COVER TYPE AND FIRE CONTROL 9 
highest fuel content of any type and fires sweep through them with 
great fury. Also, the progress of any given fire is far more uncertain 
and difficult to check than in the chaparral. In the brush fields the 
heavy layer of humus retains smouldering embers for long periods, 
and these flare up at the first wind that may arise and start a new 
conflagration. 
In the chaparral the density and height of stand depend mainly 
upon the frequency with which past fires have swept the area. Where 
fires have been absent for a decade or more, there is an uninterrupted, 
impenetrable cover of multistemmed individual shrubs. The princi- 
pal species being evergreen and small!-leaved, comparatively small 
quantities of dead material fall to the ground, and the dense shade 
prevents the establishment of grasses and weeds. Fires race with 
fury through this unbroken canopy of chaparral when high winds and 
low humidities prevail. On the other hand, fires subside quickly 
during calm weather and rising humidities, and finding little fire- 
holding fuels on the surface, frequently burn themselves out. Thus, a 
fire in chaparral may at one moment be a raging crown fire and the 
next almest completely subdued. 
The three major kinds of forest fires—those in the branches and 
tops of forest trees, or crown fires; those confined to the subordinate 
cover of grass, weeds, and litter, or surface fires; and those that run 
through the duff, or ground fires—occur at times in all types. Never- 
theless, without overgeneralizing, each cover type may be character- 
ized in fire control by the frequent occurrence of one kind of fire, in 
the following manner: 
Crown fires: 
Chaparral type. 
Brush type. 
Surface fires: 
Grass type. 
Woodland type. 
Western yellow pine type. 
Mixed conifer type. 
Douglas fir type. 
Ground fires: 
Sugar pine-fir type. 
Fir type. 
Fires which race through the crowns of dense stands of brush or 
chaparral have very properly been classified as crown fires in this 
region, although the term ‘“‘crown fire” has been almost entirely 
restricted in other regions to fires which run through the crowns of 
trees. Crown fires develop where a continuous closed canopy is 
found, and the chaparral and brush types are distinctly of such 
character. Only rarely in the daytime, more commonly at night, 
do fires burn as surface fires in these types, particularly the chaparral, 
and then only to burst into crown fires with the first considerable 
decrease in relative humidity or increase in wind velocity. Crown 
fires sometimes occur in the virgin forest types, but if so they are 
localized, except in the severest fire years; they are more common in 
second-growth forest. Crown fires generally spread the most 
rapidly, surface fires next, and ground fires least; and the degree of 
damage is ordinarily in the same sequence. The rapidity with which 
crown fires spread is indicated by the average of 55 acres per hour 
38207°—29——_2 
