Fire, if properly controlled and managed, can be a valuable tool to manipulate vegetation composition, structure, and fuel loads on rangelands (and other wildland ecosystems). Managed fire can create and maintain a mosaic of plant communities, at appropriate locations on the landscape, to provide valuable ecological services that benefit many rangeland resources. Among these are:
- “cleaning” or removing unwanted vegetation from landscapes to reduce fuel loads and the risk of large catastrophic fires
- changing the relative balance between herbaceous (grasses and forbs) and woody plants to improve forage availability for livestock or wildlife
- changing the structure or size class of plants across appropriate distances to benefit wildlife
- creating variability both within and among plant communities for wildlife and other rangeland resources
- promoting seed germination and the regeneration of desired plants; and
- maintaining the hydrologic cycle to promote the infiltration of water and to prevent rapid runoff and accelerated erosion.
The use of fire as a management tool requires that it be applied at the correct frequency (return interval), intensity (heat release), and spatial scale so that burned and unburned patches remain on the landscape. A mosaic of burned and unburned patches typically results in a mix of early- to late-seral communities, which benefits more rangeland resources than a homogeneous landscape of either early- or late-seral plant communities.
The Importance of Fire Intervals
Prior to European settlement of North America, most American rangelands burned periodically. The return interval between fires varied widely among different rangeland ecosystems (e.g., tall-grass prairie, sagebrush steppe, ponderosa pine forest) but was relatively constant within an ecosystem. Some pre-settlement fires were caused by lightning, but others were intentionally set by Native Americans to achieve vegetation management objectives. Because fire, regardless of the source, was a recurring ecological event with a relatively constant interval, most rangeland vegetation has adapted to periodic burning. After a fire, the vegetation usually forms early-seral plant communities. If there are few unburned "islands," wildlife species that need mid- to late-seral plant communities to complete their annual life cycle cannot inhabit the area or they have very small populations. The same ecological effect occurs when fire return intervals become so short that mid- and late-seral plant communities never become established. Excessively long or short fire return intervals create uniform plant communities across large landscapes and tend to exclude (or dramatically reduce) the wildlife species that need a diverse habitat structure created by a mosaic of early- to late-seral plant communities.
Modern fire return intervals on most rangelands are either longer or shorter than the pattern that evolved prior to settlement. This has resulted in undesired vegetation changes that often adversely affect rangeland resources. Fire return intervals that are substantially longer than the evolved interval lead to a substantial increase in shrubs and/or trees (woody fuel) and a decline in many desired grasses and forbs. The long life span of most shrubs and trees, combined with the slow decomposition (compared to herbaceous vegetation) of their dead branches, stems, and leaves creates large fuel loads and a fuel ladder that can carry a fire into the upper canopy of the vegetation. Horizontal and vertical continuity of the vegetation (fuel), combined with excessive fuel loads, can result in large, intense wildfires across tens of thousands of acres or more. In the western United States, it is becoming increasingly common for individual fires to burn 100,000 to 200,000 acres or more.
Public land management agencies and some private owners of range and forestland are increasing their effort to thin the woody vegetation on land they administer or manage. Their goal is to reduce wildfire risks and create plant communities that are more resilient to insects, disease, and the inevitable wildfire that will occur. Treatments that thin shrub- and tree-dominated communities not only reduce fuel loads but can also provide large volumes of woody biomass for an alternative fuel for society. According to a recent national study, about 8.4 billion tons of dry biomass resides on range and forest lands, but only 60 million dry tons can be removed annually with current fuel reduction methodologies (USDOE and USDA, 2005). At the current removal rate, it will take at least 140 years to reduce woody fuel loads; thus, additional large-scale catastrophic fires are inevitable in many areas.