Current fire models are designed to model the spread of a linear fire front in dead, small-diameter fuels. Fires in predominantly living vegetation account for a large proportion of annual burned area nationally. Prescribed burning is used to manage living fuels; however, prescribed burning is currently conducted under conditions that result in marginal burning. We do not quantitatively understand the relative importance of the fuel and environmental variables that determine spread in live vegetation. Laboratory fires have been burned to determine the effects of wind, slope, moisture content, and fuel characteristics on fire spread in fuel beds of common chaparral species. Four species (Manzanita sp., Ceanothus sp., Quercus sp., or Arctostaphylos sp.), two wind velocities (0 and 2 m s-1) and three slope percents (0, 40, or 70%) were used. Oven-dry moisture content of fine fuels (< 6.25 mm diameter) ranged from 30% to 105%. Forty-nine of 90 fires successfully propagated the length (2.0 m) of the elevated fuel bed. A logistic model to predict the probability of successful fire spread was developed using stepwise logistic regression. The variables selected to predict propagation were wind speed, slope percent, moisture content, fuel loading, and relative humidity.

[This publication is referenced in the "Synthesis of knowledge of extreme fire behavior: volume I for fire managers" (Werth et al 2011).]