D 
COVER TYPE AND FIRE CONTROL 93 
incendiaries. The high rate of incendiarism in the Douglas fir type 
is exceptional and is explained by the point of view of the local popu- 
lation in the northern group of forests—a condition not duplicated 
elsewhere. In the fir and sugar pine-fir types the incendiary problem 
is insignificant. 
RELATIVE EASE OF HANDLING LIGHTNING FIRES AND MAN-CAUSED FIRES 
In the earlier analysis of these 10-year data it was found that of the 
three major general risk causes—lightning, camper, and incendiary— 
lightning fires were the most easily handled, except where very large 
numbers occurred at one time. A partial explanation for this is that 
some rain ordinarily accompanies lightning storms, thus checking 
the initial spread and giving the organization a chance to suppress the 
fires. An additional reason now appears to be the relative scarcity 
of lightning fires in the chaparral and brush types, where fires are 
most difficult to control. The lightning fires are thus generally timber 
fires in character, rather than crown brush fires. 
The most difficult fires to handie were found to be those caused by 
incendiaries. The outstanding reasons are evidently that incendiary 
fires are typically brush and chaparral fires and that they are purposely 
set under the most difficult circumstances. 
Camper fires are more abundant in timber than in brush and chapar- 
ral (though by no means absent in the latter types) and so are inter- 
mediate in difficulty of control between lighting and incendiary fires. 
Man-caused fires as a group form a very much higher part of the 
total fires of the low elevation types—chaparral, grass, and wood- 
ete wae of the higher elevation types of Douglas fir, sugar pine-fir, 
and fir. 
The relative proportion of the different types found in a forest is of 
tremendous importance in the fire problem, affecting the kind of fires 
that start and consequently their behavior after start and difficulty of 
control. ‘This explains to a considerable degree the inherent differ- 
ences in fire control between adjacent forests and forest groups. 
CONCENTRATION OF RISK 
So far the numbers of fires starting have been discussed as though 
they were uniformly distributed within the total area of each type. 
This is a useful concept in comparing the fire problem in the several 
types, but it is more accurate to recognize that everywhere fires, both 
lightning and man-caused, start in well-defined zones, which cover 
but a portion of the total forest area. 
Concentration of risk is well illustrated in the northern group of 
forests, for which the percentage of each type in actual risk areas was 
planimetered from type and fire occurrence maps. (Table 11.) The 
grass and woodland group has the largest part of its total area sub- 
jected to risk, followed by the brush-chaparral group, the western 
yellow pine-mixed conifer group, the Douglas fir group, and sugar pine- 
fir and fir group. Comparable figures for the other national forest 
groups indicate similar conclusions and are therefore not included. 
In this particular, as in many of the other elements of the fire 
problem, the nontimbered and, except for the brush fields, relatively 
low-value types, have a higher rating than the timber types, averaging 
46.6 per cent, as compared to 41.9 percent. This difference, however, 
