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Operations to reduce fuels in western forests.

Sunlight beats down on the surface of the earth, supplying 1 kilowatt/[m.sup.2] of power when the sun is directly overhead. In forests and other biomes that contain photosynthesizing organisms, a small fraction of this power input converts carbon dioxide and water to biomass.

What happens to this forest biomass?

Prior to human intervention, fire and decay organisms converted most biomass back to carbon dioxide and water. The same agents are at work today, but several aspects have changed. Humans harvest much of the wood, utilizing it for fuel or construction. The fire and decay that oxidizes this harvested wood act outside the forest.

Harvesting and fire suppression have changed fire regimes in many forests, resulting in less frequent but more intense and destructive fires. For example, prior to the European settlement of California, fires burned in most Sierra Nevada forests at median intervals of 10 to 20 years, while data from the 20th century indicates that the return interval has increased to over 100 years. In the past, most fires burned on the surface. Now, due to a number of factors, especially the higher amounts of fuel that accumulate over the longer intervals, many fires are so intense that they kill all or most of the trees in a stand. They destroy habitat and increase erosion in addition to negating much or all of the value of the trees for wood products. The 2002 Biscuit Fire in southwestern Oregon burned 1,000 square miles, mostly in national forests.


Why not remedy the problem by using fire at frequent intervals?

There is little doubt that this technique would work in the long term, but most of us may not be happy with the consequences. At present, fuel loads are so high that introducing fire on a broad scale is likely to result in many stand-replacing crown fires. Then, of course, there is the human footprint, including communities, residences within the urban-wildland interface, etc. Fire can be prescribed, but is difficult to control. Especially on private lands, managers are unwilling to take the risk and liability of having a fire escape and cause the loss of property and possibly lives. Smoke impacts health, aesthetics, and safety, and therefore the time "windows" during which managers may burn are being reduced.

Physical removal or modification of fuels offers another alternative

Beginning in the 1980s, a new market for biomass-to-electric energy in California created opportunities to thin many forest stands. While this market has declined due to cheaper prices for alternative electric energy (the recent energy crisis being a non-typical glitch), recent concern about catastrophic fires has sparked several research projects, especially in the western United States.

Three key aspects of forest fuels are air-accessible surface to mass ratio, vertical position within the stand, and amount. Fine fuels such as needles and branches burn faster per unit mass than do dense elements such as the trunks of large trees. For the same reason, "fluffy" material such as branches on small trees will burn more intensely than a compacted layer of chips produced from those small trees.

The amount of loose, fine fuel on or near the soil surface strongly affects the intensity and rate of spread of ground fires. Ladder fuels--smaller materials in the understory--influence whether surface fires progress into the crowns. Clearing small trees and underbrush is the best way to reduce ladder fuels. Fines in the crown affect the risk of a crown fire progressing horizontally.

While it's impossible to completely fireproof a stand (unless all biomass is removed), experience and simulations of fire behavior have shown that fuel reduction can be effective. The 2002 Cone Fire in northeastern California (set accidentally by a logging operation) burned through the crowns of areas that had not been treated, then dropped to the surface or was extinguished altogether at the boundaries of thinned units, some of which also had been prescribe-burned.

What equipment and methods are available for fuel reduction treatments?

Two systems used for conventional logging have been extensively tested and utilized. The "whole-tree" system, a mechanical feller/buncher, cuts and piles trees which are then skidded with rubber-tired skidders to roadside for processing into logs and/or chips. The fines are removed with the rest of the tree. The "cut-to-length" system utilizes a mechanical harvester to fell, limb, and buck trees into short logs. These logs are transported by a forwarder (essentially an off-road truck) to roadside. Tops and limbs are left in the woods but in windrows that are compacted by the traffic of the harvester and forwarder, rendering them less dangerous than in their previous locations as part of the fuel ladder. Both of these systems were developed for logging of trees that were large enough to have commercial value. Research has shown that they can handle more of the smaller trees per hour, but this does not compensate for the reduction in tree volume, resulting in higher costs per unit volume. In addition, the value of a tree per unit volume decreases with tree size. The smaller trees that constitute the main elements of fuel ladders therefore can at present only be removed at a net cost, whereas larger trees provide a positive net income.


Mastication--another option

Mastication--using horizontal- or vertical-axis flails or cutters to chop small-diameter trees and brush in the understory and to compact surface fuels--is another option. No material is removed from the forest, so fuel loading is not reduced, but the relatively compact layer of chopped material on the surface will burn less intensively (although longer) than the same material if uncut.

Small base machines, such as the ASV, with multiple interchangeable attachments for felling, processing, skidding, and masticating, have been tested for use on small ownerships such as parcels around homes. The single base machine eliminates the high fixed cost of transporting a standard mix of single-purpose machines to the site.

A relatively new machine of Scandinavian design--the Timberjack Bundler--is being tested this year in the United States. It collects tops and limbs, left by cut-to-length logging, and compresses and wraps them into bundles that can then be transported by a forwarder.

Any of these mechanical operations can be followed by prescribed fire to reduce the amount of surface fuel.

None of the existing equipment and methods are ideal for removing small trees, because they were designed to handle single trees. Using them for small understory trees is like using a hand clipper to mow a lawn: it works, but not very efficiently. An excellent opportunity exists for developing new equipment to economically remove the small trees, brush, and possibly surface fuels that contribute so much to fire danger.

The current scenario

Managers must, in most cases, pay (on the net) to remove the small trees, underbrush, and surface fuels that constitute the largest fire danger. The benefits in reduced fires can be substantial, but the costs of reduction are concrete and the benefits nebulous, meaning that relatively few dollars are made available for fuel reduction. Three options are: 1) focus fuel reduction on only the top-priority areas, i.e. those near high-value property and those that can most effectively suppress large fires. Shaded fuel breaks are examples of the latter; 2) remove some larger trees so the commercial value can offset some or all of the cost of fuel reduction; 3) develop new equipment to remove ladder and/or surface fuels at little or no net cost. As many of the greatest needs for fuel reduction are in national forests rather than on industrial timberlands, the U.S. Forest Service could greatly assist in solving the fuel problem by investing in a program to develop such new equipment.

ASAE member Bruce Hartsough is professor and chair of the biological and agricultural engineering department, University of California-Davis, Davis, CA 95616-5294 USA; 530-752-0103, fax 530-752-2640,
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Author:Hartsough, Bruce
Publication:Resource: Engineering & Technology for a Sustainable World
Date:Jan 1, 2004
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