Canopy fuel characteristics mapping and crown fire initiation and spread

Background/Question/Methods Crown fire occurrence and spread in woodlands are potentially limited by tree species composition and canopy structure.  This information is partially captured by measurement of canopy bulk density (CBD), which when mapped can be utilized to determine potential for crown fire behavior and effects.   Inclusion of spatially-explicit measures of CBD in fire modeling can affect land management decisions related to habitat conservation and urban fire risk assessment.  In this study, CBD was measured CBD at 74 sites in central Texas woodlands on two wildlife refuges. These data were then utilized to map CBD over the landscape based on a multiple linear regression model derived from comparison of LANDSAT 7 ETM+ data with the CBD data. To investigate the influence of spatially heterogeneous CBD on simulated fires, 80 simulations were run which used average CBD and mapped CBD values as control and treatments, respectively for  winter and summer weather scenarios.

Results/Conclusions Spatial extent of crown fire occurrence and active crown fire occurrence was significantly higher (p=.008 and p<.001, respectively) for simulations that used average CBD inputs in summer fire scenarios, while only active crown fire occurrence was significantly higher (p<.001) in winter fire scenarios. In both control and treatment simulations, mean fire areas were significantly larger in summer burns (p=.001). In crown fire area, summer burns showed significantly greater crown fire extent with heterogeneous (p<.001) and homogeneous (p=.002) canopy layer treatments than did winter burns. Reaction intensities with heterogeneous canopy mapping were significantly higher (p<.001) in summer burns than winter burns. Summer burns also showed significantly higher (p<.001) heat per unit area compared to winter burns when both landscape sets were modeled with heterogeneous and homogeneous canopies. Spatially heterogeneous canopy fuels provide natural crown fire “braking” that can lead to landscape-level diversity enhancement. Heterogeneity of crown fuels is important to capture in models for prescribed crown fire scenarios. Winter burns are shown to produce less crown fire occurrence and lower probability of active crown fire spread than summer burns.