IMPACTS OF TIMBER HARVESTING ON SOIL ORGANIC-MATTER, NITROGEN, PRODUCTIVITY, AND HEALTH OF INLAND NORTHWEST FORESTS

Soil organic components are important factors in the health and produc tivity of Inland Northwest forests, Timber harvesting and extensive si te preparation (piling, windrowing, or scalping) reduces the amount of surface organic material (woody residues and forest floor layers) ove r large areas. Some wildfires and severe prescribed burns can have sim ilar consequences, Such organic matter reductions can have important i mplications for soil chemical, biological and physical properties. A n umber of studies have linked substantial reduction in mycorrhizae deve lopment and tree growth to high levels of soil disturbance, or removal of organic horizons, Timber harvesting also removes a large percentag e of coarse woody debris, which has unknown ramifications on soil prod uctivity. Current woody residue guidelines in this region recommend le aving <10 to 125 Mg ha(-1) on site to replace woody materials lost dur ing harvesting operations. Large amounts of soil nitrogen (>500 hg ha( -1)) can also be lost from timber harvesting and site preparation, esp ecially when using prescribed flue. The time required to replace this lost nitrogen may range from <10 to >275 yr, and depends on the severi ty of site treatments, presence or absence of nitrogen fixing plants, and amounts of atmospheric deposition. Maintaining adequate amounts of organic matter on some forest sites in the Inland Northwest may tempo rarily increase the risk of wildfire or favor the activity of certain insects or disease fungi. However, carefully planned prescribed burns and mechanical site preparation can be practiced on most sites with re latively low impacts on soil organic levels, while accomplishing Vie i mportant forest management objectives of fuel reduction, seedbed prepa ration, and reducing competing vegetation, Organic matter management w ill be the most difficult on very dry sites, with their historically l ow soil organic and nitrogen content, and high fire potential, The mai ntenance of adequate soil organic matter levels is critical for sustai ning forest health and productivity under the variable moisture and te mperature conditions of this region, Thus, soil organic components wil l become more important in the future as ecosystem management systems are developed for western forests.