Favorable Stand Attributes for Bioenergy Production

Not all forest stands will have biomass readily available in sufficient quantity for immediate utilization. An undocumented but widely used rule of thumb is that at least 6-7 dry tons per acre are required. Furthermore, topography and other physical factors are likely to constrain the economic utilization of forest biomass. Despite this, many forest stands can be potential sources of biomass for bioenergy at some point in the management cycle and its integration into silviculture can assist in meeting management goals.

Several major sources of residue-biomass are available from forests. Logging residues, thinned trees, under-utilized tree species, and other stand improvement measures are the major sources likely to be available from forests (see Biomass Harvesting). Catastrophic events such as hurricanes, fires, or major insect outbreaks also can result in sources of biomass. Short-rotation tree bioenergy crops could also be grown by landowners. Municipal wastes and secondary residues from forest industries and manufacturers will not directly influence forest management and are not included in this discussion.

Site attributues in terms of land-use, forest type, species composition, historical management, and site characteristics have a direct bearing on the potential for forest bioenergy production. In general:

• Logging residues usually come from clearcutting stands on relatively flat topography, with easy access to roads that are suitable for large chip vans. The maximum slope for tracked machines is about 40% and for rubber-tired machines is about 20%. Soil factors may also influence harvesting operations and on very wet or infertile sites may prevent removal of harvest biomass.
• Smaller amounts would be available in even-aged seed tree and shelterwood harvests in naturally-regenerated stands. Often they would include hardwoods that are more energy-dense. Similar standards for road access and slope would be needed as for clearcutting.
• Thinning stands to promote the growth of the remaining trees or to reduce losses from fire, insects, or disease can provide biomass from tree boles, some crown material, and occasionally understory shrubs. However, the overall economics of biomass harvesting should be evaluated in view of its long-term impact on the forest stand. Damage to residual trees must be avoided. Again, good access, gentle topography, and resilient soils are important prerequisites.
• Stands under uneven-aged silviculture are less likely to be favorable for bioenergy operations. The group selection method, where whole tree harvesting is used, might be possible as the residues and perhaps thinned trees could be accumulated at roadsides.
• Clearing shrubs and small trees for fire prevention and control, perhaps associated with thinning operations, would be limited to special situations such as along major roads or near buildings.

High Fire Risk Stand

Low Fire Risk Stand

• Large areas of natural stands that have been degraded by poor exploitation could be rehabilitated and produce biomass commercially (Andersson and others 2002). Such stands have often been high-graded several times by loggers taking only the valuable and most accessible trees and leaving the rest. The result is naturally regenerated, poorly- to fully-stocked stands of low commercial value. Harvesting for bioenergy may be included in programs to improve and restore the long-term value of these forests.
• Short-rotation bioenergy tree crops will generally be grown on moderately fertile, relatively flat topography.
• Various agroforestry practices, such as shelterbelts or silvopastoral systems, may offer opportunities to provide tree biomass for bioenergy.
• Trees and shrubs with high wood density, such as oaks (Quercus spp.) and many other hardwoods, produce higher energy-density chips than most conifers (Hakkila and Parikka 2002). Transpiration drying, to lower the moisture content of the chips, also increases the energy density. A major benefit of high energy density chips is reduced transport costs per quantity of delivered energy (See Harvesting for more details).

Additional information regarding sources and quantities of biomass can be found in the Sources and Quantity of Supply section in the Economics module.