Productivity
The productivity of a forest ecosystem can be measured according to a variety of outputs or values. For example, productivity might be measured in units relevant to wildlife populations, carbon sequestration, water, or timber. In the context of managing forests for bioenergy feedstocks, it is essential to measure forest productivity by quantifying the accumulation of biomass over time in above- and below-ground components of trees. Tree biomass is typically sorted by material that can be harvested and utilized for various merchantable end-products and material that remains in the forest after harvesting. The productivity of specific forest sites is the result of the interaction of several factors, including climate, soil quality, crop management or silviculture practices, and tree genetic potential. The combination of soil and climatic factors contributing to plant growth and development is generally refered to as "site" productivity. It can be described as biomass accumulation as a function of time. The graphic above depicts a hypothetical forest biomass production curve (Burger 2002). As resources are readily available during early phases of stand development, plant biomass accumulation and generally stand production increases exponentially. A leveling off of the rate of accumulation and decrease in production rate typically occur as resources become limited and trees mature until the point at which the carrying capacity of the site is reached.
As characterized by Dyck and Cole (1990) (at left) there are generally three means of increasing the inherent biomass production capacity of a forested site including: (1) selecting plants that can more efficiently convert site resources into biomass; (2) increasing the availability of site resources to plants; and (3) controlling tree density per acre through time to maximize harvest yield. Genetically improved trees can be planted to increase the potential of the stand to efficiently utilize site resources. Fertilizer and weed control are effective for increasing the availability of site resources such as soil water and nutrients essential to crop plants. Soil tillage can also be used to increase the carrying capacity of waterlogged forest sites by increasing aeration in the rooting zone of the soil. Silvicultural systems have been developed for the major Southern forest types which control planting conditions, stand density, and spacing from plantation establishment to final harvest in order to achieve various stand management objectives, including high biomass yields. Many of these practices are described for Southern forests by Fox and others (2004) and Allen and others (2005).
Adhering to the principles of sustainable forest management implies that forest ecosystems will be managed to maintain environmental, economic, and social criteria or values. Environmental criteria for most international agreements, including the Montreal Process, include indicators for site productivity and soil quality (MPCI 2005). Therefore, it becomes essential to understand the biological and physical factors that affect soil and site productivity.
Soil productivity is the capacity of a soil to contribute to the production of a crop, whether it is agricultural crops or forest biomass. Physical, chemical, and biological properties of the soil all affect its productivity.
Different combinations of physical and chemical properties affect the productivity of soil. Some of these properties, including soil depth, parent material, and slope position, cannot be changed easily with management practices, although deep ripping can be accomplished mechanically to increase the effective rooting depth of a soil. Other properties including soil structure, organic matter content, nutrient content, and temperature can be modified by forest practices, which in turn can help to improve the productivity of the soil. Soil productivity is highly influenced by several factors including: soil temperature, water-air balance, and soil fertility (Burger 2002).
Soil temperature can be greatly affected by forest management practices (Burger 2002). Removing organic layers and exposing mineral soils can result in higher surface temperatures. In colder climates, root and shoot growth can be stimulated in this manner. In other climates, it may be necessary to retain or add organic layers to help reduce the soil temperature and stimulate growth. Harvesting has the potential to affect the distribution of organic soil horizons through machine movement and distribution of unutilized forest biomass.
The water-air balance of soil can also be manipulated through forest management practices. The balance between water and air in soil pore space affects water and nutrient uptake and root respiration (Burger 2002). Plant root function and health are optimal when soil moisture content is near "field capacity" (right), since there is both adequate water and air for root survival. The depth to saturated soil on forest sites is an important measure of the volume of soil in which the water-air balance is optimal for plant health. Water table depth can be affected by forest management activity. For example, the water table can rise in relatively low positions on the landscape after clear-cut harvesting as watershed foliage is removed and tree uptake of water is reduced during a period of revegetation. Alternatively, the water table can be lowered during periods of stand development when foliar surface area is high and water utilization by the crop is high. Water tables can also be lowered by installation of drainage ditches and drain tiles. However, such engineered solutions may be highly prohibitive in forest management operations in the Southern United States.
The term "soil fertility" is used generally to refer to the total availability, concentration, and amount of essential plant nutrients. The essential nutrients that tend to limit forest growth and development include nutrients needed in large quantities (macronutrients) such as nitrogen (N), phosphorous (P), potassium (K), calcium (Ca), and magnesium (Mg), and those for which only trace amounts are necessary (micronutrients) such as boron (B), zinc (Zn), and copper (Cu).
Nitrogen availability often limits the growth of forests in the South (Brady and Weil 2004; Burger 2002; Fisher and Binkley 2000). Trees mostly absorb inorganic forms of nitrogen from the soil; however, the majority of forest ecosystem nitrogen is contained in soil organic matter. Therefore, sustaining the supply of nitrogen to plants requires consideration of the critical physical, chemical, and biological factors that convert organic forms of nitrogen to inorganic forms. The nitrogen cycle (at right) shows how nitrogen is transformed as it passes through the atmosphere, soil, and plant tissue.
Trees utilize nitrogen in the form of nitrate (NO3-) anions and ammonium (NH4+) cations, and in some cases as organic compounds. Soil micro-organisms are responsible for converting organic forms of nitrogen to inorganic nitrate and ammonium (mineralization process), and so productive sites often have soil and climatic conditions which are conducive to high rates of biological activity. On sites with insufficient amounts of available nitrogen, chemical fertilizer containing nitrogen may be added, if economically feasible. In appropriate circumstances, other common ways of adding nitrogen to soils include amendment with animal manure, wastewater biosolids and effluent, and planting nitrogen-fixing plants such as those in the legume family; these methods require very specialized consideration and are not common in forestry. Careful management is required to avoid excessive loss of nitrogen due to leaching, which in some cases might be a source of water pollution (Brady and Weil 2004). Nitrogen deficiencies in trees can be identified by yellowish foliage, thin stems, and a stunted appearance.
Phosphorous is needed by plants to help supply energy for the completion of many biochemical processes, including uptake and transportation of plant nutrients. Phosphorous availability limits forest site productivity on some soils (Burger 2002). The phosphorous cycle (at left) shows the organic and inorganic forms of phosphorous and their relative availability for plant growth. Easily soluble inorganic soil phosphorous is the most readily available for plant use. Phosphorous deficiencies generally occur due to a low phosphorous content in very old, highly weathered soil parent materials, or where high levels of iron and aluminum oxides and hydroxides chemically bind with phosphate ions, and reduce phosphorous availability to plants. In general, only a small amount of phosphorous is found in soil parent materials and most phosphorous in forest soils is in organic forms. The application of chemical fertilizer to forests to increase phosphorous availability is relatively common in the South, is necessary for adequate forest growth on some soils (Fox and others 2004), and can be very effective at amending inherent site deficiencies. Poor yields, stunted growth, and plant mortality can be attributed to a lack of phosphorous.
For some forest sites, fertilizer additions of N and/or P or other essential elements may enhance forest growth where economically justified (Allen and others 2005; Fristoe and Gothard 1998; Fox and others 2004). When properly applied, fertilizer can greatly enhance the productivity of a forest site. Nutrients should be applied on the basis of diagnosed deficiencies identified at a specific site. Improper application of fertilizer can lead to negative environmental effects and may violate Best Management Practices and state environmental regulations for forest management (Shepard 2006). More information related to soil productivity and forest fertilization can be found at the Alabama Forestry Commission website and in the literature of the NCSU Forest Nutrition Cooperative (http://www.forestnutrition.org/history.htm).
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