In the News

Forest Thinning Projects Require Careful Elimination of Fuels
date posted: December 22 2005

Lake Tahoe Report #91
Nov. 8, 2004
By John Cobourn and Heather Segale

Reducing catastrophic wildfire risks in the Tahoe Basin is a major priority of federal, state and local fire management agencies and the Tahoe Regional Planning Agency. A significant amount of progress has been made, particularly in the last seven years. Previous articles have described many aspects of this work, including prescribed fire and other forms of treatment. Prescribed fire is usually done after other necessary steps are completed, including thinning overgrown stands.

In many parts of our forests, the clear-cutting of trees in the late 1800s, followed by fire suppression throughout the 20th century, has resulted in unnaturally high densities of small-diameter trees. Trees in these stands compete for scarce moisture, and become weak and susceptible to insect infestation and mortality. Such conditions have contributed to the high level of hazardous dead fuels in Tahoe’s forests and elsewhere in the Sierra. Once wildfires start, they burn hotter and more rapidly than they would in a healthier, more natural forest.

Since 1987, more than 36,000 acres of national forest lands have been treated in the Tahoe Basin. Several thousand more acres of nonfederal lands also have been treated. Thinning of small-diameter trees is an essential first step in forest health restoration. Where the terrain allows, mechanical thinning using a mechanical harvester is highly efficient. However, Tahoe Basin terrain is very steep in many parts, which limits access. Hand crews are used to thin in many areas where terrain, sensitive soils or water quality impacts are a concern.

What happens to the small trees removed in thinning? If they are left on the ground, they dry out and greatly add to already high fuel loads. The trees that are cut can be sold, but the market for trees smaller than 12 inches in diameter is not strong enough for large sales. This means foresters usually must burn or mechanically chip and masticate the small trees they remove.

Most small-diameter material currently is burned in piles. Pile burning is the most cost-effective and rapid means of disposal. It also fully eliminates the fuel material, reducing the risk of high intensity wildfires. Hand crews tend the burn piles to prevent the fire from escaping. The work does result in smoke, but far less smoke than is produced by wildfires.

Use of chippers and larger masticators is an alternative to pile burning. These machines do not actually eliminate the hazardous fuels, but instead change their shape. Because chippers and masticators are either towed or vehicle-mounted, they can only be used on flat to moderate slopes, and in areas where they will not damage soil and water quality. Chippers and masticators do not produce smoke, but do produce very loud noise – particularly the masticators that process bulkier materials. Because these machines do not process fuels as quickly as prescribed fire, the noise they produce is experienced for a much longer time by nearby residents than the smoke that is produced from pile burning. Unfortunately, both methods have objectionable side effects.

The masticated or chipped wood typically is spread in or around the area where the thinning work has been done. However, there are questions about the long-term effects that large-scale chip spreading may have on soil and water quality. There is also concern over the potential for a wind-driven wildfire to spread rapidly through areas with large quantities of dry chips on the ground. The burning chips could then be carried down wind, further spreading the fire.

Another way to eliminate fuels from thinning projects is to chip them and burn them in “biomass generators.” This emerging technology can turn cellulose-based material, such as trees or scrap lumber, into energy that can heat structures or water, or be used to generate electricity. Biomass generators burn hot and clean, and can be installed at a processing facility or moved from site to site. However, this technology also has its limitations. Among them is the difficulty of establishing the steady, reliable supply of fuel needed for commercial viability. In order to access large quantities of such fuels in our forests, costly new roads probably would have to be built. Roads also are known to be a major cause of soil erosion and water quality degradation in any watershed.

Rex Norman, USDA Forest Service, concludes, “There is no single solution for what to do with the materials left over from thinning and other fuels reduction work. In the long run, there will continue to be multiple ways of reducing the overburden of hazardous fuels. These will include an increasing market for smaller-diameter trees, the use of prescribed fire, chipping, masticating, and increased biomass opportunities.”