INTRODUCTION 



The specific effect of living herbaceous plants and woody shrubs on fire behavior 

 has been difficult to quantify (Deeming and others 1972) . Intuitively we have known 

 that live fuels intercept some of the radiant and convective energy from a fire, thereby 

 interfering with the preheating of adjacent, unburned fuel elements. That is, the live 

 fuels act as a heat sink. The result is a reduction in fire intensity and forward rate 

 of fire spread. 



If moisture content is high enough, living fuels will not burn, which in effect 

 reduces the available fuel load. However, as the moisture content of the living fuels 

 decreases, at some point these fuels can be desiccated and ignited within the flaming 

 front, effectively increasing the available fuel load. The live fuels are then no 

 longer a heat sink, but a heat source. Depending on the amount of live material, a 

 rapid and often significant increase in fire intensity and spread can occur (fig. 1) . 

 Inclusion of these live fuel moisture effects improves the seasonal response of fire 

 danger ratings. 



Phenological processes define the general moisture profile of many plant species 

 during a growing season. This general profile is modified by weather conditions to 

 produce a specific seasonal moisture profile. Thus, particularly with respect to her- 

 baceous fuels, the relative severity of drought can combine with phenological curing 

 processes to produce a seasonal decrease in the live/dead ratio. From a fuels stand- 

 point, curing of herbs and forbs affects the transfer of material from the living to 

 the 1-hour timelag (1-h TL) class in addition to decreasing the moisture content of 

 remaining live plants. 



The 1972 National Fire Danger Rating System (NFDRS) (Deeming and others 1972) 

 partially accounted for these effects by using subjective estimates for the condition 

 of two classes of living fuels: 



1. Leaves and small twigs of perennial woody shrubs 



2. Herbaceous plants. 



The following section briefly reviews how these estimates were made and some 

 problems that resulted. 



LIVE FUEL MOISTURE ESTIMATION - 1972 NFDRS 



Woody Fuels 



Only general levels of moisture in the twigs and foliage of living woody plants 

 were considered in the 1972 NFDRS. The moisture levels varied according to three sub- 

 jectively assigned growth stages, (Deeming and others 1972) but the loading of live 

 woody fuels remained constant. 



1 



