Loading is an important property of fuel because it expresses an amount of fuel 

 energy or potential fire intensity. Taken alone, however, it is not an adequate 

 predictor of fire behavior because other fuel properties, such as moisture content, 

 porosity, particle size, and chemical composition, also determine the amount and rate 

 of fuel energy release. 



Porosity is the inverse expression of compactness. Porosity relates to the 

 spacing of fuel particles and affects ignition time and combustion through its influence 

 on oxygen supply and radiant energy transfer between particles. Several studies using 

 wooden cribs and pine needle beds have shown that various measures of burning rate 

 increase with increasing porosity up to a certain point (Curry and Pons 1940; Gross 

 1962; Byram et al. 1964; Anderson et al . 1966). According to Byram, increasing the 

 stick spacing in a fuel complex increases the transmission of radiant heat in the 

 direction of the unburned fuel. But if spacing is too great, unburned particles may 

 not receive enough heat for ignition. If sticks are too closely spaced, sustained 

 burning will not be possible because of restricted airflow. Maximum fire spread 

 yccurs between these limits. 



Recent research shows that burning characteristics are related to aA(a dimension- 

 less variable) (Rothermel and Anderson 1966). Both particle size and porosity are 

 incorporated in aX. Burning characteristics of different fuels possibly may be rated 

 on the basis of aX , although more research is needed to demonstrate this conclusively. 

 Past combustion studies involving aX have dealt only with a single particle size. 

 Mixtures of particle sizes certainly need study as well. 



The significance of particle orientation in combustion of forest and range fuels 

 is probably minor compared to other fuel properties, although its exact role is not 

 understood. Particle orientation does affect the aerodynamic drag in a fuel complex 

 and probably affects exposure to incident radiation. The forced convection heat transfer 

 coefficient for blue spruce (Picea pungens Engelm.) foliage has been shown to vary 

 according to orientation of foliage with respect to direction of airflow (Tibbals et 

 al. 1964). 



The fuels studied were collected in western Montana and occur widely throughout 

 the West and are highly flammable. No effort was made to inventory fuel characteristics 

 throughout areas where these two types occur; rather, we sought to obtain an idea of the 

 range in values of fuel properties encountered in these types of fuel and to test 

 techniques of measurement. 



SAMPLING PROCEDURES 

 Ponderosa Pine Forest Floors 



Forest floor material was sampled in 13 ponderosa pine stands within 20 miles of 

 Missoula, Montana. The stands ranged from open to closed in density. They were 52 to 

 99 years of age and occurred on poor, medium, and good sites (figs. 1 and 2). Pine 

 needles and miscellaneous particles, which include staminate flowers, cone scales, 

 cones, bark flakes, branches, grass, and flat leaves, comprised the forest floor (fig. 3). 



In each stand, five plots, 1 foot square, were randomly located and photographed 

 in stereo to provide a permanent visual record of the fuel before disturbance. Depth 

 of the litter, or L layer, ^ was measured at four positions (fig. 4) using a rule sliding 



'^The layers of the forest floor are defined in a glossary of terms approved by the 

 Terminology Committee of the Soil Science Society of America (1965) as: L layer--the 

 surface layer of the forest floor consisting of freshly fallen leaves, needles, twigs, 

 stems, bark, and fruits; F layer--a layer of partially decomposed material with portions 

 of plant structures still recognizable; and H layer--a layer occurring in mor humus 

 consisting of well-defined organic matter of recognizable origin. 



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