ROLE OF FIRE IN CALIFORNIA PINE FORESTS 17 
CONTROLLING FACTORS 
As one might expect, the degree of crown injury appears greatest 
in the shortest trees and least in the tallest trees. That this relation 
exists on the particular area studied is evident from the height column 
of Table 5 though the relation is far from perfect. Similar figures, 
as shown in Table 6 were obtained from two plots laid out in a fire- 
pamoeed area of pure western yellow pine on the Modoc National 
orest. 
TABLE 6.—Influence of height on crown injury 
[Western yellow pine; Modoc National Forest] 
Plot 1 Plot 2 
Diam- Diam- 
Trees Trees 
- ter Crown . eter Crown 
Height 2 ae meas- | Height ae eas- 
a injury rae phen injury ured 
————————— sero oO ees ee | | | | | 
Feet Inches | Percent | Number| Feet Inches | Per cent | Number 
1 Oy fale 2 0 6 44 1 
85 8 8 1 
86 14 49 2 85 18 62 1 
90 16 55 6 95 20 37 3 
97 22 26 3 107 22 10 5 
102 20 48 2 112 28 20 4 
106 24 26 1 115 24 19 5 
115 26 24 5 118 34 17 3 
122 28 31 2 122 26 7 5 
123 34 41 1 127 30 23 4 
125 40 11 1 130 32 0 i 
130 38 37 1 135, 38 0 1 
140 32 27 2 140 36 17 1 
ee 2 DRS aS ae a ee [ee 150 40 20 2 
Though the progression is not entirely regular, it is evident that 
the degree of crown injury varies inversely with the total height of 
the trees. This relative susceptibility to crown injury is of great 
importance not only in its influence on relative damage to different 
species In mixed stands, but because it confirms what has already 
been shown, that the poorer the quality of the site, and conse- 
quently the shorter the trees, the greater is the susceptibility of the 
forest to crown injury, and to retarded growth or even outright death 
directly from heat. If the scale of protection is based on the value 
of the resources, it will obviously be less intensive on poor sites than 
on good ones, in young stands than mature ones. ‘The likelihood of 
heavy damage, and consequently the need for intensive protection, 
are greater on the poorer sites and in the younger forests. 
EFFECT ON VARIOUS SPECIES 
The relative resistance to heat killing of the crowns of several of 
the important California conifers has been noted. The experimental 
lot on the Shasta National Forest discussed in connection with 
able 5 affords some very interesting side lights on this point, partic- 
ularly in those cases in which the intensity of the fire is not sufl- 
cient to destroy the crowns completely. By determining the average 
ercentage of crown killed for each species, the result, as shown in 
able 7, is an excellent index of the relative susceptibility, since in 
none of the trees studied did the fire actually consume the crowns. 
2027°—24——2 
