Types of Natural Fires: Surface Fires, Ground Fires, and Crown Fires

Surface fires

Surface fires burn the upper litter layer and small branches that lie on or near the ground. Surface fires usually move rapidly through an area, and do not consume all the organic layer. Moisture in the organic horizons often prevents ignition of the humus layer, and protects the soil and soil-inhabiting organisms from the heat. The heat pulse generated at the burning front of these fast-moving fires does not normally persist long enough to damage tissues underneath the thick bark of large trees. However, it will girdle the root collar of small trees and shrubs, and reduce small-diameter branches and other fine surface fuels (Nyland 1996).

Ground fires

Ground fires normally smolder or creep slowly through the litter and humus layers, consuming all or most of the organic cover, and exposing mineral soil or underlying rock (Davis et al. 1959; Kimmins 1987). These fires usually only occur during periods of protracted drought when the entire soil organic layer may dry sufficiently, but they may burn for weeks or months until precipitation and low temperatures extinguish the fire, or they run out of fuel.

Some of the effects of ground fires are deleterious. For example, they generally kill large and small trees because of the long and high temperature heat pulse generated. They release considerable amounts of nutrients from the burned fuels, destroy many small organisms and fungi that live in the humus and organic layers, consume seed stored in the litter, and kill roots in all but deep soil layers. They increase the chance of surface flow and erosion on slopes, and leave a baked and hardened seedbed that may prevent rapid revegetation. Increased surface runoff across the exposed surface may carry away ash and dissolved nutrients, making conditions even less favorable for plant growth. Foresters normally consider ground fires unsuited for site preparation, and restrict prescribed burning to periods when the humus remains too moist for complete combustion (Nyland 1996).

Crown fires

Crown fires occur when surface or ground fires ignite slash piles and dead or living lower branches of standing trees, tree crowns becomes engulfed in flames, and the fire spreads to nearby trees. Crown fires occur in forests during periods of drought and low relative humidity, particularly in areas with heavy accumulations of understory material called ladderfuels (e.g., fallen trees, logging slash, and combustible understory vegetation). Crown fires generate tremendous heat that rises in a strong convection column, drawing in brisk surface winds that fan the flames even more. Heated air blowing across the flames also warms and dries the fuels ahead of the fire, and releases volatile gases from vegetation ahead of the flaming front. Crown fires have environmental effects similar to those of ground fires, killing all trees and shrubs in their path and usually consuming the surface organic layers. Similar to ground fires, crown fires only stop by running out of fuel, or if precipitation cools the fire and wets the fuels sufficiently to inhibit burning. Overall, crown fires have little value for site preparation within forested ecosystems, and have not become common part of silviculture (Nyland 1996).