Effects of timber harvest on stream nutrient concentrations in Idaho

The Mica Creek Experimental Watershed is a paired and nested watershed study in Shoshone County in northern Idaho. Participating organizations include Potlatch Corporation, University of Idaho, and NCASI. Partial funding support has been provided by USDA’s National Research Initiative.

A recent paper from the Mica Creek study is titled “Nutrient concentration dynamics in an inland Pacific Northwest watershed before and after timber harvest” (Forest Ecology and Management 257:1663-1675). The authors are J.A. Gravelle (University of Idaho), G. G. Ice (NCASI), T.E. Link (University of Idaho) and D.L. Cook (NCASI). They found that post-harvest increases in NO3 + NO2 concentrations were statistically significant but small in magnitude (<1 mg L-1). The abstract follows.

“The nutrient loads of water draining forested watersheds are generally lower than the loads in water draining basins with other dominant land uses. Commercial forest management activities including timber harvesting, site preparation, road construction, and maintenance can alter the chemical properties of headwater forest streams, and there are concerns this can result in cumulative effects at downstream locations. Monthly water samples were collected from 1992 to 2006 in the Mica Creek Experimental Watershed (MCEW) in northern Idaho. This period of record included a pre-treatment time interval from 1992 to 1997; post-road construction period from 1997 to 2001; and post-harvest period from 2001 to 2006. Samples were analyzed for total Kjeldahl nitrogen (TKN), total ammonia nitrogen (TAN), nitrate + nitrite (NO3 + NO2), total phosphorus (TP), and orthophosphate (OP). Statistically significant increases (p < 0.001) were observed in NO3 + NO2 concentrations following both clearcut and partial cut harvest practices. Downstream of the clearcut harvest activity, mean monthly increases of 0.29 mg-N L-1 were observed. Statistically significant increases were also observed at sites further downstream, but changes were smaller than those immediately below the harvest sites and reflected dilution and possibly instream processing and/or uptake. Continued monitoring at these sites will help evaluate nutrient concentration trends during stand regrowth and hydrologic recovery.” 

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