestry practice than clearcutting in this region. Changes in stream water chemistry following a partial cut are reported here from a 10 ha study catchment in a northern hardwood forest in the Catskill Mountains of southern New York, and are compared to those of a nearby 48 ha reference catchment. The lower two thirds of the treatment catchment was harvested in February-April 2002 by a shelterwood method, such that 33% of the basal area of the catchment was removed. Stream NO_3~-, NH_4~+, Ca~(2+), K~+, and total dissolved aluminum (Al_(to)) concentrations increased significantly after the harvest. Stream Ca~(2+), Mg~(2+) and NH_4~+ concentrations peaked 5 months after the initiation of the harvest, NO_3~- and K~+ concentrations peaked 6 months after cutting, and Al_(to) concentrations peaked 1 year after cutting. Streamflow was not significantly affected by the harvest when compared to the flow of three nearby streams. Export of NO_3~- in stream water increased five-fold the year after the cut, and briefly exceeded atmospheric inputs of inorganic nitrogen during 4 months in the fall of 2002. Changes in stream NO_3~- and K~+ concentrations were less than predicted by the relative basal area removed compared with those of a recent nearby clearcut. In contrast, changes in Ca~(2+), Mg~(2+) and Al_(to) concentrations were approximately proportional to basal area removal in these two cuts. Stream chemistry returned to values close to those of the pre-cut period and to reference values by early spring of 2003, just over a year after the initiation of the harvest, except for NO_3~- concentrations, which remained elevated above background 18-20 months after completion of the cut.