Measurement of Spread Model Inputs 



Loading by fuel ipartio'le size class. — The amounts of fuel in 0- to 1-, 1- to 3-, and 

 3- to 5-cin. diameter size classes and the needle class for ponderosa pine were calculated 

 from sample determination of the proportions of total fuel in each size class (fig. 2). 



Three branches per plot were randomly collected, dissected into the size classes, 

 and the proportions of total branch weight were determined for each size class. These 

 proportions were used to calculate fuel weights by size class for ponderosa pine 

 because the proportions appeared to be the same on all plots. The amount of fuel in 

 each size was actually sampled on each Douglas-fir plot using the planar intersect 

 technique (Brown 1971) because the amount of 0- to 1-cm. size class material appeared 

 to vary considerably from plot to plot. 



Partiale surfaae-avea-to-volume ratios. — For the 0- to 1-cm. Douglas-fir, an average 

 particle surface-area-to-volume ratio (a) was determined by taking random diameter 

 measurements. For a cylinder, a is 4 divided by the branch diameter. Diameters for 

 the ponderosa pine and the other Douglas -fir size classes were taken at 10-cm. intervals 

 along the three sample branches per plot. Standard errors of the estimate averaged 2.9 

 percent of the mean diameters for both species and all size classes. The a for needles 

 was obtained from measurements on needle cross sections (Brown 1970) . 



depth. — Twenty-four measurements of depth were systematically taken just prior to 

 ignition of each plot. Standard errors of the estimate ranged from 1 to 5 percent of 

 the mean depths. Depth values were used to compute packing ratios. The packing ratio 

 is the ratio of fuel volume-to-volume occupied by fuel and it indicates compactness of 

 the fuel for internal calculations in the spread model. 



6 



