Historically, fires have played an integral role in the vitality of our forests. Fire changes the landscape by altering population densities, removing some species and allowing others to thrive, and reducing surface fuels on the forest floor. These changes can lead to diverse ecosystems that are resilient to disease and pest outbreaks. Native tribes in the Pacific Northwest used fire to manage forests and grasslands to enhance wildlife habitat and create open hunting conditions. Fire also benefited these tribes by allowing more habitable space for desirable food species, such as camassia, roots (bitterroot, balsamroot), and berries (mainly huckleberries). In the late 19th century*, fires—due to land use changes—were excluded from the landscape and more recently by land managers to protect the public and other valuable resources. As a result, forests that had once evolved and adapted to frequent, low-severity fires have undergone dramatic change. Following a century of fire suppression, the forests have become overcrowded with dense populations of small trees and have an overabundance of surface fuels. Consequently, these landscapes are at risk of uncharacteristically large and severe wildfires.
*The California fires in the 1880s actually started suppression concerns, but actual management through suppression began after the 1910 fires in the West.
Fire regimes in the Pacific Northwest
How often did fires occur on the landscape? ‘Fire regime’ is a term used to describe patterns of fire that are presumed to have repeatedly occurred historically in a landscape, and includes both the frequency and severity of fires. There are five fire regimes in the Pacific Northwest (figure 1). Each regime varies in the amount of time (fire-return interval measured in years) between fires and the fire severity. In forest types like the ponderosa pine forests of eastern Oregon, the fire-return interval was every 0–35 years, and fires burned at low to mixed severity (figure 1 and table 1, see Fire Regime I). Coastal forests in this region, however, burned less frequently and had a greater fire-return interval (200-plus years). As a result, those coastal forests burned with a stand-replacement severity (figure 1 and table 1, see Fire Regime V).
‘Fire severity’ is a term that describes the effects of fire on soil (sometimes called burn severity) or on fuels and vegetation, and measures the degree to which an area has been altered or changed by fire. Fire severity descriptors may include loss of or change in organic matter, both aboveground and below ground, with indicator measures such as bark char and foliage scorch. Fire severity often is incorrectly used interchangeably with fire intensity. Fire intensity is a measure of the energy (rate of heat) released during a fire. Fuels, weather (wind, temperature and humidity), and topography are important in determining the rate of heat released by a fire.
| Fire Regime | Frequency | Severity |
|---|---|---|
| I | 0–35 years | low to mixed |
| II | 0–35 years | replacement |
| III | 35–200 years | low to mixed |
| IV | 35–200 years | replacement |
| V | 200+ years | replacement/any severity |
Fire behavior
There are two triangles to consider when understanding basic fire behavior. The first triangle is the fire triangle composed of fuel, oxygen and heat (figure 2). In order for fire to occur, these three components are needed. Fuels and ignition sources (heat) are the most common limiting factors in the fire triangle. Historically, ignition was caused by lightning and by Native American burning. Today, the leading natural cause of wildfire is lightning. Humans are also major sources of ignition, either directly from arson or untended campfires, or indirectly from machines, vehicles and power lines. The behavior of a fire is affected by fuel, weather and topography; these components influence the fire’s rate of spread, flame length, torching, crowning, spotting and whirling (figure 3).
Fuels
Fuel is the common component in the fire triangle and fire behavior triangle. Fuels are comprised of vegetation (alive or dead) and other combustible material. There are four types of fuels—ground, surface, ladder and crown.
There are many characteristics of fuel, such as fuel particle size, fuel loading, fuel arrangement and continuity and fuel chemical makeup. But one of the most important characteristics is fuel moisture content. Fuel moisture is influenced by relative humidity, temperature and precipitation. Moisture content is determined by weighing fuels’ before and after percentage. The formula for that calculation is Wet Weight minus Dry Weight divided by Dry Weight multiplied by 100 (wet weight – dry weight/dry weight x 100). Live fuels have moisture contents from 30–300%, while dead fuels usually range from 2–30%. Fuels are categorized by the length of time it would take the fuels to reach the moisture content at which the fuels are in equilibrium with the air around them.
Fire managers are often most concerned about 1-hour fuels because they are the most likely to respond quickly to changes in weather. Examples of 1-hour fuels are grass, leaves, mulch and litter. Conversely, larger-diameter fuels such as brush piles take up to 1,000 hours to respond to changes in weather conditions.
Weather
Hot, dry and windy weather increases the risk of wildfires. As the temperature rises, the relative humidity (the amount of moisture in ambient air) decreases. The less moisture available in the air, the drier the fuels become. Windy conditions desiccate fuels further and increase the oxygen supply. A Red Flag Warning is issued when warm temperatures, very low humidities and strong winds are expected to combine to produce an increased risk of fire danger. This warning is used to inform firefighting and land-management agencies that conditions are ideal for wildland fire combustion.
