Subsurface or ground fires burn on or beneath the forest floor. Fuels consist of duff (topsoil, partially decayed leaves, peat and tree needles) and decayed woody material at a depth that may vary from 8-10 cm (3-4") in dry areas to 1 m (3') in wet areas. Because these types of combustible material (ground fuels) are compact, have limited oxygen supply and are protected from wind, a persistent slow burning fire is produced and both fire suppression and mop-up become difficult.
The anatomical parts of a forest fire are: Pocket(s) or Bay(s) — A marked indentation in the fire perimeter, usually located between two fingers. Finger(s) — An elongated burned area(s) projecting from the main body of the fire resulting in an irregular fire perimeter. Flanks — Those portions of the fire perimeter that are between the head and the back of the fire which are roughly parallel to the main direction of spread. Head — That portion of the fire perimeter having the greatest rate of spread and frontal fire intensity which is generally on the downwind and/or upslope part of the fire. Back, Base or Heel — That portion of the fire perimeter opposite the head; the slowest spreading part of the fire. Island(s) — Area(s) of unburned fuels located within the fire perimeter. Point of Origin — The location within the fire perimeter where ignition first occurred. Large fires may exhibit multiple fire types, in different locations, at any given time.
Initial Attack: The action taken to halt the spread or potential spread of a wildfire by the first firefighting force to arrive at the wildfire. Expanded Attack: Expanded Attack, or Sustained Action, refers to those fires which require more time, and more resources to extinguish. They are generally larger fires, which may dig-in and require more ground work to put out. Modified Response: A fire that has been deemed beneficial to land management objectives, and suitable for fire responders to modify the goal of immediately extinguishing the fire. Trigger points are flagged and a fire may be allowed to burn to a certain point before any action would be taken. Sometimes a series of trigger points are set and different action taken at each stage. The area is carefully monitored and resources allocated as necessary, and as directed by a fire management plan.
Surface fires burn on and above the forest floor, but not in the crowns of trees. Fuels include ground debris, grasses, vegetation, windfalls, brush, slash, young trees and lower branches of standing timber. Forest growth up to 7.5 m (25') may be affected. The spread rate of fires depends on the density of these fuels, continuity and size of trees and underbrush, the slope of the terrain and the weather. With surface fires, fire suppression usually includes burning off fuels in front of the fire. Mop-up may vary from easy to difficult depending on the terrain and fuel types. Crown Fires burn in the upper foliage and the crowns of standing timber in conjunction with surface fires. In some cases, a running crown fire may develop. This is a fire that travels through the tops of the trees ahead of the surface fire, and generally travels as fast as the wind pushing it. Intermittent crown fires burn up towards the tops of trees as a result of intense heat from the burning material below. This “candling” effect occurs in light timber stands where the canopy is not continuous; and in open stands where the trunks of trees are covered with dead limbs, mosses or lichens that provide a ladder for the fire to climb. Candling will also occur in isolation where only the crown of one tree will burn. “Spotting,” where embers are thrown ahead of the fire can also cause intermittent crown fires.
Mop-Up: Mop-Up is the actions required to take a fire from control to having a secure perimeter or extinguishment. It begins after the fire has been brought under control, and before suppression work is reduced to patrol. Mop-up may involve cold-trailing (feeling with one’s hands for remaining hotspots) and extinguishing all smoldering materials with water or soil. On a large fire this could be a 30-100 foot strip around the fire, where the middle is allowed to burn out—often due to safety concerns. Patrol: Patrol involves watching for fire spotting outside the fire line while the fire is still active. After mop-up is complete, patrollers watch for any smoke or fire that may rekindle after mop-up is completely. A fire in patrol status is a fire that has been extinguished but is being monitored. Out: A fire is declared Out when it has been determined to be completely extinguished.
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