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A series of laboratory fire spread experiments were completed to analyze the effect of terrain slope on marginal burning behavior of live chaparral shrub fuels that grow in the mountains of southern California. We attempted to burn single species fuel beds of four common chaparral plants under various fuel bed configurations and ambient conditions. Seventy-three (or 42%) of the 173 fires successfully propagated the 2.0 m length of the elevated fuel bed for slope percent ranging from â€“70% to 70%. There exists a critical slope above which fire spread is successful, and below which fire spread is unsuccessful. Critical slope varied widely with fuel type, moisture content, and fuel loading. Upslope and downslope orientations affected marginal burning behavior differently. In examining a special fuel bed slope set-up, it was found the upslope fire spread depended not only on the increased radiant heat transfer but also on the aerodynamic effect created by the interaction of the flame with the inclined surface. Under certain conditions, the convective heat induced by this interaction became the dominant mechanism determining fire spread success. A stepwise logistic regression model was developed from the data to predict the probability of successful fire spread. It is expected that this model may be helpful in providing guidelines for prescribed fire application.