14 BULLETIN 12%, U. 8S. DEPARTMENT OF AGRICULTURE 
TABLE 4.—Relative susceptibility of species to direct heat killing 
Ham Station fire Howard fire 
Species Volume | Trees | Volume | Trees 
killed |. killed killed killed 
; Per cent | Percent | Per cent Per cen 
Westernuyellow pines $325. 2 eT Oey eee ee ey 54. 0 72. 0 45.0 | 62. 0 
SUP GTO ine ae OO Ve ee AS ep ee oe eee, Seen ae oe es 56. 3 70. 0 60. 0 | 64. 0 
Mii os vite toe ees VAIbS 1S 75,8 92.0 | 72.0 | 78.0 
ineensewcedarer Wie ies ee oe rt) oir a ke Sete st vl] 97.0 85. 0 
The reason for the greater resistance of the pines to heat, as com- 
pared with the fir and cedar, is not difficult to find. White fir, though 
it averages a greater height than the pines, has very mflammable 
foliage, and its needles are unable to endure as high a degree of heat 
as the needles of the pine. The needles on the pines cluster around 
and enfold the terminal buds and protect them from the heat. These 
buds, too, have thick scales. But the buds of the fir have thin scales 
and are only slightly protected by the needles. The crowns of white 
fir and cedar, the two most tolerant species’ in the mixed conifer 
forest, are both longer and denser than those of the relatively intol- 
erant pines, for the lower branches, being able to withstand the shade 
of those above, continue to live, whereas the lower branches of the 
pines are shaded off. Therefore, the possibility of a hot surface fire 
setting the crowns afire and then being able to spread from branch 
to branch and finally to strip the foliage is much greater in fir and 
cedar than inthe pines. A further condition influencing the relatively 
high susceptibility of the cedar is its smaller stature. In the areas 
under consideration, the average merchantable height of mature 
cedars killed was only 65 feet, as compared with 104 feet for white fir 
and 94 feet for the pines. 
The factor of susceptibility thus plays an important part in the 
succession of species in the pine region where there are repeated fires 
For example, on the east side of the Sierras in the yellow pine-white 
fir type, where enormous areas have been subjected in the past to 
repeated surface fires, the present merchantable stand is almost pure 
western yellow pine, but the reproduction that is appearing abun- 
dantly since the inauguration of systematic fire protection is predom- 
inatingly white fir. Im an undisturbed state of nature in this type 
the inevitable tendency is for the tolerant species (white fir) ulti- 
mately to dominate the stand to the exclusion of the intolerant spe- 
cles (western yellow pine), because the tolerant fir will reproduce in 
the shade of the mature forest and thus get astart on the pine. The 
recurrence of fires is responsible for interrupting the normal succes- 
sion so generally. 
The most important conclusion concerning relative susceptibility 
to heat killing is that the more tolerant the species the higher its 
degree of susceptibility to a fire of given intensity. 
RELATION TO SITE QUALITY 
Not only is specific susceptibility an exceedingly important factor 
in determining changes of composition within a given mixed forest, 
but it also furnishes a clue to the relative amount of damage in 
7“ TPolerance”’ is the ability toendure-shade. White fir is spoken of asa tolerant species because it can 
grow under heavier shade than, for instance, western yellow pine, an intolerant species. 
81.0 73.6 
