A root fire is a distinct and dangerous type of wildfire that occurs beneath the ground surface, often unnoticed. Unlike typical surface fires that consume grasses and low brush, or crown fires that race through the tree canopy, a root fire burns in the soil layer. This hidden nature makes it a persistent and challenging threat to manage in fire-prone regions. It is technically classified as a ground fire, defined by the consumption of organic material below the surface litter.
Defining the Subsurface Combustion Mechanism
Root fires are sustained by smoldering combustion, which is fundamentally different from the flaming combustion seen in an open fire. Smoldering is a slow, low-temperature, flameless form of fire that occurs when oxygen availability is limited. Temperatures can range between 250 and 700 degrees Celsius, sufficient to sustain the reaction even with minimal air circulation.
The primary fuel source for this subsurface burn is the organic material in the soil, including dried tree roots, partially decomposed organic matter known as duff, and deep deposits of peat or humus. Ignition happens when heat from a surface fire or a lightning strike transfers downward through the soil. The fire then transitions underground as surface flames pass, consuming the subterranean organic load.
The fire spreads slowly, often only a few centimeters to a few meters per day, but it can persist for months or even years because the surrounding soil acts as insulation. The fire follows dry, continuous fuel sources, burning along tree roots and through air gaps or tunnels created by burrowing animals. Burned-out roots can form “chimneys” that allow enough air circulation to sustain the smoldering.
Unique Dangers and Hidden Spread
The hidden nature of root fires creates unique hazards for both the public and firefighting personnel. A significant danger is the potential for hidden movement and re-ignition. Since the fire travels invisibly underground, it can cross established fire lines and resurface far from the initial burn area, sometimes kilometers away. This behavior has earned them the nickname “zombie fires” because they can appear extinguished only to return later, sometimes reigniting major wildfires in subsequent seasons.
Another serious risk is the sudden collapse of trees, known as snags. As the fire consumes the root structure anchoring a tree, the base becomes unstable. This makes the tree susceptible to falling with minimal provocation, such as a gust of wind.
The subsurface burning also creates voids beneath the ground where organic material has been consumed and turned to ash. These voids are known as “ash pits,” often filled with hot ash and potentially two or three feet deep. Stepping onto the surface above one of these pits can cause the ground to collapse, leading to severe burn injuries.
Locating and Extinguishing Root Fires
Locating and suppressing a root fire requires specialized techniques that go beyond standard surface firefighting methods. Firefighters cannot rely on visible flame or heavy smoke, as the fire is often only indicated by minimal smoldering or a faint wisp of smoke rising from the ground.
Detection Methods
The most common detection tool is the use of thermal imaging cameras (TICs), which detect infrared radiation emitted from the ground. TICs allow crews to scan a burned area and identify specific heat pockets, or hot spots, indicating active smoldering underground. Traditional methods, such as “cold trailing,” where crews carefully feel the ground for unusual warmth, are also used to pinpoint hidden heat sources.
Suppression Challenges
Extinguishing a root fire is challenging because simply dousing the surface with water is often ineffective. The water tends to run off or evaporate before it can penetrate deep into the smoldering fuel bed. Suppression requires saturation, involving massive amounts of water, often mixed with a surfactant or wetting agent, to ensure deep penetration into the soil. Physical containment is also employed, where crews dig deep trenches down to the mineral soil, removing all organic fuel to create a barrier.