Forest soil productivity can affect how fast trees grow and what kinds of trees grow.
The characteristics of a forest soil are defined by varying combinations of mineral particles, organic matter, water, and air. Soil productivity is influenced by levels of mineral nutrients available from the rocks from which that soil is derived. For example, limestone-derived soils tend to have a higher pH, allowing nutrients to be more available and in turn, to be more fertile. Soils derived from granite tend to have a lower pH, which locks up nutrients and so tends to be less fertile. The lack of practical, economically feasible means of increasing soil productivity underscores the importance of maintaining existing soil nutrients.
Soil nutrients can be lost through leaching and timber harvesting. Acid deposition and other forms of air pollution leach certain soil nutrients, especially calcium. These losses may equal or exceed losses from timber harvesting over the length of the rotation. Exposing soil can result in small amounts of harvest-induced leaching. Soil type and the amount of trees removed also influence the amount of leaching. Prompt revegetation can minimize soil nutrient losses.
Nutrient loss from timber harvesting is affected by the portion(s) of a tree taken, the harvest method, and the frequency with which a stand is harvested. More frequent harvests and a higher percentage of fiber removed during harvests increase the amount of nutrients removed. Whole-tree (biomass) harvesting removes more nutrients than bole-only harvests, because the tops, limbs and leaves serve as a significant reservoir for many nutrients.
Though nutrient loss is a concern with biomass harvests, current knowledge is limited regarding the effect of intensive harvesting on soil productivity. Two studies at the Hubbard Brook Experimental Forest in Thornton, N.H. didn't find perceptible soil nutrient loss from whole-tree clearcutting. However, nutrient response is site-dependent and difficult to apply from one soil to another and additional research on a variety of soils is needed. Erosion control and other best management practices maintain soil integrity and minimize losses.
The greatest concern for nutrient depletion arises when the more intense practices are applied repeatedly on sites already low in nutrients (e.g., most coarse-textured sands, some shallow-to-ledge soils, and some soils with high seasonal water tables). In general, whole-tree harvests by the clearcut method on short rotations (e.g. 40 years) should produce the greatest nutrient losses.
Erosion and soil compaction may also diminish soil productivity for tree growth. Timber harvesting can cause soil damage by disrupting topsoil, mixing soil layers, creating deep ruts, or compacting soil layers. The primary factor contributing to soil erosion is the exposure of bare soil.
A typical soil is 45 percent mineral material, 25 percent air, 25 percent water, and 5 percent organic material. Half the feeder roots in a forest are found in the top 6 inches of soil. Roots need both air and water and activities that compact the soil, eliminating space for air and water, lower a site's productivity.
Repeated passes of heavy equipment over certain types of soil, especially during wet conditions, can compact soil pore space, reducing the availability of air and water necessary for trees, impeding root growth, and allowing the entry of pathogens that cause root diseases. To some extent, natural soil processes such as freeze/thaw cycles and activities of soil organisms help restore compacted soils to near preharvest conditions. The rate of recovery depends on soil type, soil depth, and degree of compaction.
Low-fertility soils, those with a high silt, clay, or organic matter content, and soils shallow to bedrock may be more subject to erosion and compaction or have most of the fine roots very near the surface, where they may be easily damaged.
Maintain long-term soil productivity.
2.2 Forest Structure; 3.1 Timber Harvesting Systems; 3.2 Logging Aesthetics; 4.3 Forest Management in Riparian Areas; 5.4 Logging Damage; 6.2 Cavity Trees, Dens and Snags; 6.3 Dead and Down Woody Material; Appendix-Important Forest Soils.
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