Function
The "function" of soils refers to the roles that soils play in the environment, e.g. supporting plants, transmitting water. Five main functions of soils characterize the relevance of soils to forest management (at right): 1) medium for plant growth, 2) recycling system, 3) water supply regulator, 4) soil organism habitat, and 5) engineering medium (Brady and Weil 2004).
Medium for plant growth. Soils play a critical role in supporting the physical structure of plants, controlling water movement, providing essential nutrients, controlling temperature, and providing adequate aeration for living roots. Each of these roles are essential for normal plant growth and development, and many of these can be controlled by foresters and forestry activity.
Soils provide adequate support for trees when soil depth and physical structure allow strong, anchoring root systems to develop. Shallow and wet soils do not typically support trees during strong winds or heavy snowfall as well as deep and well-drained soils.
Soil moisture content and aeration levels must be balanced since plant roots require oxygen for normal respiration and must take up water to maintain normal plant growth and development. Soils that are too wet or dry are therefore not desireable for optimal tree growth and ecosystem health.
Soils are typically well buffered with respect to temperature fluctuations due to the insulating effects of surface forest floor organic matter and temperature regulating properties of soil moisture. Forested soils also have more moderate seasonal fluctuations than bare soil in similar ecosystems due to the temperature moderating effects of the stand of trees. Summer high temperatures and winter low temperatures are therefore more moderate under forests than in open areas. The temperature moderating effects of forests can contribute positively to root development and biological activity in soil organisms in temperate and tropical ecosystems. In boreal ecosystems, soil activity often increases when forests are cleared.
The majority of essential elements needed by trees for normal growth, reproduction, and development are taken up by roots from the soil. In addition to supplying essential nutrients, soils can also contain chemicals which are toxic to plants (phytotoxic) at certain concentrations. Soils that are a good medium for plants continuously supply trees with essential elements in the proportions necessary and appropriate for normal seasonal growth and development. It is common for Southern forest plantations to be fertilized to improve levels of available nutrients and forest growth rates.
Recycling system.The complex physical, chemical, and biological properties of forest soils are created and maintained by incorporation and decomposition of myriad organic compounds in leaf and branch fall, dead roots, and other organic matter. This "recycling" function is an important characteristic of soil productivity maintenance in both unmanaged and managed forests and must be recognized for sound forest management (see Productivity). Because of the great variety of organic matter in forests, e.g. herbaceous plants, tree bark, branches, cones, seeds and foliage, forest soil organism communities have evolved capacities to recycle these compounds. Because of these inherent characteristics soils have the capability to recycle some human waste materials such as wastewater treatment plant effluent, sludge (biosolids), vegetable processing residues, animal manures, some pulp and paper mill sludges and ash from combustion boilers. Such potential recycling systems must be carefully evaluated to ensure that wastes are applied in appropriate amounts and do not contain toxic and environmentally-harmful elements or compounds that could "poison" soils, waters, and plants.
Water supply regulator. Most water in streams, rivers, lakes, and ground-water aquifers enters the earth through soils. Rain or snow-melt penetrating a forest canopy and soil surface percolates through soil layers, often very slowly, and physically and chemically interacts with soil components resulting in chemically-altered water in streams and aquifers. Water from forested landscapes is slowed and delayed in its passage to streams, thus "regulating" stream flows. Removal of forest vegetation eliminates some or much of this regulating function and reduces time of water contact with soil components. Re-establishing forests restores these functions. Vegetated versus bare soils greatly reduce erosion and runoff rates and help protect many aquatic resource values. Soil management as part of forest management is clearly important to preserve water supplies.
Soil organism habitat. Forest soils provide a great variety of habitats for numerous large-, small-, and microscopic-sized organisms. Tiny mites and insects, ants, centipedes, earthworms, and small mammals make their homes in soils where they plow, chop, shred, and move soil components. Bacteria and fungi proliferate in forest floor litter layers and within soils where they decompose organic residues. In these processes, organic matter and inorganic soil particles are intimately mixed and bound together to create plant-nurturing soil structure. Specialized fungi create mutually-beneficial structures with roots (mycorrhizae) that facilitate nutrient and water uptake by trees. Organism habitats are maintained by additions of organic matter and maintenance of soil structure and porosity, all of which can be influenced by forest management.
Engineering medium. Soils mediate flows of water and support roads, machines, and structures. Their composition, strength, porosity, and water content influence their utility in engineering functions. Appropriate designs and management of forest roads and machines are important to sustainable silvicultural and harvesting operations.
Soils must have adequate bearing strength to avoid rutting and compaction during harvesting and forwarding operations. Season moisture contents often range from dry to wet with corresponding bearing strengths ranging from high to very low or negligible. Forestry operations should take seasonal soil moisture levels into account, and also select machine and tire designs which provide adequate flotation and soil protection. Forest engineers constructing roads desire soils with adequate bearing strength, shear strength, compressibility, and stability to permit regular traffic of forwarding machinery with heavy loads. Soils not possessing these qualities require expensive engineering technological solutions to meet operational standards. Southern soil physical properties are highly variable across the landscape, so forest managers are encouraged to becom knowledgeable about their specific resources.
Encyclopedia ID: p1273
