Why Defensible Space



What is defensible space and why do we need it?

Defensible space is a properly landscaped perimeter usually 100′ from a structure that reduces the possibly of a fire spreading to that structure. Firefighters will defend the structures that have defensible space first, as the ones that do not have it are less likely to be saved. In most areas, defensible space is also enforced on vacant lots. The reason for this is to reduce the chance of wildfires spreading from one property to another and to give firefighters more room with which to work. This landscaped area of defensible space is based on the reduction, proper spacing and pruning of combustible fuels. In the Sierras, these combustibles usually consist of pine and fir trees as well as manzanita, snowbrush, sagebrush, and bitterbrush. Not all woody trees and plants are combustible though. Most deciduous trees like aspen, for example, have a high resistance to burning and are not usually considered a threat if they are near your home. Pine needle build up on roofs and on the ground also pose a threat. In a wildfire, burning embers can sometimes travel over a mile through the air. If one of these lands in a pile of dry pine needles on your roof, it can spell disaster for you house. Fire can also travel through pine needle build up on the ground and create a ladder fuel effect, spreading the fire to low lying bushes and then eventually into mature trees. When a fire gets into a mature tree, it is then considered a canopy fire and becomes an aerial fight for the firefighters. This is when forest fires become disastrous.

angora fire

An aerial view of the Angora Fire aftermath, which destroyed more than 300 structures in South Lake Tahoe in June of 2007

Here in the Sierra Nevada mountain range, we have a vast amount of wildland urban interface (WUI). WUI is defined as the area where structures and other human development meet or intermingle with undeveloped wildland. 100 or more years of fire suppression in the western United States has created a dangerous situation in our forests. Before there was fire suppression in our forests, there were natural fire cycles as evidenced by fire scars on many mature trees throughout the Sierra Nevada.

Cat Face on Western Juniper

A fire scar on an old growth western juniper (Juniperus occidentalis) in Tahoe National Forest. These open fire scars are called a "cat face".

Old Growth Cedar Fire Scar

Evidence of fire on an old growth cedar tree (Calocedrus decurrens) in the Tahoe area












These fires were usually caused by lighting strikes and would act as natures way of reducing understory build up. By burning this understory build up, competition for water, sunlight and nutrients was reduced for mature trees. Since these low intensity fires occurred every 10 years or so, the density or basal area of the forests were kept in check. This meant that the fires would stay on the ground and burn off the pine needles, grass, saplings, and low lying brush. The taller, mature trees were resistant to these fires and usually got healthier as a result of the reduced competition.

Low Intensity Fire Aftermath

Aftermath of a typical low intensity fire in the Tahoe area. The low lying flammable brush is gone and new plants are thriving. The trees have lost some lower limbs, but are still in good overall health.

Since the days of fire suppression, these plants and organic material that would normally have been burned by these small brush fires were allowed to grow unchecked. The pine needle layer on the ground got thicker and thicker, the brush got more and more dense and the saplings grew to become mature trees. This became very detrimental to our forests. Higher forest density means more competition for precious water, sunlight and nutrients amongst the trees. Not only does this mean a wildfire can spread more rapidly and with greater intensity, but the trees become stressed in their natural environment, predisposing them to attacks from parasitic plants, fungus and insects. This is why we have not only seen catastrophic wildfires in California and the west coast, but massive outbreaks of parasitic insects like the fir engraving beetle.

With this volatile state of our forests and more wildland urban interface than ever before, the need for defensible space has not only become the smart thing to do, it has become a law (PRC 4291) in the state of California. Alpen Tree Experts has over 10 years of combined defensible space and landscaping experience. We understand the value of your trees and the aesthetic appearance of your property. Defensible space done by Alpen Tree Experts is not only fire safe, but we leave your property looking better than before we started. That’s our promise.

Talk to us about maintaining your property which has already been landscaped for defensible space. Just like any good landscape, even defensible space needs maintenance. It doesn’t take long for trees and brush to grow back and the pine needles to build up again. Having maintenance done once a year can not only save you money, it gives you more peace of mind. If you wait until your property is re-inspected, not only will you be looking at a bigger bill, your house can be in danger in the meantime. We can set up a yearly maintenance schedule for you and you can relax knowing that your property is in good hands.


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