Fire behavior predictions are furnished in the same format for unlopped and lopped 

 debris (as shown in fig. 9) . A further discussion of model output and a guide to inter- 

 pretation of the fire behavior numbers are in the Northern Region's Users' Guide. 6 



Fuel Bed Depth Prediction 



To predict fuel bed depth, predictions of slash in tons per acre are converted to 

 number of 1/4- to 1-inch intercepts through two manipulations. First, weight per unit 

 area is converted to number of 1/4- to 1-inch intercepts by species, using a constant 

 multiplier that is inversely proportional to the product of wood density of the 1/4- 

 to 1-inch pieces and their mean square diameter (Brown 1974). Table 5 gives the 

 conversion factors for 11 western conifer species. 



Table 5. --Factors for converting from 1/4- to 1-inch size class 

 fuel loading to "expected number of intercepts per 

 foot of random transect perpendicular to the ground" 

 for 1/4- to 1-inch size class limb wood of different 

 species 



Species of tree : 



Factor for tons/acre : 



Factor for lb/ft 2 



Ponderosa pine 



0.4309 



9.39 



Lodgepole pine 



.5871 



12.79 



Western larch 



.4382 



9.55 



Douglas-fir 



.5608 



12.22 



Grand fir 



.7295 



15.90 



Subalpine fir 



.5784 



12.60 



Western redcedar 



.6400 



13.94 



Western white pine 



.5258 



11.46 



Western hemlock 



.5950 



12.96 



Engelmann spruce 



.7089 



15.45 



Whitebark pine 



.5258 



11.46 



Next, number of intercepts are combined for different species. Equations have been 

 given that relate bulk depth to the count of 1/4- to 1-inch fuel intercepts in two 

 crossed, 2-foot vertical planes (this relationship is the same as that for a randomly 

 placed single 4-foot vertical plane) . The equations for the pines and for other 

 conifers are different, so in some cases a method of combining them is necessary. 



The depth prediction equation can be written for slash of one species type as: 



6 = a/x" 

 o 



where 



6 Q = initial bulk depth 



x = expected number of intercepts of 1/4- to 1-inch fuel particles in a randomly 

 placed 1-foot vertical plane. 



See footnote 3. 



17 



