How to Create Defensible Space?

You’ve decided you need to create a defensible space around your home. How do you do it? Every site is unique and the decisions about how to handle the vegetation should be made on that basis. However, there are a number of generally applicable guidelines that will help.

If your property is in Douglas County, you probably have one or more of these plant species to deal with: Gambel oak, Ponderosa pine, Douglas fir, mountain mahogany, buffalo grass, Rocky Mountain juniper, and other, less common species, such as Englemann and blue spruce and lodgepole pine. The two species that most require attention are Ponderosa pine and Gambel (also known locally as scrub) oak.

Historically, fire has played a significant role in the natural history for both these species. Naturally occurring fires, before European settlement, kept smaller trees and the amount of brushy species, such as scrub oak, limited. This created a more open and park-like setting than what is seen today. Since firefighters began suppressing fires, the forest has become overgrown and much denser than it would under natural conditions.

There are basically two types of fires: ground fires and crown fires. A ground fire is typically a slower moving fire that, as its name implies, stays on the ground and consumes grass and brush. Fire intensity is not very high and only fine fuels, such as grass, are completely removed. Grasses are annuals and will usually grow back after a ground fire without any problem. In fact, grass usually returns the next growing season in better condition, assuming there is sufficient precipitation, due to nutrients released to the soil from burning. Brush species will also regenerate, although it will take longer and will not be as dense as it was before the fire. A ground fire will not affect larger Ponderosas while smaller pines will be consumed. This means less competition for water, sunlight and nutrients, allowing the larger pines to get even larger. This is how the open park-like stands were created.

A crown fire is a ground fire that found a way to get into the tops of trees, even the large ones, and moves from crown to crown. There is almost always more wind at the tops of trees than there is at ground level; this means the fire can move faster. When a tree’s crown is burned the tree dies, regardless of its age or size. Crown fires usually cannot be stopped by suppression efforts because of the fire’s speed and ability to spot ahead. Spotting is when burning embers are picked up by the wind and blown ahead to new unburned fuels. If the wind is fast enough and humidity is low enough a fire can travel faster than a person can run, especially uphill. The Hayman Fire was a perfect example of this. When a crown fire is finished, the fire has consumed everything and you have to start from scratch. If the fire was hot enough, even the soil is destroyed and nothing will grow for quite some time. Burned soil becomes hydrophobic, meaning the top layer of the soil cannot absorb water until it is conditioned and amended, an expensive and labor intensive process.

What do we as homeowners do to protect our homes and property from wildland fire? There are two primary methods to keep a ground fire from getting into the crowns or to keep a crown fire manageable.

The first is the removal of ladder fuels and the second is fuel spacing. Ladder fuels are the primary way ground fires get to the crowns. A typical ladder fuel is a scrub oak growing under a pine. A typical scenario is for a large pine’s limbs to grow immediately over, or even inside, an oak. A ground fire can slowly burn along in the grass, reach the oak and then go up the pine’s branches and into the crown. Remember: fire moves faster when it is going up because heat rises. This is because fire dries out and pre-heats fuels uphill and they ignite more rapidly. When the fire goes up the tree it is exposed to the higher wind speed in the canopy and is pushed downwind at a much faster speed. Limbs reaching down to the ground are also ladder fuels, so are small trees and stacks of firewood.

The other mitigation technique is to provide crown spacing between trees. Crown spacing is the distance between the limbs of adjacent trees. Crown spacing is measured in feet and can be a negative number if the crowns are overlapping! Below are two examples of crown spacing. These are overhead views, like being in a hot air balloon floating over the forest:

This is an example of positive spacing.	This is an example of negative spacing (interlocking crowns).

Crown spacing does two things: it reduces the intensity of a fire, because there is less fuel to burn, and it slows the rate of spread of a fire that is already in the crowns when it enters the property. Providing adequate crown spacing might also take a fire out of the crowns and bring it back to the ground. Both of these factors help in the suppression of the fire. Any attempt to suppress a crown fire is difficult and dangerous; therefore anything that reduces risks or makes suppression efforts easier is worth doing.

Foresters and firefighters describe the amount of flammable material on a property in terms of “fuel loading” and it is quantified as tons per acre. This component can be compared to your fireplace. If you put a lot of wood in you will have a hotter fire and one that will burn longer. If a property has a high fuel load (lots of logs in the fireplace) it will burn hotter and longer and be more destructive. Fuel loading, however, is only one factor to be considered.

The composition of the fuels is also very important. The finer a fuel is, such as grass or oak leaves, the more easily it can be ignited. Which is easier to start burning, a sheet of paper or a two-by-four? If your property has a lot of fine fuels as part of the fuel load, any ignition source is more likely to succeed in starting a fire and the quicker it can travel to and ignite larger fuels such as trees.

A very important factor in defensible space and wildfire mitigation planning is fuel continuity. This relates to ladder fuels and to “rate of spread." Rate of spread is the term that describes how fast a fire moves horizontally. Fuel continuity is one of the most important factors the Wildfire Mitigation Specialists look at when conducting a preliminary or final inspection of your property. Continuity is either vertical or horizontal: vertical continuity is usually just another way to describe ladder fuels. Horizontal continuity deals with the presence or absence of fuel breaks that affect the fire’s ability to spread across the ground. Disruptions to horizontal continuity act like speed bumps on a road, slowing a fire’s spread, thus offering an opportunity to suppress it and protect your home.

Building materials are another important component to protecting your home. Obviously, this is most easily accomplished when designing your home, but some post-construction changes can also be made, such as installing a more fire resistant roof when it is time to replace the old one. Fire resistant materials may compensate for the limitations you may have in creating D-Space for your home. Even when you do your best at modifying the fuels surrounding your home, your property is still subject to the surrounding forest. Possibly your neighbors haven’t gone to the same level of effort you have, or possibly your property is simply too large to economically manage. In these cases your building materials are a critical part of your strategy.

Public Education

The Douglas County Building Division offers assistance to the residents of Douglas County in a number of ways. The Wildfire Mitigation staff is available to confer with homeowners or homebuilders about existing homes or proposed residential structures. They are professional foresters with a great deal of experience in forest health, fire suppression and mitigation. If you have questions about the condition of the vegetation on your property, feel free to call to arrange a time for an appointment.

The Wildfire Mitigation staff is also available to meet with Home Owner Associations (HOA) or Metro Districts to assist in neighborhood plans for forest health and wildfire mitigation on a neighborhood-wide basis. The Wildfire Mitigation staff works closely with the Colorado State Forest Service and local fire districts and will coordinate with those groups to offer coordinated presentations to the HOA or Metro boards of directors or to the neighborhood residents.

If you have any questions about this program, please call the Douglas County Wildfire Mitigation Specialists at 303.660.7497.