18 BULLETIN 12%, U. S. DEPARTMENT OF AGRICULTURE 
TABLE 7.—Relative susceptibility of species to crown injury 
[Shasta National Forest] 
: Amount | Average | Growth- 
Species ‘ hee of crown; growth | reduction 
| killed j|reduction| index! 
| 
; Feet Per cent | Per cent 
Miestorn. yellow.pine..- .-- 22-423 Se 64.9 , FA 1.00 
SSLOTEE Te 0) 1 02 ea eases weeds hare ee he rea a ee 8 Eee LS 78. 0 31.3 38. 9 1.75 
DWoglas fir 25) steed zee eet pe teen db Be ee se 64. 5 | 38. 7 43.3 1. 58 
RH EG HSEICOGAT (0. S25 ee Oe ow oe ere een Ree | 61.5 | 40. 2 39.7 1.39 
erorage's s2a2 ii. See ae eee eile lad be Pir |e Hosa ees | 35.9 | 1.43 
| 
1 On basis of crown injury, and taking western yellow pine at unity. 
Of the four important species represented on the plot, all but sugar 
pine had practically identical heights. Western yellow pine showed the 
owest percentage of crown injury, Douglas fir the next lowest, and 
incense cedar the greatest. The sugar pine, with practically the 
same degree of crown injury as western yellow pine, but with a con- 
siderably greater total height, apparently is intermediate in resistance 
to injury between western yellow pine and Douglas fir. Table 7 
further supports the conclusion that resistance to heat killing varies 
inversely with the tolerance of the species. 
The last column of the table, showing the relative response of the 
four important species to a given degree of crown injury, indicates 
that western yellow pine suffers least and sugar pine most, while 
Douglas fir and incense cedar are intermediate. Of all the species 
studied western yellow pine is not only the most resistant to crown 
injury from heat but is the least responsive in reduction of growth 
to such injuries as are received. 
INDIRECT PHYSICAL DAMAGE TO MATURE TIMBER 
INSECT INJURY 
Certain other secondary injuries from fire are not immediately 
evident or simple of proof and, indeed, can be exactly determined 
only after detailed study over a period of years. Chief among these 
are losses from insects and wood-destroying fungi. 
Of the insects which cause the greatest losses to pines the genus 
Dendroctonus is the most important and most prevalent. Though a 
light infestation is practically always present in the pine forests, lum- 
bermen and foresters are chiefly concerned with this and other forest 
insects only when an epidemic develops. Insect epidemics have been 
known for many years, but the reasons for the sudden outbreaks have 
been exceedingly obscure. Only recently has it been discovered that 
insect epidemics are frequently precipitated or intensified by forest 
fires. 
One of the first large burns to be studied with this probability in 
mind was the Mormon Hill fire, in the Sierra National pee which 
occurred October 15, 1916, covering about 640 acres in the western 
ellow-pine type. The direct loss due to the fire was comparatively 
light, though many trees were injured. The history of the area was 
followed in detail by one observer for the seasons 1916 to 1919, in- 
