INTRODUCTION 



Knowledge of the variations in heat con- 

 tent of wildland fuels is important in predict- 

 ing fire behavior or assessing the relative flam- 

 mability of different fuel complexes. The heat 

 content of the fuel, and the availability of this 

 heat content, partly determines the intensity 

 with which the fuel burns. (Intensity is usual- 

 ly expressed as the rate of heat release per 

 unit area per unit time; that is, B.t.u. per 

 square foot per second.) Heat content availa- 

 bility also affects the fire's rate of spread. 

 Studies are now underway to determine the 

 effect of heat content changes on flammabil- 

 ity.' Several species are being sampled season- 

 ally to acquire data on fuels that differ in heat 

 content. 



The relation of heat content to ether ex- 

 tractive content is a part of this study. "Ether 

 extractives" is a broad term that covers var- 

 ious waxes, oils, terpenes, and fats present in 

 most plant fuels in varying amounts. Because 

 these compounds do not undergo the com- 

 plex pyrolytic reactions prior to combustion 

 that are characteristic of the carbohydrate 

 constituents of fuels (primarily cellulose), the 

 extractives are more readily available. Also, 

 much of the extractive material appears to be 

 deposited on or near the surface of the plant 

 parts, especially the leaves. The extractives 

 collectively contain about twice the heat con- 



^Defined for these studies as a combination of ignit- 

 ibility, combustibility, and suslainability. 



tent of the extracted fuel. All indications are 

 that extractives could play an important role 

 in fire spread and intensity and that their sea- 

 sonal variation is partly responsible for varia- 

 tion in heat content of fuels. 



The specific effect of extractive content on 

 fire behavior and flammability is presently an 

 open question. In a study of herbicidal treat- 

 ment on fire intensity, the U. S. Forest Serv- 

 ice, Rocky Mountain Forest and Range Ex- 

 periment Station (1963), found that chemi- 

 cally dried manzanita {Arctostaphylos pun- 

 gens H.B.K.) did not burn as readily as un- 

 treated manzanita, although its moisture con- 

 tent was much lower. The treated material 

 contained about one-half as much ether ex- 

 tractives as the untreated material. On the 

 other hand, Mutch (1964) found the ignition 

 time for powdered ponderosa pine (Pinus 

 ponderosa Laws.) to be much longer than that 

 of powdered sphagnum moss {Sphagnum sp.), 

 although the pine had over four times more 

 extractives than the moss. (The higher density 

 of the pine may have influenced these re- 

 sults.) Pilot ignition time for both fuels, how- 

 ever, was increased by the removal of extrac- 

 tives. Philpot and Mutch (1968) found that 

 guava {Psidium guajaua L.) leaves that had 

 been treated with herbicide did not burn as 

 fast as leaves that had died naturally. The 

 treated leaves were found to have 18 percent 

 less extractives, as well as other chemical 

 differences. 



1 



