ROLE OF FIRE IN CALIFORNIA PINE FORESTS 13 
Climatic factors also exert an important influence in determining the 
damage that fires will do in a particular stand of timber. Excessive 
dryness and high winds favor rapid spreading and intensity of fires. 
Normally, summer fires occur under drier conditions than those of 
spring and late fall. 
The stage of growth of the forest at different seasons is also a 
factor in damage. In spring or early summer when the tree is grow- 
ing actively, a tree is more likely to be killed by a given volume of 
heat than during late summer or fall when the tree is less active and 
the heavily protected winter buds have been formed. 
The density of the stand itself is a factor which must not be over- 
looked. Open stands not only offer less interference to the sweep of 
wind than do dense stands, but usually have an understory of highly 
inflammable grass, brush, or reproduction which utilizes the space 
and moisture. A dense, closed stand of timber, on the other hand, 
will more readily develop a true crown fire. Such fires are the rule 
in dense, even-aged, second-growth stands where there is an uninter- 
rupted tree canopy. 
PREVALENCE OF HEAT KILLING 
One of the most striking features brought out in Table 3 is that in 
every fire but one a certain percentage of the burned area shows 
heavy loss from heat killing, heavy loss here being defined as the 
outright death of 50 per cent or more of the merchantable timber on 
any area. Heavy damage was found on as little as 1.4 per cent and 
as much as 100 per cent of the total burned area of the different fires 
studied, with a general (weighted) average for all of 15.3 per cent. 
Table 3 thus shows that no nee fires occur without a certain amount 
of heat killing. What will happen in any particular stand is a ques- 
tion of degree rather than of kind. 
The average (weighted) loss of merchantable material for the areas 
studied is 1,570 board feet to the acre. This loss, it should be noted, 
represents the complete or nearly complete wiping out of small patches 
of the stand rather than a uniformly distributed loss over the entire 
area. 
SUSCEPTIBILITY OF VARIOUS SPECIES 
Though heat killing has been considered for the forest as a whole, 
without distinction between different species, it is only where true 
crown fires sweep through a mixed stand that complete destruction 
results. 
In studying the effects of ground fires it is important to note to 
what extent the species differ in their resistance to injury. Per- 
centages from two sample plots in heavily burned areas are given in 
Table 4 for the mixed conifer type. It will be observed that the loss 
of both western yellow and sugar pines is strikingly less, in volume 
and in number of trees, than is the loss of either white fir or incense 
cedar. In both these mixed stands the trees of different species grew 
in close contact with each other, so that these figures give a very fair 
picture of the relative susceptibility of the different species to hot 
surface fires. 
