Woody Fuel and Duff Consumption by Prescribed Fire in Northern Idaho Mixed Conifer Logging Slash

Book excerpt: Experimental burns were conducted on 36 plots in mixed conifer logging slash in northern Idaho to investigate consumption of duff and woody fuel. Prescribed fires were conducted in spring and fall, in YUM (yarded unmerchantable material) and non-YUM clearcuts and seedtree cuts. Preburn and postburn fuel loadings were measured, and fuel moistures were sampled shortly before burning. Preburn duff depth ranged from 1.2 to 2.6 inches. Preburn woody fuel loadings ranged from 28 to 86 tons/acre. Sixty-four percent of the duff was consumed. Most of the small woody fuel was consumed, along with an average of 15.5 tons/acre of large woody fuel. Postburn mineral soil exposure averaged 66 percent. Residual loading of large woody fuel averaged 31 tons/acre. Methods for predicting duff and woody fuel consumption are reported. For predicting percentage of duff reduced, the best independent variables were shallow duff moisture content and unit average diameter reduction of large woody material. For predicting duff depth reduction, the best independent variables were average duff moisture content and preburn duff depth. Diameter reduction of large woody fuel was best predicted using moisture content and average preburn diameter as independent variables. Guidelines are included for fire prescription development. These include recommendations for amounts of fuel to retain and to consume and discussion of tradeoffs in setting fire objectives. Equations are presented for computing moisture contents.