Organic Matter Disturbance
Soil organic matter moderates soil temperature, increases water infiltration and holding capacity, and serves as food and energy for soil organisms. Plant nutrients are also held within soil organic matter, which is the most important source of nitrogen in forest ecosystems. The removal or decrease of soil organic matter, especially forest litter layers on the soil surface, as a result of forest management practices, is generally thought to have negative consequences for environmental sustainability, and therefore should be carefully managed by those responsible for design of biomass production and harvesting systems. Site preparation operations, such as windrowing or root raking, which remove forest floor layers and harvesting residue (at right), have been shown to reduce nutrient availability and stand productivity in the following rotation, and therefore should be conducted in such a manner that minimizes forest site organic matter removal.
Conservation of site production potential requires careful consideration of the amounts of organic matter and nutrients removed with merchantable tree boles, branches and foliage during harvesting operations, and the amount of organic residues retained on site to return organic matter to the soil and to benefit successive rotations of trees. Research has shown that, while the branches, foliage, and unmerchantable tops potentially removed during whole-tree harvesting represent a relatively small proportion of the total biomass produced over the whole rotation, it may be large when compared to the amount of available nutrients on a site.
Tree crown biomass is relatively nutrient rich, and should be retained on site if nutrient deficiencies are predicted in the following rotation. For example, in young loblolly pine trees, the greatest percentage of nutrients is held within the needles, with a smaller percentage of nutrients in stembark, branches, and stemwood (Metz and Wells 1965) (at right). However, during a full rotation a greater amount of organic matter is returned to the soil through tree mortality, root turnover, and litterfall during a full rotation than is removed at harvest time. There is a greater concern for organic matter depletion when growing short rotation woody crops, yet these crops are generally grown on highly fertile soils, and crop nutrition management typically involves regular fertilizer additions (Shepard 2006).
In general, traditional forest harvesting including whole-tree harvesting has been shown by research to cause little or no detriment to site productivity potential in terms of soil organic removal unless followed by mechanical disturbance of forest floor layers, soil tilling, or intense burning (Johnson and Curtis 2001).
Encyclopedia ID: p1276
