Originally Published: May 12, 2017 6 a.m.
In Yavapai County, almost any area surrounded by natural wildland vegetation faces some hazard due to fire. In mountainous regions, such as Prescott, fire hazard increases due to topography and increased vegetation density.
Local forests are predominantly populated by Pinyon pine and juniper trees, and Chaparral, and they are usually found on slopes between 3,000 to 7,000 feet. When fires do occur, they are typically moderate to high intensity, and have the potential to destroy many structures and much of the local vegetation.
The word “chaparral” means vegetation composed of broad-leaved evergreen shrubs, bush, and small trees, which together often form dense forest. Typical chaparral is generally deeply rooted, leathery-leaved evergreen shrubs. The most common local species being shrub oak, mountain mahogany, and manzanita. Yavapai’s interior chaparral is a fire-adapted ecosystem. Because its extensive root systems are able to pull moisture and nutrients from a large area, chaparral brush can regenerate quickly, with shrub densities regaining within five to seven years following a fire.
As chaparral density has increased over the years because of urban fire suppression techniques, the grasses that most readily carry fire have gradually disappeared, thus making frequent wildfires less likely. However, when wildfires fires do burn, they result in fast-moving high intensity fires that are most severe in spring and early summer.
Because of its more flammable nature and its potential for extreme fire behavior, chaparral must be carefully managed around homes. It is especially dangerous when growing down slope from a house and where flame lengths can reach as much as 47 feet high.
Creating wildfire survivable space (also referred to as defensible space) involves finding the right balance between reducing fuels and maintaining a diverse plant community that protects the soil from erosion and provides suitable habitat for wildlife species.
Three factors affect wildfire behavior: the fuel type; continuity and density; and weather and topography.
All plants burn under certain conditions; but, they burn at different intensities and rates of consumption. Grasses exists in two states – green and dry. When green, moisture content is high enough to prevent or decrease fire spread. Dry grass, on the other hand, promotes extreme fire spread rates. Brush fires spread slower than grass fires, but burn at a higher intensity.
Timber burns as surface fires and crown, or canopy fires. Surface fires consume fuels on the forest floor without burning many trees, although trees may burn individually. Crown fires occur when large areas of trees are totally consumed in the canopy. Crown fires and convective air currents create and lift burning materials or embers, and winds carry them horizontally for hundreds or even thousands of feet ahead of the actual fire. Embers can then fall onto horizontal surfaces, such as combustible roofs, decks and dry vegetation around structures.
Weather is also a major factor that affects fire behavior and can change dramatically in a short period of time. Winds determine fire direction and rate of spread, pushing flames into adjacent fuels, and facilitating rapid ignition. Humidity has an effect, and before combustion can occur, fuels must reach ignition temperature (approximately 450° F). Fires also produce large amounts of radiant energy which can preheat or even ignite structures. However, radiant energy decreases dramatically within a 50 to 100-foot distance of a structure.
Chaparral is a major component in Arizona’s wildlands fire fuel, both as pure brush and where it mixes with ponderosa pine and pinyon-juniper trees, and it is often maligned as “worthless shrub land.” Creating survivable space in natural chaparral will significantly reduce wildfire risk, while also enhancing the beauty of a home and conserving water in the landscape. Unmanaged vegetation adjacent to a structure provides continuous and abundant fuels, which can ignite flammable building surfaces. Creating defensible space and fuel breaks around a structure is specifically intended to reduce this threat.
Next time we will look at how homeowners can protect themselves from wildfire by developing their own defensible space.
Paul Scrivens is a Firewise Assessor and Committee Member in The Ranch at Prescott.