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Fuel is accumulated live and dead plant biomass. Four characteristics of biomass fuels significantly affect wildfire behavior and fire effects:

  • The moisture content of the fuels;
  • The size (diameter) of the fuels;
  • The distribution of the fuels (both horizontally and vertically); and
  • The total quantity of the fuels.

Moisture Content of Fuels

Moisture content may be the most important single property controlling flammability of a given fuel source. The moisture content of green foliage fluctuates widely, depending largely on weather. Dead biomass also contains water, but at much lower levels than green foliage. Fuel with a low moisture content of just 20%-30% can be ignited by a match, a chainsaw spark, or even lightning. Yet once a fire is started, even biomass with 100% moisture can burn, especially if the fire is driven by high winds.

Steep slopes and high winds reduce fuel moisture ahead of the fire and raise temperatures, making the fuel more flammable. High winds can also carry firebrands, spreading the fire across very wide firebreaks.

Diagram showing how slope affects fire behavior, burnig fire uphill fed by draft and preheating rolls downslope,causing feaster ignition and spread

Size of Fuels

The sizes of fuel particles are very important in determining fire behavior and effect. Fine and small fuels are needles, grasses, leaves, and small twigs. Small fuels have the greatest influence on the ignition and are crucial to the spread of fire. Removal, burning and decomposition are the only means for reducing fine and small fuels. 

Large fuels include twigs larger than pencil-diameter, branches, and logs. Larger fuels contribute primarily to fire intensity.

Surface fuel is composed of small shrubs, grasses, and plant debris lying on the surface of the ground. Surface fuel is what allows fire to spread continuously across landscapes. 

Ladder fuels are intermediate sized trees or shrubs that provide a means for a surface fire to climb into the tops of trees or tree crowns. This is called ladder fuel because it provides a “ladder” for fire to climb from the ground to the crown.

Crown fuels are made up of branches and foliage of the trees and large shrubs over 6 feet in height. Continuous crown fuel enables fire to spread through the crowns of trees.

Distribution of Fuels

Fuel distribution refers to its arrangement horizontally and vertically.

“Fuel ladders” are continuous fuels, especially of fine and small fuels, between the ground and the tree crowns. Ladders help spread fires into the canopy, turning a surface fire into a crown fire, by providing continuous fuels between the ground and the canopy.

Quantity of Fuels

Fuel loads include dead biomass such as needles or leaves, branches that have fallen, and older dead trees, and undergrowth like grasses, shrubs, and small trees. Fuel can be removed by a variety of means, thereby reducing fuel quantity.

What you can do to reduce fuels around your home?

  1. Clean leaf and needle fall that accumulates in foundation plantings, next to buildings and under decks. Take special care to clean out dead leaves from shrubs next to buildings. Better yet, replace those shrubs with fire resistant plants. Download the Guide to Firewise Plants (599k PDF).
  2. Clean the roof and gutters of leaves, needles and other debris. Clean accumulations of leaves from window sills.
  3. Look at the vertical arrangement of your vegetation. Is there continuous fuel (grass, leaves, branches) reaching from the ground to the crowns of the trees? Eliminate this ladder fuel by mowing tall grass, trimming shrubs, and pruning the lower branches off trees up six to ten feet.
  4. Look at the trees. If the trees are predominantly evergreens, which are highly flammable, a ten-foot minimum space between the crowns (branches of adjacent trees) should be maintained. This keeps fire from jumping through the crowns. Also make sure you maintain this distance from tree to house. You may need to remove a few trees.
  5. That firewood pile so conveniently placed by the back door should be moved outside the home ignition zone by March each year. Sparks from a wildfire can easily catch in firewood piles, and the intense heat of those burning piles next to the house will catch the house on fire.
  6. Clean up the home ignition zone. Remove old cars, lumber piles, downed trees and other debris. Is there enough space for firefighters to protect the backside of the home? Remove obstructing debris and trees and make sure fences have easily accessible gates.
  7. Clear a 10 foot space around propane tanks. Keep this space in gravel, rock or short, well-watered grass. Propane tanks should be located at least 10 feet from the home.

The Pine Brush Pit is located approximately 1/2 mile east of Hwy 87 on Control Rd South of Pine. The Brush Pit is open March through November every year (weather dependent). Residents can discard their brush for free. Leaves and pine needles in bags must be emptied and the bags taken back home with you. No household trash.

Your donations to PSFR are appreciated so we can keep the Brush Pit open.

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