Sustained Yield and Stand Biomass Assessment

Sustained Yield

Sustained yield is what a forest can produce continuously at a given intensity of management (Helms 1998). This supply should be able to be maintained into perpetuity and is often calculated as an annual or regular periodic output. For bioenergy, it is the continuous supply of biomass for feedstock to an industrial plant for energy production through conversion. Relatively large forests can readily produce sustained yield on an annual basis regulated by appropriate management systems.

For NIPFs the output and control of biomass is complicated by the size and diversity of ownership, each with specific goals, and by the lack of knowledge by owners of the concepts of sustained yield and harvest control. With the exception of owners growing short rotation woody crops, many small NIPF owners with less than 300 to 400 acres may find it difficult to plan for an annual sustained yield of biomass, although periodic yields would be easier to accomplish. Hicks (1998) suggested that for many of these small landholders, uneven-aged silviculture may adequately meet their objectives, while allowing periodic harvesting. Cooperatives may also offer another mechanism by which NIPFs can guarantee a sustained supply to an industry.

Biomass Assessment

To assess the bioenergy potential of a stand or forest, it is necessary to estimate the quantity of biomass available. This will be most important where a large program is being considered. Although the discussion of forest inventory procedures is outside the scope of this Encyclopedia, we will offer a brief introduction to the topic. In-depth discussions of appropriate forest inventory procedures are provided by Avery and Burkhart (2002) and Clutter and others (1983).

Simple sampling procedures for estimating stand biomass have been described by Phillips and Saucier (1981). Where a quick estimate is required, they recommend using point sample plots using an angle gauge (e.g. a 10-factor prism) and recording the number of trees by major species groups. The estimate of biomass yield is obtained using factors determined by Hughes (1978) and reported in Phillips and Saucier (1981). To obtain more detailed estimates by species and using diameter and height, fixed radius plots should be used. For trees >= 5 inches in diameter at breast height, measure all trees for diameter and height, by species, on 1/20 acre plots. Smaller trees from 1.0 to 4.9 inches should be measured in a 1/100-acre plot in the center of the larger plot. These bounded plots should include between 15 and 40 trees. Calculate the biomass using published tables or equations. It is important to consider the merchantable size limits of the harvesting system and to adjust figures for losses during harvesting so that estimates of harvestable biomass, rather than total above-ground biomass, can be obtained. Biomass yield tables (green weights by diameter and tree heights) for tree species in the Southern U.S. have been produced. See papers GFRP60 (for hardwoods) and GFRP79 (for southern pines(Pinus species)), together with other yield tables (listed at http://www.gatrees.org/Resources/Publications/ForestMarketing/researchpapers.cfm). A comprehensive database of diameter-based biomass regressions of dry weights is given by Jenkins and others (2004).

To estimate the available woody biomass or volumes of residues on logged sites, foresters often use line-transect methods (Martin 1976). Normally shrubs and other small residues, such as branches and foliage, are sampled using small plots or quadrats.

However, where harvesting for bioenergy is an accepted practice, many owners and contractors do not conduct biomass inventories, but rely on their experience to decide on whether the operation will be successful. The benefits of accurately knowing the available quantities are outweighed by the cost of making the measurements. Where utilization for bioenergy is considered an integral part of silviculture and forest management, the decision may not need the added precision of separate stand assessments.