14 BULLETIN 1495, U. S. DEPARTMENT OF AGRICULTURE 
questionable. Because cover type is only remotely influential in 
controlling the occurrence of special-risk fires, it is necessary to omit 
this category from the present discussion. 
The other major causes, lightning, camper, and incendiary, may, 
on the contrary, be regarded as causes of general-risk fires (‘‘un- 
known” are also grouped in this classification), because their distri- 
bution is widespread and because they may occur practically anywhere 
within the forest area. ‘These will be the only causes considered in 
this study, save on particular areas and in the total of the burned 
area by types. 
NUMBER AND SEASONAL DISTRIBUTION OF FIRES BY TYPES 
The knowledge that in general the length of fire season decreases 
progressively from the low-altitude chaparral type to the high- 
altitude fir, is important in building a protection organization to meet 
the varied needs of a national forest. But specific information on the 
average number of days’ lag in the opening of the fire season from one 
major type to the next is far more useful, because the financial 
structure of the Forest Service requires the use of average dates as a 
basis for allotment of funds. The usefulness of reliable average 
opening and closing dates is great, even though variations in length 
of fire season occur from year to year. 
SEASONAL DISTRIBUTION OF FIRES 
Over the 10-year period (Table 5) only a few fires have occurred in 
March, and these have been nearly entirely in the western yellow 
pine and Douglas fir types, with a very few in the mixed conifer 
and brush types. Neither in the high altitude fir and sugar pine- 
fir nor in the foothill types of chaparral and woodland have fires 
been noted in March. In April substantially the same is true, except 
that a few fires have occurred in the woodland and grass types. In 
neither of these months have there been enough fires to justify special 
measures in addition to the protection afforded by the regular force. 
In the low elevation grass and woodland types the grass and weeds, 
the principal fuels, are still green, but in the western yellow pine 
type the needles are becoming dry. 
In no type is the danger point of one fire per 100,000 acres in 10 
days reached in the first 10-day period of May. The fir and sugar 
pine-fir types are still entirely free from fires. In the second 10-day 
period of May the brush type closely approaches the need for special 
protection measures, with 0.85 fire per 100,000 acres, and, in the 
northern group of forests considered alone, past that point. In 
the western yellow pine and mixed conifer types the number of fires 
increases markedly, but does not reach the danger point. The fir 
type has a very few fires. Jn the third 10-day period western yellow 
pine, mixed conifer, and Douglas fir exceed the rate of one fire per 
100,000 acres. Chaparral reaches a figure of 0.99 fire per 100,000 
acres; but in the fir and sugar pine-fir types there still is but a scat- 
tered occurrence of fires. 
In the first 10-day period of June, there is a general increase but 
no marked change. The fir and sugar pine-fir are still well below 
the danger line (0.18 and 0.63); but finally reach that line in the 
second 10- day period of June. 
