99 BULLETIN 1294, U. S. DEPARTMENT OF AGRICULTURE 
that even scorching of the bark appeared to invite rather than to 
repel attack. In detailed examination of the 134 fire-scarred trees 
on the Snake Lake burn it was found that large areas of inner bark 
surrounding the fire scar may be killed by a fire that does not con- 
sume the outer bark or leave any external evidence of injury. It 
seems not improbable that such fire-injured trees should be peculiarly 
susceptible to attack in view of the apparent preference of forest in- 
sects for weakened trees. On these trees it was found that pitch 
tubes were far more numerous on the blackened bark which had 
been subjected to intense heat than they were on the unscorched or 
lightly scorched bark of the same tree. On a number of trees a count 
of the number of pitch tubes showed that the number on the badly 
scorched portions was more than twice as great per unit of surface as — 
on the-unscorched. 
Though not conclusive, this study confirms the deduction that in- 
sects successfully attack those trees and those portions of trees that 
‘ have been most seriously affected by fire. This conclusion is in har- 
mony with the known biological facts governing the resistance of organ- 
isms to disease or attack by their enemies. 
FUNGI 
The virgin forest, subjected to repeated surface fires for centuries, 
has been exposed to what has been aptly called cumulative risk. 
That is, the older the stand the more opportunities there have 
been for the trees to be injured, and the more serious subsequent 
injuries have proved. One serious result of cumulative risk has been 
the susceptibility of the damaged trees to ravages of wood-destroying 
fungi, which have gained entrance to the trees through open fire 
scars, and have caused a serious loss of merchantable timber. 
Decay in white fir.—In the detailed studies of white fir (Abies concolor) 
by Meinecke (17) it has been shown that only in very rare cases did 
the Indian paint fungus (Hchinodontium tinctorum) obtain a foothold 
in uninjured trees. Of 59 firs wounded by fire, he states that only 
11 had decay not traceable to the fire wound. Out of a total of 109 
cases of decay, the causes of which were definitely determined, 48 
were due to fire, 25 to frost, 23 to lightning, and 13 to other causes. 
The seriousness of decay from infection by various causes was 
rated by Meinecke as follows: Lightning least, fire and minor causes 
next,and frost most serious. By combining the factors of frequency of 
infection by different causes and the seriousness of the decay, Meinecke 
obtained the following relative ratings: Fire 144, frost 100, lightning 
23, others 39. A major conclusion reached is that fire, chiefly 
through the formation of basal wounds, is the most important single 
ee resulting in the prevalent unsoundness of white fir in the virgin 
orest. 
Decay in incense cedar.—Boyce’s studies (2) of dry rot in incense 
cedar caused by Polyporus amarus are equally conclusive in demon- 
strating the effect of cumulative risk on the merchantability of this 
species. Surface fires start a train of circumstances the effects of 
which can not be fully determined until many years afterwards. That 
incense cedar with advancing age is subject to cumulative risk from 
dry rot is strikingly shown in Table 10. Indirectly, the same figures 
further emphasize the cumulative risk in fire scarring, for fire scars 
are by far the most important point of entry of fungus. 
