Surface Fire, Crown Fire and Spotting: How Wildfire Moves

What are the main types of wildfire?

Fire scientists sort fire by where it burns. The extension guide Reducing Fire Risk on Your Forest Property, from Oregon State, Idaho and Washington State universities, describes three types: ground fire, surface fire and crown fire. A single wildfire can include all three, and the guide notes that the mix of types gives clues to the overall severity of the fire.

The same guide explains that fire behavior, meaning rate of spread and intensity, depends on three factors that make up the fire behavior triangle: the amount and arrangement of fuel, topography and weather. A change in any one alters the fire's behavior and its type. Fuel is the only one a landowner can change, which is why the rest of this Montana Wildfire Guide is about fuel.

What is a ground fire?

A ground fire burns in the duff, the layer of decomposed needles and organic material on the forest floor, and produces little or no visible flame. The PNW guide says ground fires can burn out stumps and follow decaying roots and buried logs, can smolder for days or weeks with little smoke, and can emerge later to ignite surface fuels.

For landowners the lesson is in slash piles. The guide warns that piles containing too much soil can let fire smolder for weeks or months as a hold-over fire, then re-emerge. That is one reason burned piles need to be checked well after the flames are gone; our guide to burning slash piles covers it.

What is a surface fire?

A surface fire burns needles, grass, herbaceous plants, shrubs, small trees and saplings with a visible flaming front. The PNW guide notes that it can ignite large woody debris and duff that keep burning long after the flames pass, and that its severity ranges from low to high: a high-severity surface fire kills most trees. It also says surface fires with flame lengths under four feet can be controlled by ground crews.

Surface fire is the fire most western Montana dry forests evolved with. Montana DNRC's west-central landowner guide says frequent surface fires historically kept ponderosa pine and western larch forests open and favored those thick-barked trees. A surface fire through a well-thinned stand of big pines is often survivable for the trees; the same fire reaching a woodpile against a house is not survivable for the house.

What is torching, and what is a crown fire?

Torching is a surface fire moving up into the tree crowns. The PNW guide says crown fires are either passive or active. Passive crown fire involves the torching of individual trees or groups of trees, and torching is the precursor to an active crown fire. Crown fire becomes active when enough heat is released to preheat and burn fuel above the surface and fire spreads from one crown to the next through the canopy.

The guide names four factors in the transition from surface to crown fire: foliage moisture, surface flame length, the height to the base of tree crowns, and the density of the crowns. Ladder fuels raise flame length and lower the crown base; crowded trees raise crown density. Our guides to ladder fuels and thinning conifers are about breaking exactly those links.

Why is crown fire so dangerous for homes?

Because of what it produces, more than where it burns. The Colorado State Forest Service notes that crown fires are intense, often move rapidly, can burn large areas and produce embers that can travel great distances and start spot fires well ahead of the main fire. A crown fire is an ember factory.

The flames themselves have a shorter reach than people expect. Forest Service fire scientist Jack Cohen reported crown fire experiments in which wooden walls 10 meters from the forest edge ignited only when flames touched them, and walls at 30 meters were not scorched. A crown fire a few hundred feet away usually threatens a house through embers, and through whatever those embers light near the house. See how homes catch fire in a wildfire.

What is spotting?

Spotting is fire jumping ahead of itself. The PNW guide describes it as firebrands, glowing embers, being lofted up and ahead of the main fire front, igniting multiple spot fires that then feed back into the main fire to create very extreme and dangerous conditions. CSFS says embers can be carried more than a mile ahead of the flaming front, and Cohen notes firebrands capable of ignition can originate from a fire several kilometers away.

The PNW guide also notes that burning snags can torch and loft embers ahead of the main fire. Spotting is why a home can be threatened before the fire is visible from the yard, and why the first five feet around a house matter more than the forest a mile away. Our guide to Zone 0 covers that strip, and dead standing trees near homes covers snags.

What is a fire whirl, and what is extreme fire behavior?

The PNW guide describes a fire whirl as an upward-spinning column of air carrying flames, smoke and embers aloft. Whirls often form in heavy fuels on the downwind side of ridges and in extreme conditions can twist off entire trees. The National Weather Service uses the term extreme fire behavior for a wildfire likely to burn out of control, and lists criteria including a high rate of spread, prolific crowning or spotting, fire whirls and a strong convection column.

When the NWS issues a Red Flag Warning, it means critical fire weather is occurring or expected shortly; a Fire Weather Watch means those conditions are possible but not imminent. A fire burning under those conditions does not behave like the textbook examples, which is why preparation has to be done before the season, not during a warning. Our Ready, Set, Go evacuation guide covers what to do when a warning is out.

Which fuels drive each type of fire?

Fuel size decides how quickly fuel ignites and how long it burns. The PNW guide's appendix classes dead fuels by diameter and drying time. One-hour fuels under a quarter inch, such as needles, twigs, grass and small shrubs, ignite easily and support initial spread. Ten-hour fuels up to an inch carry fire and heat larger fuels. Hundred-hour and thousand-hour fuels, up to eight inches, sustain fire and add to its duration and severity. Logs over eight inches do not do much for spread but can burn for hours or days, heating soil and killing roots.

The guide adds that logging slash is made up of the smaller sizes and represents a high hazard, and that shrubs with volatile oils, such as bitterbrush, burn with higher intensity. Grass and needles drive surface fire speed; ladder fuels and crowded crowns drive crown fire; snags and heavy slash drive embers and long burning. Each needs a different treatment, which is why a good fuel reduction plan is not just one kind of cut.

What does this mean for my property?

Match the work to the type of fire you are trying to prevent. Close to the house, the goal is to give embers nothing to light and keep surface fire away from the walls. Within about 30 feet, the goal is to keep any surface fire short and cool. Beyond that, the goal is to keep fire on the ground: no ladders, gaps between crowns, and dead material reduced. Our guide to defensible space zones in western Montana puts that into a worklist.

HJ Property Care's fire mitigation service works on all three: needle and debris cleanup near structures, ladder fuel removal and limbing, and selective thinning with the slash chipped or hauled away, across Missoula and Ravalli counties. Where dead trees are involved, our hazard tree removal service takes them down safely.

Call HJ Property Care & Tree Service at (406) 215-7319 or request a free estimate and we will walk the property with you.

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