STREAM CHEMISTRY IMPACTS OF CONIFER HARVESTING IN WELSH CATCHMENTS

Hydrochemical data have been collected for between 6 and 9 years from forest harvesting experiments in small catchments (>10 ha) at Plynlimon and Beddgelert, Wales, UK. Felling resulted in rapid increases in NO3- and K+ concentrations at both sites. A maximum of 3.2 mg N L(-1) was observed at Plynlimon about one year after the start of felling. Concentrations declined to control stream values (0.5 mg N L(-1)) after 5 years. At Beddgelert, NO3- concentrations in the manipulated catchments remained above those in the unfelled control catchment for three years, before declining below control values. The NO3- pulse was related to increased rates of mineralization acid nitrification in the soil after felling. The initial increase in K+ concentration after felling at Plynlimon was followed by a slow decline, but concentrations were still above those in the control stream after 5 years. From 4 to 8 years after felling at Beddgelert, K+ concentrations fell below and then generally remained lower than control values. The NO3- pulse after felling at Plynlimon sustained inorganic anion concentrations above those in the control stream for the first 18 months after felling. As the NO3- pulse declined, inorganic anion concentrations decreased to below those in the control stream about 4 years after felling. At Beddgelert, the smaller increase in NO3- concentrations had less of an effect on inorganic anion concentrations which decreased after felling relative to values in the control stream. The increase in NO3- was associated with temporary streamwater acidification in the felled catchments due to the increased rates of nitrification and nitrate leaching. At Plynlimon, streamwater filterable Al concentrations declined after felling, but controls on Al behaviour are complex and not explained by simple equilibrium relationships with Al(OH)(3) or by variations in inorganic anion concentrations. At Beddgelert, felling had no effect on stream water filterable Al concentrations. Felling at Plynlimon led to a large reduction in streamwater Cl-, Na+ and SO42- concentrations. At Beddgelert reductions in SO42- and 'sea salt' ion concentrations were less clear, reflecting the smaller proportions of the catchments which were harvested. Felling had no deleterious effects on water quality, apart from a temporary slight further decline in stream pH at Beddgelert. Increases in NO3- concentrations were short-lived and concentrations were well below drinking water standards. Filterable Al concentrations were already higher than statutory standards, but were not increased or decreased through felling.