Wildfire! A Primer on Behavior
By Louisa Evers, Golden Eagle Audubon Board Member
Recent wildfires in the Boise Foothills and southwest Idaho forests and grasslands have made people curious about wildfire behavior. This blog will introduce the fire triangle and discuss fuel abundance and future blogs will cover other fire science subjects. I have a professional background in fire ecology and many years experience assessing fire risk and predicting fire behavior.
The Fire Triangle
The fire triangle provides the baseline for understanding fire behavior. It’s pretty simple: heat, fuel, and oxygen.
Heat can come in many forms including lightning, a campfire that was not completely extinguished, fireworks, or sparks from any number of sources. Air temperature plays an important role. Wildfires start more readily when the temperature is 90 degrees than when it’s 32 degrees. Therefore, a small amount of external heat can start a fire on a hot day. More external heat is required to start a fire on a cold day.
The next leg is the fuel. In the case of wildfires, this means vegetation – grass, brush, and trees. How readily fuels will burn depends on their abundance, usually measured as tons per acre, how they are arranged both horizontally and vertically, and how much moisture they contain.
The last leg of the fire triangle is oxygen. Our atmosphere is approximately 20% oxygen, plenty to support a fire. The way the fire triangle works is if you remove one side of the triangle, fire can’t burn. We can’t do anything about the amount of oxygen.
Spraying or dropping water on a fire reduces the amount of heat but is usually of limited effect. Fire retardant alters the ignition temperature of the vegetative fuel that receives enough of the retardant, so also attempts to adjust heat. However, the main way wildfires are fought is to reduce fuel abundance by construction of a fireline down to mineral soil.
Fuel Facts
Abundance of fuel is driven by many factors depending on the type of fuel. Grass abundance is driven by winter and spring snow and rain that support growth. Brush abundance with respect to wildfires depends on size of the plants and the species. Some species of brush contain volatile oils and chemicals and burn more readily than species that don’t contain these things. Examples of species that burn more readily include the many species of sagebrush and snowbrush, Ceanothus velutinus. Examples of species that do not burn readily include snowberry and willows. Large old shrubs are also more likely to contain easily-flammable dead parts, especially in the middle of the plant.
Tree abundance is driven by species and composition. As with brush, tree species contain different amounts of volatile oils and chemicals. Conifers typically contain volatile chemicals while hardwoods typically do not. The types of conifer needles, how many trees per acre, and tree size are important measures of fuel abundance as well as what is present underneath the trees. Long-needle conifers, especially ponderosa pine, create what firefighters call a fluffy fuelbed, which facilitates fire spread on the ground surface, while short-needle conifers, such as Douglas-fir create a very flat fuelbed, which does not support rapid fire spread. In forests, both horizontal and vertical continuity are important. Lots of small trees tightly packed, what firefighters call doghair stands, are more likely to support rapid fire spread with very long flame lengths. In contrast, a stand of widely spaced large trees may have horizontal fuel continuity but not vertical continuity, limiting fire spread and flame length.
How moist a given fuel type is also impacts fire behavior. Moisture levels usually depend on the time of year and the weather in preceding months. Grasses usually dry out the earliest of the major fuel types. Whether the dominant grass is an invasive annual grass or a native bunchgrass is also important. The invasive annual grasses usually form more continuous fuelbeds and dry out by late spring or early summer. Native bunchgrasses usually form less continuous fuelbeds and dry out by mid-to late summer. Fuel moisture in brush and trees typically reach their lowest levels late in the summer.
The weather in the preceding months also matters. Years with low snowpack and hot dry spring weather mean fuels have less moisture and burn earlier. Years with this drought stress are most likely to experience fire. Ironically, the volatile chemicals in brush and trees also are adaptations for coping with drought. As drought stress increases, so do the concentrations of these volatile compounds. Drought stress can also result in tree and brush mortality and increase the abundance of available fuel, especially if the drought lasts many months or years.
In contrast, years with average or above-average snowpack and cool, wet springs mean fuels dry out later. Especially wet years may mean that wildfires are few, far between, and small as not enough fuel is available to support the development of large wildfires.
Editor’s Note: Watch for Louisa’s next blog to learn more about Fire Science