Topography
Topographic factors that influence fire behavior include: slope steepness and position of fire on a slope; aspect and elevation; and natural topographic barriers, such as roads and rivers. Fire tends to spread faster up hills as heat from the flames dries out the plants upslope, making them more combustible. South-facing slopes have drier fuels because of a higher intensity of solar radiation. Fire managers use topography to inform the use and location of resources when fighting a wildfire or planning a prescribed burn.
Prescribed fire
Over 11 million acres (40%) of forestland are in need of restoration in the Pacific Northwest region (Haugo et al. 2015). Coupled with predicted hotter and drier summers, these landscapes are at risk for uncharacteristically large and severe wildfires. Prescribed fire is the controlled application of fire to the land and is one tool used by land managers to achieve specific management goals.
Because fire behavior is dependent on fuels, weather and topography, prescribed fire enables land managers to manage fuels in a situation where weather and topography are predictable. Prescribed fires are beneficial because they:
- Reduce fuel buildup: By reducing the available amount of fuel on the ground, including woody debris left behind after harvesting and thinning, future fires in these areas will be less intense, resulting in a decreased probability of catastrophic wildfires.
- Improve wildlife habitat and forage for grazing: Prescribed fire is used to create a varied land and vegetation pattern that provides diverse habitat for plants and animals. Openings in the landscape created by fire encourage the growth of grasses and other grazing materials for grazing wildlife.
- Manage competing vegetation: Historically, fire-prone and fire-adapted forests that have experienced fire exclusion have been invaded by weeds and fewer fire-tolerant species. Reintroducing fire removes some of these invasive plants while allowing native, fire-adapted plants to thrive.
- Control disease: Fire is a natural control of disease and pathogen outbreaks in plants.
- Increase aesthetics: Burning can increase flowering annuals, open stands and improve aesthetics.
- Cycle nutrients: Fire stimulates important chemical reactions for nutrient cycling, such as releasing nitrogen and phosphorous into the soil for uptake by many plant species.
Before a prescribed fire is set in place, a burn plan is written and submitted to local forest or agriculture departments for approval. The burn plan sets guidelines for how the area will be treated, and how risk will be mitigated through evacuation and contingency plans. The burn plan also includes ideal weather parameters for the prescribed burn, which are determined using computer simulations. Land managers look for days that are cool and clear, and dry enough to start a fire but with enough moisture from previous rainfall to ensure fuels have very low moisture content. Smoke management is also an important part of the burn plan, as smoke is one of the biggest challenges for fire managers.
Common concerns
Smoke
Fires (prescribed or wildfire) produce a mixture of gases and fine particles from burning trees and other plant materials. This smoke is a valid public concern. Smoke can cause health problems and decrease visibility on nearby roads. Land managers work to mitigate the negative impacts of smoke by looking carefully at what they plan to burn and the proximity to homes and roads in the planned burn area. While prescribed fires do produce smoke, they prevent heavy fuel accumulation that would send a larger amount of smoke into the air should a wildfire occur. Other concerns surrounding prescribed fire are tree mortality, aesthetics and wildlife.
Tree mortality
Even fire-sensitive species can survive some fires. Prescribed burns are also set during weather conditions that should prevent large flame lengths.
Aesthetics
Plants resprout quickly after a fire. By the following year, the only evidence of a fire may be some charring on trees.
Wildlife
There are usually very few wildlife fatalities during fires. Most animals are able to remove themselves from the threat. There are many species that rely on fire to improve food resources and habitat conditions, including species that use dead trees for foraging and nesting.
Summary and conclusion
Fire is an important natural disturbance that helps maintain the health and diversity of our forests and grasslands. Wildfires, however, are behaving differently than historically described due to a number of factors, including human actions. Prescribed fire is one of the many tools available to help maintain healthy, resilient forests. Carefully managed prescribed fire can help maintain fire’s natural role across the landscape and reduce the hazard of catastrophic wildfire caused by excessive fuel buildup.
For more information
Videos
Prescribed fire: A multi-purpose tool. Produced by the Northwest Fire Science Consortium.
Catching fire: Prescribed burning in Northern California. Produced by Klamath-Salmon Media.
Forest fast break: Forest fire. Produced by Oregon Forest Resources Institute.
Forest fast break: Fire safety. Produced by Oregon Forest Resources Institute.
Forests born on fire. Produced by Wild Nature Institute.
Websites
Washington Prescribed Fire Council
Oregon Prescribed Fire Council
Coalition of Prescribed Fire Councils
References
Haugo, R. et al. 2015. “A new approach to evaluate forest structure restoration needs across Oregon and Washington, USA”. Forest Ecology and Management 335(1): 37-50.
