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Prescribed burning , also known as “controlled burning,” refers to the planned use of fire in different environments for a wide range of reasons, including reduction of wildfire severity, protecting grasslands from forest incursion, and simulating prior fire regimes. Although the term was developed only in the late 20th century, the practice of prescribed burning has been applied throughout human history.
The word prescribed is used as these fires are set only once certain environmental parameters are achieved. In industrialized societies, the parameters are generally based on quantitative thresholds for a range of factors such as wind speed and direction, air temperature, relative humidity, and moisture content of vegetative fuels and soil. Together with attributes such as local topography, these influence fire behavior, intensity, and rate of spread. Thresholds are usually determined empirically following research into fire behavior and are established to ensure that the fire can be contained while achieving management objectives.
However, generations of experience also provide the basis for knowing where and when to burn for best results. Indigenous peoples have long employed fire for environmental management. After 40,000 years some Australian Aboriginal communities continue to use fire to achieve a range of habitat and cultural outcomes. For example, fires are set to stimulate germination of grasses that attract kangaroos and other game for hunting and to thin out trees and shrubs to ease movement and habitation. Native Americans also burned selected areas regularly to favor grass species over trees and shrubs and improve grazing for game. Other societies share long histories of using fire as a land management tool. In Europe, fire was used for centuries to clear land, improve its short-term fertility, or bring about changes in vegetation structure to suit farming or hunting objectives.
Central to understanding the environmental consequences of prescribed burning is the concept of the fire regime. This refers to the frequency, intensity, season, and fire types, natural or otherwise, that have occurred in an area over time. Species, especially flora, evolve responses to fire regimes that enable them to persist in the ecosystem in the face of fire, but their adaptive responses relate to the historical fire regime. Therefore, changes to a fire regime, including the exclusion of fire, may cause changes in the species composition of an ecosystem. Prescribed burning imposes human fire regimes on the environment, contributing to the shaping of the landscape as a cultural artifact.
In New World countries such as the United States, Canada, and Australia, colonial settlement and agriculture interrupted fire regimes and ecosystems associated with indigenous environmental management practices. Wary of the threat that wildfires represented to their lives and assets, European settlers tended to suppress fires in the landscape. Ironically, these changes often produced a more wildfire-prone environment. Shrubs and trees replaced grasslands and forest densities increased. Of course, heightened fuel levels meant that when wildfires inevitably occurred they did so with greater severity.
After World War II, research into fire behavior and its role in the environment made clear the need to include fire as an integral part of land management in order to restore or maintain historic vegetation patterns and fire regimes. This represented a paradigm shift, which, together with improvements in the technology to conduct and control prescribed burning, resulted in an increase in its use particularly in silvicultural practice to reduce wildfire risk and to manage forest estates. Its use has also increased as a means of reducing wildfire risk to urban environments. Prescribed burns conducted to reduce wildfire risks are often referred to as “hazard reduction” or “fuel reduction” burns. They are of low intensity so they may be contained; yet they are hot enough to consume the vegetative material that might otherwise become fuel during a wildfire. Hazard reduction burns need to be conducted every few years, depending on the vegetation, to keep fuel levels low.
The advent of frequent hazard reduction burning, particularly to protect urban environments at the wildland-urban interface, has led to concerns that some species that coevolved with indigenous or natural fire regimes will not survive altered fire regimes. As a result, research is now being undertaken to establish fire regimes that support both wildfire hazard reduction and biodiversity conservation, but some regard these objectives as incompatible. Prescribed burning undertaken specifically to achieve conservation objectives, such as preserving threatened ecosystems or particular species, may be referred to as “ecological burning.”
Other concerns about prescribed burning include: the potential for increased soil erosion; the risks of fires escaping containment lines; smoke pollution; and the costs of providing adequate resources to conduct and control burns. More recently attention has also focused on the implications of burning for the carbon cycle and climate change issues. These and other problems drive continuing research into the use of prescribed burning and the parameters associated with fire prescriptions. Prescribed burning remains a contentious and complex issue. Debate is characterized by uncertainty over both the effectiveness of hazard reduction burning and the ecological consequences of prescribed burning.
- William J. Bond and Brian W. van Wilgen, Fire and Plants (Chapman and Hall, 1996);
- John Mulvaney and Johan Kamminga, Prehistory of Australia (Allen and Unwin, 1999);
- Stephen J. Pyne, World Fire: The Culture of Fire on Earth (Henry Holt, 1995).