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Stream chemistry impacts of conifer harvesting in welsh catchments |
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| Authors: | B Reynolds P A Stevens S Hughes J A Parkinson N S Weatherley |
| Institution: | 1. Institute of Terrestrial Ecology, Bangor Research Unit, UWB, Deiniol Road, LL57 2UP, Bangor, Gwynedd, UK 2. Institute of Terrestrial Ecology, Merlewood Research Station, LA11 6JU, Grange-over-Sands, Cumbria, UK 3. National Rivers Authority, Rivers House, St. Mellons Business Park, CF3 OLT, Cardiff, UK |
| Abstract: | 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 NO 3 ? 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, NO 3 ? concentrations in the manipulated catchments remained above those in the unfelled control catchment for three years, before declining below control values. The NO 3 ? pulse was related to increased rates of mineralization and 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 NO 3 ? pulse after felling at Plynlimon sustained inorganic anion concentrations above those in the control stream for the first 18 months after felling. As the NO 3 ? pulse declined, inorganic anion concentrations decreased to below those in the control stream about 4 years after felling. At Beddgelert, the smaller increase in NO 3 ? concentrations had less of an effect on inorganic anion concentrations which decreased after felling relative to values in the control stream. The increase in NO 3 ? 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 SO 4 2? concentrations. At Beddgelert reductions in SO 4 2? 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 NO 3 ? 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. |
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