Effect of timber removal on the total nutrient fluxes from a freshwater-supplying forested watershed

Abstract

Effects of 18% thinning on nutrient fluxes were investigated in a broadleaf forest. Streamflow and suspended sediment were sampled during the calibration and treatment periods for the treatment and control watersheds to determine total nutrient fluxes through bedload sediment and runoff. Streamflow was measured, and water samples were collected from the streams of the watersheds. The volume of the bedload deposited in both weirs' pools was measured, and sediment deposits were sampled at the ends of the calibration and treatment periods. Water and sediment samples were analyzed for calcium (Ca2+), magnesium (Mg2+), Kjeldahl nitrogen (KN), sodium (Na+), potassium (K+), iron (Fe3+), and sulfur (S2-). Results showed that there was only a significant increase in KN loss from 0.01 kg/ha to 0.10 kg/ha and from 0.04 kg/ha to 2.41 kg/ha in the second and the third post-treatment years, respectively (P < 0.05). Thinning was not intensive to change nutrient outflow in the study site. Practitioner Points The 18% thinning was not intensive enough to increase the total nutrient flux, deteriorate water chemical quality, and hence threaten aquatic life in the stream water. Significant increases were found only in KN outflow after timber removal. If there is no information about the effect of timber harvest on nutrient loss in a specific watershed, the application of evenly distributed low-intensity timber harvest may be a good option to protect stream water quality in forested watersheds. Further investigations are needed about the effects of various timber harvest intensities on physical water quality (odor, color, turbidity, taste, temperature, etc.) and biological characteristics (population and biodiversity of aquatic organisms) of the stream water for making better water management plans.