Impacts of timber harvesting on soil organic matter, nitrogen, productivity, and health of Inland Northwest forests

Abstract

Soil organic components are important factors in the health and productivity of Inland Northwest forests. Timber harvesting and extensive site preparation (piling, windrowing, or scalping) reduces the amount of surface organic material (woody residues and forest floor layers) over large areas. Some wildfires and severe prescribed burns can have similar consequences. Such organic matter reductions can have important implications for soil chemical, biological and physical properties. A number of studies have linked substantial reduction in mycorrhizae development and tree growth to high levels of soil disturbance, or removal of organic horizons. Timber harvesting also removes a large percentage of coarse woody debris, which has unknown ramifications on soil productivity. Current woody residue guidelines in this region recommend leaving <10 to 125 Mg ha-1 on site to replace woody materials lost during harvesting operations. Large amounts of soil nitrogen (>500 kg ha-1) can also be lost from timber harvesting and site preparation, especially when using prescribed fire. The time required to replace this lost nitrogen may range from <10 to >275 yr, and depends on the severity 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 temporarily 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 relatively low impacts on soil organic levels, while accomplishing the important forest management objectives of fuel reduction, seedbed preparation, and reducing competing vegetation. Organic matter management will be the most difficult on very dry sites, with their historically low soil organic and nitrogen content, and high fire potential. The maintenance of adequate soil organic matter levels is critical for sustaining forest health and productivity under the variable moisture and temperature conditions of this region. Thus, soil organic components will become more important in the future as ecosystem management systems are developed for western forests.