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Publication #FOR135

Wood to Energy: Sustainable Forest Management 1

Alan Long and Michael Andreu2

Woody biomass for energy production comes from both urban environments and rural forests. Urban sources include materials from land clearing for development, yard waste from tree trimming, and storm damage debris. These woody materials represent an excellent fuel source for energy production, which might otherwise be wasted and lost when sent to landfills or piled and burned. If the amount of urban woody biomass will not be sufficient for a proposed wood-to-energy project, the remaining feedstock requirement must come from agriculture or rural forestry sources. The three sources of woody biomass from a forest are (1) thinning to reduce the number of small, competing trees (for forest health, wildfire risk reduction, ecological restoration, and timber stand improvement); (2) short-rotation plantations grown specifically for fuel production; and (3) residues left after harvesting for other wood-based products. (See the fact sheet, Sources and Supply, for more information. All of our materials are available at http://interfacesouth.org/woodybiomass.) Many people are concerned that the forests could be overharvested and the ecosystems impaired by the constant demand of a wood-to-energy system. This fact sheet addresses the sustainability of forests when extracting woody biomass for energy production. More detailed information about forest sustainability is summarized in the online forestry encyclopedia, at http://www.forestryencyclopedia.net/Encyclopedia/bioenergy.

Sustainability is evaluated from several perspectives, one of which is the ability of the soil to continually support tree growth, which is a function of soil structure and nutrient levels. A second perspective is the ability to sustain forests across the landscape; this is particularly important in areas such as the southeastern U.S. which is rapidly urbanizing. Nearly 75 percent of the South’s forestland is in non-industrial private ownership and a majority of those landowners are interested in forest sustainability. In many areas of the southeastern U.S. the value of the land is higher if it is sold and developed rather than if it is maintained as a functioning forest. Therefore, if landowners can generate income by selling forest residues after regular harvesting operations while maintaining the forest’s productivity, they may be more willing to hold onto their forests rather than sell the land. Therefore, woody biomass for the production of energy provides an additional market for products and services from their land, and another reason to sustain the forests.

Forest Biomass Sources

Forestry residues are usually the non-merchantable portion of a tree (branches, leaves or needles, and small or broken stems) left after a harvesting operation. These residues are normally piled to get them out of the way of subsequent tree planting activities or they are spread across the site and either burned or left to gradually decompose. Since the leaves and small twigs typically account for 25 to 50 percent of the nutrients in a tree (close to 50 percent in young trees) (Gresham 2002), nutrient loss can be a concern if these residues are taken off-site to be used for energy. One way to assure that the nutrients from leaves or needles go back into the soil is to allow the residues to dry out and drop the leaves or needles before the woody stems are removed from the harvest site. Leaves and needles can also be removed from woody stems by various machines which use chains or other mechanisms to physically separate foliage and branches before the larger woody pieces are chipped and removed. Either of these practices can help retain nutrients for replenishing forest soils.

Currently, few forest plantations are grown specifically for woody biomass production in the southeastern U.S. Where they do exist they are typically grown on old agricultural fields or land reclamation projects (e.g., reclaimed mine sites) and they are harvested on very short rotations (5 to 10 years vs. 15 plus years) compared to plantations grown for wood fiber. In both cases these woody biomass plantations represent an addition of forested land in the landscape.

As with utilizing woody residues, harvesting of trees in biomass plantations may extract only the wood or it may also include the leaves. Several studies in the South have shown that total nutrients in the stems and leaves represent only about 5 percent of the total nitrogen in an ecosystem, and a much smaller portion of the total amount of other nutrients (Gresham 2002). Since whole tree harvesting is not widely practiced in the southeastern U.S., the concern for loss of nutrients from the site is minimal because it is only economically feasible to remove the woody stem. Therefore it will not affect the rest of the nutrients, which are in understory vegetation, litter, decomposing plant parts, tree roots, soil organic matter, and the soil itself. In instances where it does appear that nutrient loss is a concern, most of the removed nutrients can be replaced by fertilizer applications, as described in the next section.

Management Practices for Sustaining Forests

Several forest management practices and guidelines focus on maintaining the productivity of a forest site as well as improving forest product value and long-term forest management objectives. On some sites, forest fertilization may be used one to several times between planting and harvesting to supplement soil nutrients. It is usually applied by tractor-mounted spreaders, although fixed-wing aircraft and helicopters are also used. Nitrogen and phosphorus are the primary nutrients applied at rates that approximate what is removed in harvest. Supplemental fertilization is also used following thinning in order to produce larger tree diameters and higher product values. Other ecosystems that produce food and fiber, especially agricultural crops, are typically fertilized several times during a growing season. In contrast, forest tree plantations typically require only two or three applications in twenty years.

