Fire

Figure from Loehman et al. (2020)

The projected increase in winter and spring temperatures in the southwestern United States is likely to shift precipitation from snow toward rain during the cold season, advance the timing of spring snowmelt, and elongate fire seasons, thus increasing the potential for ignitions to spread into large and/or high severity fire events . An earlier start of the vegetation growing season increases plant water requirements at a time when water may be scarce. Late spring and summer high temperatures increase evaporation from the soil, plants, and fuels, generating large areas of dry and highly flammable material that are more likely to ignite, carry fire, and contribute to severe wildfire than when fuels are more moist. In addition, the accumulation of dead woody debris and high tree density in many southwestern forested lands contributes to risk of crown fires, those that spread through tree canopies, many becoming stand replacing fires. Stand-replacing fires increase the potential to shift forest to alternative vegetation types such as grasslands or shrublands , or can facilitate the invasion of cold intolerant annual grasses that are highly flammable and show fast post-fire recovery compared to native plants. Changes in fire patterns and behavior, fuel conditions, and vegetation types can alter historical patterns of wildfires and the role of fire in ecosystems, making fire behavior more difficult to predict and resulting in significant challenges for natural resource managers and planners.

Figure from SWFireCLIME

Relevant Fire Concepts

Fire Regime: The general pattern in which fires naturally occur in a particular ecosystem over an extended period.

Fire Intensity: The released rate of heat energy during a fire.

Fire Severity: The effects of a fire on the environment, typically vegetation damage and soil impacts. Loosely, a product of fire intensity and residence time.

Fire Frequency: The number of times that fires occur within a defined area and time period.

Fire Season: The period(s) of the year during which wildland fires are likely to occur and spread.

Fire Interval: The time between fires in an area (stand or landscape).

Mean Fire Interval (MFI): The average number of years between fires in a site.

Fire Rotation: The time required to burn an area equal to a defined area of the landscape. The entire area may not burn during this period; some sites may burn several times and others not at all.

Fire Behavior: The way a fire reacts to the influences of fuel, weather, and topography. Loosely fire spread rate and intensity.

Ground fire: A fire that burns in surface organic materials such as peat or deep duff layers. They may kill roots of overstory species because of prolonged high temperatures in the rooting zone.

Surface fire: A fire that burns only the lowest vegetation layer, which may be composed of grasses, herbs, low shrubs, mosses, or lichens. In forests, woodlands, or savannas, surface fires are generally low to moderate severity and do not cause extensive mortality in the overstory vegetation.

Crown fire: A fire that burns through the upper tree or shrub canopy. In most cases the understory vegetation is also burned. The fire may be continuous or may occur in patches.

Crowning: Active fire movement through the tree canopy.

Torching: The burning of the foliage of a single tree or a small group of trees, from the bottom up

Stand replacement fire: A lethal fire l to most of the aboveground vegetation changing the vegetation structure substantially. The fire may be crown fires or high-severity surface fires, or ground fires.

Mixed-severity fire: The severity of fires varies between nonlethal understory and lethal stand replacement fire, with the variation occurring in space or time. Individual fires vary over time between low-intensity surface fires and longer-interval stand replacement fires. In others, the severity may vary spatially as a function of landscape complexity or vegetation pattern. The result may be a mosaic of young, older, and multiple-aged vegetation patches.

Flame residence time: The time flaming exists at one point.

Fire spread rate: The time for which combustion takes place over an area.

Fuel: Living and dead vegetation that can be ignited. Categorized as herbaceous or woody and live or dead.

Fire Weather: Weather conditions that influence fire ignition, behavior, and suppression.

Cultural burning: Fire use by Indigenous stewards of the land to protect cultural or natural resources in different seasons and at different scales and intensities. Cultural burns are used for maintaining the health of ecosystems, for ceremony, for habitat protection or enhancement, and for fuel reduction.

Indigenous use of fire: Fire use by Indigenous Peoples to keep the environment in balance.

Prescribed fire: A wildland fire originating from a planned ignition following applicable laws, policies, and regulations to meet specific objectives. Prescribed burning is the application of prescribed fire.

Wildfire: A wildland fire originating from an unplanned ignition, such as lightning, unauthorized and accidental human caused fires, and prescribed fires that are declared wildfires.

Fire use for resource benefit: The management of naturally ignited wildland fires to accomplish specific pre-stated resource management objectives in predefined geographic areas.