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Wildland Fire Spread and Suppression

This article is part of the Wildland Fire Learning In Depth series. It is designed for students who want to learn more about fire. Find the complete series on the Fire subject site.
Left: Glowing coals beneath ground surface; right: visitors on a trail look towards smoke rising from the ground in a steep gully.
Fire in coal seam on the Coal Vein Trail, Theodore Roosevelt National Park.

NPS/A MCCANN

Conditions dictate how a wildland fires moves and spreads based on fuels, topography, and weather. There are three ways a wildfire spreads:

Ground fire - Fire that consumes the organic material beneath the surface litter ground, such as a peat or coal seam fire. Tree roots burning could also be classified as ground fire. In peat or coal seams, ground fires may burn for decades and are notoriously difficult to put out as they can move underground.

Two images juxtapositioned: Left - Small flames creep in a Ponderosa pine forest consuming dead needles and pine cones. Right - Large flames consume grasses in a prairie setting.
A surface fire can be very low to the ground consuming vegetative material, or can have larger flames that quickly consume vegetation on the surface.

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Surface fire - Fire that burns loose debris on the surface, which includes dead branches, leaves, and low vegetation.

Two images juxtapositioned: Left - isolated tree torching; right - aerial view of fire moving through crowns of trees in a forest.
Left - example of passive crown fire; right - active crown fire.

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Crown fire - A fire that advances from top to top of trees or shrubs more or less independent of a surface fire. Crown fires are sometimes referred to as running or dependent to show that it is either dependent (connected) or independent (not connected) to the surface fire. Crown fires can be passive, active (dependent), or independent. Passive crown fires torch trees but do not pass from tree to tree. Active crown fires are where the surface and crown fire are linked and spreading. Independent crown fires are capable of carrying fire through the trees with no link to a surface fire.
How ground fires, surface fires, and crown fires interact
A ground fire may become a surface fire or crown fire if there is a point where the coal seam or peat burns upward toward the surface and comes in contact with surface vegetation, such as grasses, brush, or trees. Likewise, a surface fire may become a crown fire through vegetation that connects the fuels on the ground with fuels in the canopy of a forest – these are called ‘ladder fuels.’ A crown fire can also burn down to the surface through direct flame contact or embers being spread by wind.All three fire spreads can occur at the same time on the same fire, though ground fires are less common and restricted to specific geographic locations.
Two images juxtapositioned: Left - a firefighter using a handtool with a rubber flapper to quell small flames; right - aircraft dropping red retardant on desert vegetation.
Suppression may look very different depending on the fire - it may involve a firefighter with a handtool quelling small flames or an aircraft dropping retardant or water.

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How to suppress ground fires, surface fires, and crown fires

Ground fires are difficult to suppress given how difficult it can be to access an area burning. Water is not always helpful but can sometimes be used if it’s directly near the surface. Foam or other wetting agents can help penetrate the fuels and remove oxygen supply. Sprinklers may be good choice to have a steady stream of water flowing into the ground. Using heavy equipment to dig up and remove the fuel source can help with suppression as it removes the “fuel” leg of the fire triangle. Smothering would remove oxygen for the fire, if the access is available to use that technique.

Surface fires can be suppressed by removing fuel on the ground, either digging a fireline to mineral soil, burning ground vegetation in advance, or smothering with a handtool such as a flapper. Crown fires can also be very difficult to contain or control. Frequently, a crown fire runs out of fuel to carry the fire – e.g., there is a change in forest type. In other instances, air resources (airplanes or helicopters that use water or retardant) are used to cool the fire.

Firefighters make constant decisions based on factors such as location, weather, fuels, topography, fire activity, and resources available. Every decision is weighed against risk to people and communities and to firefighting personnel.

Not every wildfire is immediately and aggressively suppressed as some ecosystems need fire to continue to be healthy for the plants and animals that live there. If the fire is burning in an area where people, communities, or other values are at risk, firefighters take immediate and aggressive action to suppress the wildfire using every resource available.

If a fire is burning in an area away from people or infrastructure, firefighters may use natural features such as rocks, or waterways to keep the fire contained to a specific area. This approach is often the most cost effective and reduces risk to firefighters at the same time.

If it’s the hottest and driest time of year, with the potential for wind or weather that could create conditions for crown fires, then firefighters may use aggressive actions to keep the fire as small as possible. One of the tactics often used is fighting fire with fire or lighting vegetation under more favorable conditions to remove fuel in advance of a wildfire. This starves the fire of fuel, removing one leg of the fire triangle.

Part of a series of articles titled Wildland Fire - Learning In Depth.

Last updated: July 10, 2025