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forest tracts of private land that may be owned by forest industries or by other private owners. By definition, wildland fires generally involve few or no structures. Fires that involve a mixture of vegetation and structures are considered wildland/urban interface fires and are discussed below in Section 9.1.3. Fire suppression strategy for wildland fires is significantly different than for structure fires. For wildland fires, the most common suppression strategy is to contain the fire at its boundaries, to stop the spread of the fire and then to let the fire burn itself out. Fire containment typically relies heavily on natural or human-made fire breaks. Water and chemical fire suppressants are used primarily to help make or defend a fire break, rather than to put out an entire fire, as would be the case with a structure fire. Fires that are purely wildland fires, without threatening structures, nevertheless cause environmental and ecological damage. Wildland fires kill wildlife and damage habitat. Areas that have burned are also subject to erosion and landslides due to loss of ground cover. Such fires also may result in large fire suppression costs, with a potential for casualties among firefighting personnel. Historically, fire suppression strategy for wildland fires has generally been to try to minimize the acreage burned in each wildland fire, by applying the maximum available fire suppression resources and trying to contain each fire as quickly as possible. In recent years, however, fire suppression strategy for wildland fires has evolved substantially in two important aspects. First, to a greater extent than previously, wildland fires are being recognized as part of the natural ecology and natural life cycles of wildlands. Fires create open spaces with different habitats for both plants and animals than existed previously. Second, the emphasis on maximum suppression of wildland fires has resulted in many fires being smaller than would naturally occur. Because of the reduction in frequent, smaller fires, many wildland areas have developed extraordinarily high fuel loads. Thus, the potential for very large, catastrophic wildland fires may actually be increased by the effective suppression of smaller fires. In recent years, evolving strategies for dealing with wildland fires have focused more attention on fuel management. Strategies include more controlled burns and greater tolerance for allowing smaller fires to burn, with the objective of reducing fuel loads of smaller vegetation and thus reducing the potential for target fires. Wildfires may be started by natural causes, such as lightning strikes, or by human activity. US Forest Service data indicate that about 13% of wildfires are started by lightning, about 25% of wildfires are arson, while the rest are due to a variety of human causes including debris burns, discarded smoking materials, sparks from vehicles, sparks from power lines and so on. Wildfire hazard depends on three main factors: vegetative fuel load, weather, and topography. There are several parameters that define the fire potential of vegetation. Vegetative fuel loads are typically expressed as tons per acre. The greater the amount of fuel loading the greater the amount of energy that will be released in a fire. Vegetative fuels are also classified by burn index, which is a measure of the amount of energy per pound of fuel. Fuels may also be classified by potential duration of burning. For example, wildfires fueled by grass may spread very quickly, but grass contains Public Review Draft: August 6, 2004 9-6