INTRODUCTION 



The ponderosa pine (Pinus ponderosa Laws.) timber type covers nearly 9 million 

 acres in the Rocky Mountains. The needle litter cast by these trees produces a highly 

 flammable fuel when conditions are dry. The capability for fires to start and spread 

 depends largely on the moisture content of surface fuels and their response to environ- 

 mental changes. This report summarizes the results of laboratory tests to determine 

 equilibrium moisture contents and adsorption-desorption timelags below fiber saturation 

 of ponderosa pine needles. Conducting the tests at conditions below fiber saturation 

 eliminated consideration of the movement of free water and possible leaching of the 

 needles . 



Research on fuel moisture over the past several years has added useful knowledge 

 on fine forest fuels (Pech 1968; Simard 1968a; 1968b; Nelson 1969; Van Wagner 1969 and 

 1972; Mutch and Gastineau 1970; Blackmarr 1971; and Fosberg 1975). Equilibrium moisture 

 contents and timelags are the important moisture response characteristics of fine forest 

 fuels (Byram;-^ King and Linton 1963) and are required in fire-danger rating and fuels 

 appraisal systems. These two characteristics establish the moisture content at any 

 time, depending on the environmental conditions. Equilibrium moisture content (EMC) 

 is defined as the moisture content finally attained uniformly throughout a material 

 exposed to an atmosphere of fixed temperature and humidity. This occurs when the vapor 

 pressure in the fuel equals the vapor pressure in the atmosphere. Moisture response 

 time, assumed to be an exponential response, is defined as the time required for a fuel 

 to achieve 63.2 percent of the total change between its initial moisture content and 

 EMC. This can be expressed as 1 - 1/e of the difference between initial moisture con- 

 tent and EMC, where e is the base of natural logarithms. 



The test results presented here establish the EMC values at 75° F (24° C) , and the 

 sorption response times of needles and of fuel beds at three bulk densities, and report 

 the effects of solar heating upon desorption. The tests with three fuel bed bulk den- 

 sities were conducted at 80° F (27° C) , with 90 and 20 percent relative humidity end 

 points to determine if the response times were different from response times of individ- 

 ual pine needles. The bulk densities (the load per unit area divided by the depth of 

 the layer) matched findings for litter beds of ponderosa pine needles in western Montana 

 (Brown 1970) . 



Byram, George M. 1963. An analysis of the drying process in forest fuel 

 materials. Paper presented at the 1963 International Symposium on Humidity and 

 Moisture, Washington, D.C., May 20-23, 1963. 38 p. (Unpublished report on file at 

 the Southern Forest Fire Laboratory, Macon, Georgia 31208). 



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