Effects of Timber Harvest on Stream Chemistry and Dissolved Nutrient Losses in Northeast Oregon

Abstract

Abstract Streamwater chemistry and nutrient inputs and outputs of four small (24 to 118 ha) catchments in the Blue Mountains of eastern Oregon were compared before and after timber harvest treatments. Three harvested watersheds were also compared to a control watershed. Treated watersheds included 41% clearcut in two large blocks; 17% clearcut in 0.8- to 2.4-ha patches; and selection harvest. Residues were piled and burned on the watersheds with clearcuts; on the selection harvest watershed, residues were yarded to an area off-site and burned. Chemical parameters and constitutents measured were: pH, electrical conductivity (EC), nitrate-N (NO3-N), dissolved Kjeldahl N (DKN), total dissolved PO4-P (TDP), calcium (Ca), magnesium (Mg), sodium (Na), and potassium (K). Nitrate-N concentrations in streamflow increased in response to both clearcutting treatments. The maximum level observed (0.52 mg/l) was, however, far below the maximum allowable for drinking water standards (10 mg/l). Concentration of TDP increased significantly on the 41% clearcut watershed and small but significant increases in cation concentrations were observed with both clearcutting treatments. Outputs (kg · ha-t · yr-1) of N increased with both clearcutting treatments but losses were balanced by precipitation inputs. Cation outputs were not affected by harvest treatments. For. Sci. 34(2):344-358.