Porosity of Fuel Complex 



Void Volume-to-Surf ace Area 



Porosity values determined in this study may be of particular interest for relat- 

 ing laboratory research on fire propagation to field conditions. One expression of 

 porosity. A, is the amount of void space associated with a unit area of fuel surface. 

 Generally, movement of air and gases is freer as x increases because the space between 

 particles is greater. However, the distance that radiant heat must travel between 

 particles is also greater. 



Cheatgrass, averaging 12.5 cubic centimeters of void per square centimeter of 

 surface, was 15 times more porous than the ponderosa pine forest floor litter which 

 averaged 0.8 cubic centimeter per square centimeter. The greater porosity of cheat- 

 grass is no doubt more conducive to rapid fire spread than the lower porosity of 

 ponderosa pine litter. 



Particle Spacing 



Particle spacing is also a measure of the porosity of a fuel complex. Spacing 

 between cheatgrass particles was 2-1/2 times greater than was spacing between particles 

 in the ponderosa pine forest floors. Particle spacing. A, and bulk density, even though 

 they all indicate porosity, do not correlate perfectly because they are measured dif- 

 ferently. Among the three, particle spacing showed the least difference between the 

 forest floor litter and cheatgrass. 



Bulk Density 



Bulk density is an approximate measure of porosity of a fuel complex. It fails to 

 precisely express the amount of fuel complex occupied by actual fuel volume because the 

 specific gravity of fuel is disregarded. Thus, only weight, and not volume of fuel, is 

 corporated in an expression of bulk density. Nonetheless, it does indicate a degree of 

 porosity and has the practical advantage of being easy to measure. 



The bulk density of cheatgrass averaged 0.00032 gram per cubic centimeter, which 

 was 50 times less dense than the average of 0.016 gram per cubic centimeter for the 

 litter. The F layer from the forest floors averaged 0.076 gram per cubic centimeter 

 and ranged from 0.039 to 0.105 gram per cubic centimeter. This is approximately one- 

 half as dense as bulk densities reported for humus of western white pine {Pinus 

 monticola Dougl.), hemlock {Tsuga heterophylla (Rafn.) Sarg . ) , and Douglas-fir 

 {Pseudotsuga menziesii (Poir.) Britt.) forests (Mader 1953) but practically the same 

 as reported for red pine forests (Brown 1966) . Depth of the F layer averaged 3 

 centimeters . 



Proportions of Particles 



Pine needles, averaged over all stands, comprised a little over one-half of the 

 volume in the forest floor litter and approximately 90 percent of the fuel surface 

 area. Miscellaneous particles accounted for the remainder. The proportions of needles 

 and miscellaneous particles in the litter varied substantially from stand to stand. 

 Needles comprised 43 to 79 percent of the volume and 78 to 97 percent of the surface 

 area of the forest floor litter. 



Among the miscellaneous particles, branches contributed by far the most volume but 

 only slightly more surface area than cone scales as shown in the following tabulation: 



11 



