D 
COVER TYPE AND FIRE CONTROL 19 
The northern group of forests, Klamath, Trinity, Shasta, and Cali- 
fornia, have 6.93 more fires than the total group average for the 
season. (Table 5.)’ In two types only are there any appreciable 
differences in the number of fires per 100,000 acres; fir in the northern 
forests exceeds the general average by 9.53 fires, and brush by 11.99. 
Although for all the forests considered the Douglas fir type is 
intermediate between the pine types and the true firs in total seasonal 
fire rate, in the northern group the seasonal rate for Douglas fir is 
practically identical with that in the true fir type. The length of 
fire season, however, is the same as for the western yellow pine and 
mixed conifer types. 
The monthly distribution and total number of fires per 100,000 
acres for the four most important major types found in the northern 
eroup of forests agree in most particulars with the averages for the 12 
timbered forests. (Table 5.) In the north the season opens three 
weeks earlier in the brush type than elsewhere, and for the second 
10-day period of May the rate of fires is greater in the mixed conifer 
and western yellow pine types than elsewhere. 
The data for the northern group substantiate also the previous 
statement that in August all types are about equally susceptible to 
fires starting. 
NUMBER OF LIGHTNING FIRES 
Obviously the relative number of fires in different types may be 
affected not only by inherent differences in the types but also by the 
relative proportion of lightning and man caused fires. Lightning 
fires form the most homogeneous group of fires, and analysis of this 
eroup should more exactly express the inherent risk between the 
types than an average of all general-risk fires. The fact that lightning 
storms are very much more prevalent in the timber belt than in the 
chaparral and woodland types raises some difficulty, but so far as 
known the commercial timber types can properly be compared with 
each other on the assumption that equivalent opportunities exist for 
lightning fires in all. If there is a higher number of lightning bolts 
per 100,000 acres per year in one type than in another, it is certainly 
in the fir rather than in the pine belt. 
The lightning-fire data in Table 8 give the western yellow pine and 
mixed conifer types the highest number of fires. These two types 
together average 36.35 fires per 100,000 acres, whereas the sugar pine- 
fir and fir types together average but 29.68 or 82 per cent as many as 
in the pine group. The indications are that the slower drying out of 
the upper-elevation types plus the generally heavier precipitation 
known to occur there with hghtning storms tend to prevent many 
more bolts from starting fires in the fir than in the pine. This is no 
doubt true for the season as a whole, and particularly in June, when 
lightning fires in the fir group are but 32 per cent as numerous as in 
the pine group. In July the percentage is raised to 75. In August, 
the month in which all types are most nearly alike in susceptibility to 
fires starting, the number of fires in the two type groups is very similar, 
the fir group exceeding the pines by 1.21 fires per 100,000 acres. 
Thus, as with all general-risk fires, the relatively slow drying out of 
7 The northern group of forests were selected for special study because as a group they present particu- 
larly difficult problems in fire control; also this selection furnishes an opportunity for comparisons between 
the general averages obtained and a single group, 