Thinning is another forest management practice that is used for a variety of objectives in both natural stands and plantations (Figure 1). Thinning generally focuses on removal of the less healthy or less desirable trees. This creates additional growing space for the remaining trees, thus encouraging vigorous and healthy tree growth, which can be important when managing a forest to meet ecological restoration, wildlife habitat, forest health, and sustainable forest objectives. It may also be important in naturally regenerated forests where young trees are very dense and pre-commercial thinning can remove many of the small stems to favor larger ones. Lack of thinning can lead to reduced growth rates and eventual tree mortality while simultaneously increasing the risk of extensive damage from insects, diseases, and fire. Thinning is also increasingly recommended near communities as an option for reducing the risk of wildfire. When markets for low-quality, small-diameter trees are available, thinning can be an opportunity for a landowner to improve stand conditions to meet long-term management objectives as well as an opportunity to generate income. While markets may include solid wood products such as lumber and plywood, trees removed during the thinning operation are usually chipped for use in the production of pulp and paper products or oriented strand board, which has become a major component of building construction. Linking the process of thinning to a local market for wood fuel provides an additional market outlet for the small-diameter trees, and another way for thinning to be economically beneficial to landowners. Landowners who can make money early in stand development by thinning can improve their overall economic returns and make forest stewardship a profitable venture.

Figure 1. 

Thinning operation to remove small trees and improve forest health; thinning is accomplished with machines or chainsaws.


Credit:

Larry Korhnak


[Click thumbnail to enlarge.]

Best Management Practices (BMPs) in the southern states provide voluntary guidelines for landowners on how to conduct various forestry operations while protecting water and soil quality (Box 1). These practices address the use of mechanical equipment, chemicals, fertilizer, and prescribed burning. Biennial surveys in Florida have demonstrated that more than 95 percent of landowners comply with the BMPs. The BMPs also apply to forestry operations that produce woody biomass, whether from logging residues or plantations dedicated to fuel production. BMPs help forest landowners maintain healthy, sustainable forests.

Box 1. 

Best Management Practices.

Published standards in many states are designed to protect, maintain or improve water and soil quality and wildlife habitat values during forestry activities. Individual states developed their standards in response to the 1972 Clean Water Act. They are periodically revised to incorporate new research results. State forestry agencies provide BMP manuals for landowners and most state BMPs, such as those listed here, are now available online.

Florida:

http://www.fl-dof.com/forest_management/bmp/page_1.html

Georgia:

http://www.gfc.state.ga.us/ForestManagement/BMPManual.cfm

Alabama:

http://www.forestry.state.al.us/bmps.htm

South Carolina:

http://www.state.sc.us/forest/refbmp.htm

Texas:

http://txforestservice.tamu.edu/main/article.aspx?id=71

Forest sustainability has been practiced by many private landowners for decades, as their forests were harvested, replanted, protected for various objectives, and then harvested and replanted again. In fact, many southern forests are in their third, fourth, or even fifth generation since they were originally harvested in the late 1800s and early 1900s. Today, a number of programs are available to help landowners develop management plans and conduct sustainable management practices. The American Tree Farm System and Forest Stewardship Program provide assistance and guidance for small landowners. The Sustainable Forestry Initiative and Forest Stewardship Council work primarily with corporations and large landowners, although they are also developing programs aimed at small ownerships. All four programs require landowners to meet various standards for long-term sustainability before they are certified as “sustainable.” The standards also generally apply to woody biomass harvests when performed to meet other objectives, such as thinning to improve wildlife habitat and forest health or utilization of waste materials.

Summary

Through careful planning and management, woody biomass is a viable product from many of our southern forests. Forest sustainability does not have to be compromised by repeated harvests if BMPs are followed, fertilization replaces extracted nutrients, and landowners follow the various standards that focus on producing healthy and sustainable forests.

For more information about using wood to produce energy, visit http://www.interfacesouth.org/woodybiomass and read other fact sheets, community economic profiles, and case studies from this program, or http://www.forestbioenergy.net/ to access a number of other resources.

References

Gresham, C. A. 2002. Sustainability of intensive loblolly pine plantation management in the South Carolina Coastal Plain, USA. Forest Ecology and Management 155:69-80.

Footnotes

1.

This document is FOR 135, one of the series, Wood to Energy, Circular 1526, http://edis.ifas.ufl.edu/TOPIC_SERIES_Wood_to_Energy, School of Forest Resources and Conservation, UF/IFAS Extension. Original Publication Date: September 2007. Reviewed June 2015. Visit the EDIS website at http://edis.ifas.ufl.edu .

2.

Alan Long, professor; and Michael Andreu, assistant professor, School of Forest Resources and Conservation, UF/IFAS Extension, Gainesville, FL 32611.


The Institute of Food and Agricultural Sciences (IFAS) is an Equal Opportunity Institution authorized to provide research, educational information and other services only to individuals and institutions that function with non-discrimination with respect to race, creed, color, religion, age, disability, sex, sexual orientation, marital status, national origin, political opinions or affiliations. For more information on obtaining other UF/IFAS Extension publications, contact your county's UF/IFAS Extension office.

U.S. Department of Agriculture, UF/IFAS Extension Service, University of Florida, IFAS, Florida A & M University Cooperative Extension Program, and Boards of County Commissioners Cooperating. Nick T. Place, dean for UF/IFAS Extension.