Trees in the Wildlife Urban Interface
California's wildfire seasons are burning earlier, longer and hotter, affecting more and more people, especially here in the West. Years of fire suppression and chronic drought have combined to make the situation so dire. The most obvious manner in which global warming magnifies the wildfire risk—one supported by a growing body of peer-reviewed literature—is the higher temperatures we are experiencing. Warmer air draws moisture from plants, trees, and soil, increasing what’s known as fuel aridity. This provides the dry fuel and conditions that feed wildfires. Other climatic factors can also contribute, including decreased rainfall and reduced or earlier melting of the mountain snowpack.
|Human factors also increase the danger, including increased development along wilderness boundaries and fire suppression efforts that can build up fuel, making fires deadlier when they do ignite. In the last 50 years, there has been a significant expansion of development near wildland areas in the U.S., which often result in heightened wildfire risk. Much of this development comes from new homes. California has more homes in the wildland-urban interface (WUI) than any other State—3.8 million (Radeloff and others 2005).|
|A house threatened by a forest fire in central Oregon. Photo credit: US Forest Service
The natural beauty of the wildlands is often the motivation that draws individuals farther into natural areas, but it is this same beauty that clouds our perception of the risks. One of the threats of living in or near a forest is fire. Homes are potentially at greater risk when they are isolated and hard to reach, and the first responders are threatened along with the homeowners. The cost to taxpayers is huge. A recent study showed that more than 50% of the Forest Service’s budget is spent fighting wildfires, and that is in addition to local, state and other federal spending on wildfires.
What can be done to help mitigate the risk of wildfires in the WUI?
First, fire safety begins with the house construction, and no amount of altering the vegetation will prevent the kind of firestorm that Santa Rosa and other parts of Northern California experienced last fall. But there are courses of action for homeowners to take that will help prepare their homes to withstand ember attacks and minimize the likelihood of flames touching the home or any attachments. Creating defensible space will improve your home’s chance of surviving a wildfire. It’s the safeguard you create between a building on your property and the grass, trees, shrubs, or any wildland area that surrounds it. This space is needed to slow or stop the spread of wildfire and it helps protect your home from catching fire - either from direct flame contact or radiant heat. Defensible space is also important for the protection of the firefighters defending your home.
|The 3 R's of Defensible Space:
A fuel ladder is a firefighting term for live or dead vegetation that allows a fire to climb up from the landscape or forest floor into the tree canopy. Common fuel ladders include tall grasses, shrubs, and tree branches, both living and dead. It is critical to remove these ladder fuels by paying particular attention to flammable ground fuels under bushes and trees. Irrigated, well-maintained lawn and flower beds, as well as low-growing native ground covers, can be kept under a tree’s drip-line.
Ladder fuels act to spread the flames upward.
Image courtesy of ButteFireSafe.org and FireSafeHelp.com
In addition, you can create spaces between trees and shrubs with thin strands of grasses or groundcovers to break up the continuity of fuels. Decisions vary greatly depending on the steepness of the slope, the frequency of fog drip, and whether the slope is northeast or southwest facing. Maintaining this defensible space is an ongoing, year-round activity that will vary significantly from year to year depending on the weather. Plants grow, and flammable vegetation needs to be routinely removed and composted or disposed of properly. Be the steward of your own property.
Though some may see trees as a threat, when carefully chosen and placed, trees really are a benefit in the landscape and, when kept clean and healthy, are relatively fire safe. Native and fruit trees are the best choices for the garden; they enhance our living environment by sheltering wildlife, providing shade and fruit, and adding to the beauty of our homes. When we align our needs with the reality of wildfire, we can provide safe neighborhoods for our families and defensible spaces for the brave firefighters who defend our homes.
Any tree provides a large potential source of fuel for a wildfire, so select trees in your firewise landscape with the following parameters in mind. Nearer the home trees should be small and placed so that their crowns will be at least 10 to 15 feet away from structures. Leave plenty of room between trees to allow for growth; keep 10 feet between mature tree crowns. Prune tree limbs up to a height of 8 to 15 feet above the ground, and do not allow shrubs to grow up under the trees, creating ladder fuels.
Fire-prone vegetation contributes to rapid burning, high heat output, and ember creation. Eucalyptus, Monterey pines and Juniper should be eliminated completely because the impregnated oils and resins in these species burn explosively. The result of using these plants or trees in a landscape make effective firefighting nearly impossible. Conifers differ greatly in their susceptibility to fire. Ponderosa and Jeffrey pines are highly fire-resistant, along with the extremely fire-resistant giant sequoias. Understory fires are common among coast redwood forests, but the big trees themselves are seldom destroyed. With fire exclusion, the forests become more densely vegetated with more fire-susceptible species, such as white fir. Cypress and many closed-cone pines are fire-dependent, meaning that they need fire to reproduce. Oaks also differ significantly in their susceptibility to fire, from the very fire-resistant Coast live oak to the fire-sensitive Canyon live oak. It is worth your time to investigate the fire susceptibility of a tree species before planting.
Other trees found in Sonoma County that are fire hazards include Tree of Heaven (Ailanthus altissima), the Acacia species, Cupressus species, and bamboo.
In reality, no vegetation is totally fire-resistant under extreme fire conditions. The biologist Caitlin Cornwal, of the Sonoma Ecology Center, is quoted in the Press Democrat (June 23, 2018), saying, "There's this window right now where people are making really important decisions that could set their piece of ground onto a good path or a not good path for the next many years. We're really concerned about a lot of people cutting down a lot of vegetation because it will make them feel safe.... The guidance on defensible space from Cal Fire is actually quite sophisticated, and yet people often interpret it as a very simplistic: ' The more I cut, the safer I am.' There's a balance that needs to be struck."
In the case of potentially widespread fire damage - it is imperative to have an escape plan for your family and pets.
-- Kathleen Fitzgerald-Orr, Master Gardener class of 2011
Sources for this article
Carle, David. Introduction to Fire in California: California Natural History Guides. Berkeley: University of California, 2008.