Modeling fire in semi-desert grassland/oak woodland: The spatial implications

Fire-evolved forests that historically had high fire return intervals (< 100 years) in the western United States are currently overstocked with fuels due to a century or more of fire suppression and anthropogenic modification. Additionally, some western rangelands have changed composition from fire maintained grasslands to grazed shrublands. Land managers are beginning to reintroduce fire to these ecosystems as a functional component. Estimating fire behavior through the use of computer simulations is one tool to assist in planning management-ignited fire. We evaluated the sensitivity of the fire model FARSITE to the level of detail in the fuels data, both spatially and quantitatively, to better understand requirements for mapping fuels to produce accurate fire simulations. Simulated fires generated using site specific fuel models mapped at 30 m and degraded to 210 m were compared to fires simulated using standard generic Northern Forest Fire Laboratory (NFFL) fuel types. Eight classes of surface fuels were mapped by classification of satellite imagery with an overall accuracy of 0.78. A percent tree canopy cover map was created from digital orthophotos using a linear regression model with an R_adj² = 0.93 of field sampled percent canopy cover data to a tree canopy shadow model. The dominant site specific fuel model (63% cover) was found to agree with the most suitable NFFL fuel model. Site specific fuel models mapped at fine resolution were found to produce statistically smaller fire areas than those produced with generic fuel models mapped at a fine scale and site specific fuels mapped at a coarse scale. In the worst case scenario (low fuel moistures and high wind speeds) the average fire size was about 20% larger with the fuel map using NFFL fuel models than with the fine scale map using site specific fuel models.