Defensible space (fire control)

A defensible space, in the context of fire control, is a natural and/or landscaped area around a structure that has been maintained and designed to reduce fire danger. The practice is sometimes called firescaping.[1] "Defensible space" is also used in the context of wildfires, especially in the wildland-urban interface (WUI).[2] This defensible space reduces the risk that fire will spread from one area to another, or to a structure, and provides firefighters access and a safer area from which to defend a threatened area. Firefighters sometimes do not attempt to protect structures without adequate defensible space, as it is less safe and less likely to succeed.

Graphic from the Government Accountability Office showing defensible space surrounding a home

Criteria
  • A first concept of defensible space for most fire agencies' primary goal of fuel reduction is a recommended or required defensible space around a structure to extend for at least 100 feet (30 m) in all directions.[3]
  • A second concept of defensible space is "fuel reduction." This means plants are selectively thinned and pruned to reduce the combustible fuel mass of the remaining plants. The goal is to break up the more continuous and dense uninterrupted layer of vegetation.
  • A third concept of defensible space is "fuel ladder" management. Like rungs on a ladder, vegetation can be present at varying heights from groundcover to trees. Ground fuel "rungs", such as dried grasses, can transmit fire to shrub rungs, which then transmit up tree branch rungs into the tree canopy. A burning tree produces embers that can blow to new areas, spreading and making it more difficult to control a wildland fire. One guideline is for a typical separation of three times the height of the lower fuel to the next fuel ladder. For example, a 2-foot-high (0.61 m) shrub under a tree would need a spacing of 6 feet (1.8 m) to the lowest limbs of the tree. Since wildfires burn faster uphill than on flat land, fuel ladder spacing may need to be greater for slopes.[4]

Landscape use

The term defensible space in landscape ("firescape") use refers to the 100 feet (30 m) zone surrounding a structure. Often the location is in the wildland–urban interface. This area need not be devoid of vegetation by using naturally fire resistive plants that are spaced, pruned and trimmed, and irrigated, to minimize the fuel mass available to ignite and also to hamper the spread of a fire.

  1. The first 30 feet (9.1 m) is the "Defensible Space Zone," of a defensible space around a structure. It is where vegetation is kept to a minimum combustible mass. A guideline used in this zone can be "low, lean and green." Trees should be kept to a minimum of ten feet from other trees to reduce risk of fire spread between trees. Wood piles should be kept in zone 2. No branches should be touching or hanging over the roof of the house or within 10 feet of the structure to help keep the structure safe. Any dead vegetation or plants from zone 1 should be removed, and vegetation near windows should be pruned or removed.[5]
  2. The second distance of 30 to 100 feet (9.1 to 30.5 m), is the "Reduced Fuel Zone" of a defensible space around a structure. In this area of the defensible space, fuels/vegetation are separated vertically and horizontally depending on the vegetation type. This is done by: thinning, pruning, and removal of selected vegetation; and removing lower limbs from trees closer to lower vegetation and the lateral separation of tree canopies.[6] Grass height should not exceed 4 inches. Trees should be 10 feet away from each other on a flat to mild slope but should be double that on a mild to moderate slope. Shrubs should be as far away as twice its height for flat to mid-slope but 4 times its height for mild to moderate slope. Leaves, twigs, needles, clones, bark, and small branches should be removed but can be left up to a depth of 3 inches.[5] Vertical space from trees to ground should be 6 feet while the vertical distance from a tree to a shrub should be the height of the shrub times three.

An important component is ongoing maintenance of the fire-resistant landscaping for reduced fuel loads and fire fighting access. Fire-resistive plants that are not maintained can desiccate, die, or amass deadwood debris, and become fire assistive. Irrigation systems and pruning can help maintain a plant's fire resistance. Maintaining access roads and driveways clear of side and low-hanging vegetation can allow large fire equipment to reach properties and structures. Some agencies recommend clearing combustible vegetation at minimum horizontal 10 ft from roads and driveways a vertical of 13 ft 6 inches above them. Considering the plant material involved is important to not create unintended consequences to habitat integrity and unnecessary aesthetic issues. Street signs, and homes clearly identified with the numerical address, assist access also.[7][8]

Unintended consequences

The unintended negative consequences of erosion and native habitat loss can result from some unskillful defensible space applications. The disturbance of the soil surface, such as garden soil cultivation in and firebreaks beyond native landscape zones areas, destroys the native plant cover and exposes open soil, accelerating invasive species of plants ("invasive exotics") spreading and replacing native habitats.[9]

In suburban and wildland–urban interface areas, the vegetation clearance and brush removal ordinances of municipalities for defensible space can result in mistaken excessive clearcutting of native and non-invasive introduced shrubs and perennials that exposes the soil to more light and less competition for invasive plant species, and also to erosion and landslides. Negative aesthetic consequences to natural and landscaped areas can be minimized with integrated and balanced defensible space practices.[10][11]

See also

References
  1. "Archived copy" (PDF). Archived from the original (PDF) on 2011-06-26. Retrieved 2010-04-26.CS1 maint: archived copy as title (link) SBCFD-Firescaping. Retrieved 4/26/2010
  2. http://www.fire.ca.gov/communications/communications_firesafety_100feet.php CAL-FIRE intro Retrieved 4/26/2010
  3. http://www.sbcfire.com/fp/hrp/defensible_space_requlation.pdf Archived 2011-07-16 at the Wayback Machine SBCFD-regulation. Retrieved 4/26/2010
  4. "Archived copy". Archived from the original on 2010-04-19. Retrieved 2010-04-26.CS1 maint: archived copy as title (link) SBCFD-intro. Retrieved 4/26/2010
  5. "Defensible Space – Ready for Wildfire". www.readyforwildfire.org. Retrieved 2020-10-18.
  6. "Archived copy" (PDF). Archived from the original (PDF) on 2011-07-16. Retrieved 2010-04-26.CS1 maint: archived copy as title (link) CAL-FIRE-guidelines. Retrieved 4/26/2010
  7. "SBCFD Retrieved 4/26/2010" (PDF). Archived from the original (PDF) on 2011-06-26. Retrieved 2010-04-26.
  8. "Archived copy" (PDF). Archived from the original (PDF) on 2011-06-26. Retrieved 2010-04-26.CS1 maint: archived copy as title (link) SBCFD-checklist. Retrieved 4/26/2010
  9. Colautti, Robert I.; MacIsaac, Hugh J. (2004), "A neutral terminology to define 'invasive' species" (PDF), Diversity and Distributions, 10 (2): 135–141, doi:10.1111/j.1366-9516.2004.00061.x, retrieved 2007-07-11
  10. (September 21, 2006). National Invasive Species Information Center - What are Invasive Species?. United States Department of Agriculture: National Agriculture Library. Retrieved on September 1, 2007.
  11. USA (1999). Executive Order 13112 of February 3, 1999: Invasive Species. Federal Register 64(25), 6183-6186.
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