USDA Forest Service

Pacific Southwest Research Station

Pacific Southwest
Research Station

800 Buchanan Street
Albany, CA 94710-0011
(510) 883-8830
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Research Topics Fire Science

Masticated Fuels:
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Masticated Fuels Research

Fire behavior and above-ground fire effects

:Prescribed burn ignition at the Whitmore masticated fuels site. June 5, 2006.

Masticated wood is typically very fragmented with high surface area: volume ratio, which would tend to increase fire-line intensity and reduce the duration of combustion. On the other hand, masticated fuel beds are also compact, which would tend to reduce fire-line intensity and increase the duration of combustion. When we initiated the study, there were some concerns that fires might have long residence times, causing greater than desired soil heating and the potential for cambium and root damage to standing trees.

Among our objectives was to conduct prescribed burns and estimate fire behavior in order to calibrate existing fuel models or develop custom fuel models for predicting fire behavior and fire effects in masticated fuel beds. We also wanted to better understand mechanisms of tree mortality when masticated fuel around them was burned.

Prescribed burns were conducted at two sites (Challenge Experimental Forest – May/June 2005, Whitmore – June 2006) in four replicate one acre units per site. Prior to the burns, fuels around a randomly selected subset of trees were removed from around the base in order to investigate if mortality was due mostly to bole charring, root damage, or crown damage. Ignition was accomplished using strip head fires, or backing fire when fire-line intensity became too great. Flame length and rate of spread were estimated during the burns and fire damage to individual trees (crown scorch height, percentage of crown volume scorched, bark char height etc.) measured after the burns.

Key findings:
  • Under prescribed fire conditions, flame lengths were low to moderate (approx. 1 ft for backing fire, 2-3 ft for head fire) and rate of spread slow (0.2 chains/hr for backing fire, 1.3-2.8 chains/hr for head fire).
  • Rate of spread and flame length were well predicted with the BehavePlus fire model, using either the sb1 or sb2 (low load activity fuel and moderate load activity fuel) models. However, scorch height was substantial and under-predicted by BehavePlus. At the high fuel loading site (Challenge), scorch height was approximately 4 times model predictions, while at the low fuel loading site (Whitmore), scorch height was approximately 2 times model predictions. We therefore created custom fuel models (below) that produced outputs more in line with field observations of both fire behavior and effects.

    Custom fuel model inputs for three different levels of masticated loading based on behavior of prescribed burns at two sites. Use the moderate load activity fuel model (sb2) of Scott and Burgan (2005), with the following modifications:
    Low load*
    Moderate load
    High load
    1-hr (tons ac-1) 3.5 5.7 7.8
    10-hr (tons ac-1) 2.5 5.9 13.1
    100-hr (tons ac-1) 0.3 1.2 5.8
    S:V ratio (ft2 ft-3) 750 750 750
    Fuelbed depth (ft) 0.36 0.52 0.89

    *Low load is similar to the “Shasta” and “Whitmore” sites in the fuel loading photo series; the “Iron Mountain”, “Sierraville”, and “Whiskeytown” sites depict a moderate load, whereas the “Taylor Ridge”. “Challenge”, “Applegate”, “Stanislaus” and “Mad River” sites show a high load.

  • Tree mortality from prescribed burns appeared to be primarily the result of crown scorch. High soil and duff moisture at the time of the burning limited heat penetration into the soil. This suggests that prescribed burning can be successfully used to reduce masticated fuels without killing residual trees if fire-line intensity is kept low using conservative firing techniques. Greater use of backing fire, narrowing of distance between strips with strip headfires, or burning when air temperature is low might be options, especially if the residual trees are relatively small. Burning when soil moisture is high may also help reduce damage to trees.
Last Modified: Dec 19, 2016 02:47:56 PM