Predicting Equilibrium Moisture 

 Content of Some Foliar Forest 

 Litter in the Northern Rocky 

 Mountains 



Hal E. Anderson 



INTRODUCTION 



The past half century has seen many attempts to 

 quantify moisture content, MC, of wildland fuels and to 

 incorporate these values into systems for predicting fuel 

 flammability and potential fire behavior. Hardy (1983) 

 describes the work of Harry T. Gisborne, who was study- 

 ing fuels in the 1920's, and who eventually devised a fire 

 danger meter that depended on the moisture content of 

 wood dowels (Gisborne 1948). 



Several studies have investigated moisture content 

 to determine its influence on wildland fuel flammability: 

 of wood (Simard 1968); of fine fuels such as hardwood 

 leaves (Dunlap 1932); fine fuels in southern forests 

 (Blackmarr 1971), Australian forests (King and Linton 

 1963), and forests of eastern Canada (Van Wagner 1972); 

 and fine fuels entered into the early United States 

 National Fire Danger Rating System, NFDRS (Keetch 

 1966). Air temperatures, near 23 °C (80 °F) and relative 

 humidity from 20 to 90 percent have often been used to 

 study fuel moisture content and flammability. 



Study results generally indicated that moisture content 

 of wood varies among tree species (Simard 1968) and that 

 the moisture content of wood is usually lower than that 

 of nonwoody fuels such as grass, needles, and leaves 

 (Anderson and others 1978; Van Wagner 1972). Never- 

 theless, the current NFDRS (Cohen and Deeming 1985) 

 is based on the moisture content of wood sticks, drawn 

 fi-om the Wood Handbook (USDA FS 1974). Adsorption 

 and desorption are represented by a single curve over the 

 entire range of humidity and fuel-surface temperatures. 

 Moisture values drawn from such curves would typically 

 under predict EMC for nonwoody fuels and would over- 

 predict their flammability and potential fire behavior. 

 The research reported here was initiated to determine 

 EMC's of various nonwoody, foliar litter fuels over a range 

 of temperatures and humidities and thus improve the 

 accuracy of flammability and fire behavior predictions. 



Determining an EMC for each fuel type that poses a 

 fire potential would be an unreasonable and unnecessary 

 task, particularly if a small set of fuel groups with similar 

 EMC's could be calculated. Nelson's research (1983, 1984) 

 on the sorption of water by cellulosic materials provides 

 that capability. Nelson developed an empirical model, 

 based on the observed exponential relationship between a 

 thermodynamic variable, Gibbs fi*ee energy, and the EMC 

 of a cellulosic material. The Gibbs free energy per gram 



of sorbed water is measured in terms of absolute tempera- 

 ture, degrees Kelvin, and relative humidity, RH. Nelson 

 (1983, 1984) noted that earlier researchers had reported 

 the change in Gibbs fi-ee energy as a function of moisture 

 content, MC, for cellulosic materials (Babbitt 1942; Kelsey 

 and Clarke 1956; Stamm and Loughborough 1935). In 

 addition, Anderson and McCarthy (1963) had reported an 

 exponential variation for the differential heat of wetting 

 as a function of moisture content. 



While illustrating the possible formation of foliar litter 

 groups in terms of EMC's, the research described here 

 also provides another test of the applicability of Nelson's 

 equation to materials with less cellulose content than 

 wood. Woody materials have cellulose contents near 

 75 percent, while conifer needles have contents near 

 40 percent (Susott 1980; Susott and others 1975). 



METHODS AND MATERIALS 

 Field Procedures 



Foliar litters consisting of grasses, deciduous leaves, 

 and conifer needles were collected in western Montana 

 and northern Idaho for study. One sampling was done 

 in the fall after the current-year leaf and needle cast, col- 

 lecting only the recently cast dead foliar litter. Another 

 sampling was done in the late spring collecting just the 

 weathered foliar litter. The general areas of sampling are 

 shown in figure 1, with the following specific collections: 

 western white pine {Pinus monticola Dougl.) at the Priest 

 River Experimental Forest in northern Idaho; lodgepole 

 pine {Pinus contorta Dougl.) and Douglas-fir (Pseudotsuga 

 menziesii [Mirb.] Franco), in the headwater drainages of 

 Fish Creek in Montana; Engelmann spruce (Picea engel- 

 mannii Parry), subalpine fir (Abies lasiocarpa [Hook.] 

 Nutt.), western redcedar {Thuja plicata Donn.), and grand 

 fir {Abies grandis [Dougl.] Lindl.), in the general area of 

 the Powell Ranger District in Idaho. Litter samples col- 

 lected in the vicinity of Missoula, MT, were ponderosa 

 pine {Pinus ponderosa Laws.), quaking aspen {Populus 

 tremuloides Michx.), western larch {Larix occidentalis 

 Nutt.), and cheatgrass {Bromus tectorum L.). The fuels 

 were brought to the Intermountain Fire Sciences Labora- 

 tory in Missoula, MT, where they were mechanically 

 sorted, cleaned without washing, and placed in cool, dry 

 storage until the testing was done. 



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