pine which was 1 to 2 percent lower than other conifer needles. Grasses and both 

 evergreen and deciduous hardwood leaf litter maintain higher EMC's than ponderosa pine 

 needles. The difference in EMC values becomes greater as the relative humidity in- 

 creases above 60 percent and the EMC divergence can be as high as 9 percent moisture 

 content. Van Wagner (1972) concluded that all kinds of leaf litter have EMC's about 

 3 percent higher than the common woods; this difference in EMC is felt to be definitely 

 important in fire-danger rating. 



The sorption data for various pine needles are within 2.5 percent moisture content of 

 each other in 53 percent relative humidity and 75° to 80° F (24° to 27° C) . However, when 

 other fine fuels like grasses, hardwood leaves, and wood splints are considered (Stamm 

 1964; Blackmarr 1971; Van Wagner 1972), the variation becomes sufficient to reduce the ac- 

 curacy of fire-danger estimates. Examination of the data at 53 percent relative humidity 

 showed the adsorption EMC values ranged from 8.3 to 12.3 percent and the desorption 

 values ranged from 10.9 to 15.5 percent for a total range of 7.2 percent moisture con- 

 tent in EMC. This suggests a composite estimate of fine fuel moisture may be no closer 

 than +3.6 percent moisture content to the real value. 



Comparing the fine fuel moisture content isotherm of the Canadian Forest Fire 

 Weather Index with the 1-hour timelag fuel moisture of the United States' National 

 Fire-Danger Rating System (NFDRS) shows a similar difference, approximately 4 percent, 

 in predicted moisture content. This appears to be due to the type of fuels considered 

 in establishing the moisture content response. The Canadian system is based on the 

 litter fuels in their coniferous forest (Van Wagner 1974; Van Wagner and Pickett 1975) 

 and the United States' system is based on studies of fine woody materials (Fosberg and 

 Deeming 1971). The ponderosa pine needle data (fig. 7) agrees with the Canadian iso- 

 therm at low humidities, but holds to lower moisture contents at humidities above 30 per- 

 cent and agrees with the National Fire-Danger Rating System 1-hour fuel moisture content. 



10 20 30 40 50 60 70 80 90 100 



RELATIVE HUMIDITY (PERCENT) 



Figure 7. — Comparison of EMC values for ponderosa pine needles with fine fuel moisture 

 content curve of Canadian Fire Weather Index and the USDA Forest Service NFDRS 1-hour 

 timelag fuel moisture content data. Weather conditions — 80° F and cloudy. 



11 



