D2 BULLETIN 1200, U. S. DEPARTMENT OF AGRICULTURE. 
From these figures it is readily seen that a crown fire will spread 
most rapidly in summer in either mature timber or young growth, 
- and that young growth in this respect is not so great a fire hazard 
as mature timber. Young growth, during the driest season, is not 
as inflammable as the mature timber in the spring. 
The fire danger in a clean mature forest as compared with that 
in a young stand of Douglas fir depends upon the inflammability of 
the needles, which in turn is in direct relation to the amount of 
water contained in them. A green stand of young or mature Doug- 
las fir will not burn until the needles are dried to the point of in- 
flammability. The rate at which the needles are dried determines 
the rate of spread of the fire, and when the heat ahead of the fire 
is not intense enough to dry the needles a crown fire stops. For the 
same reason a crown fire will start only where there is enough in- 
flammable débris beneath the stand to heat the air and dry the 
needles before it. For these reasons a slash area adjacent to a green 
stand either of young growth or mature timber always constitutes 
a dangerous risk. (Pl. XVII.) - 
Experiments show that in a mature stand of Douglas fir with 32 
trees to the acre, 1.500 gallons of water per acre are evaporated from 
the needles before the inflammability point is reached: in a stand of 
young growth 12 years old with 20,000 trees per acre, 2.300 gallons 
per acre are evaporated; and in a stand 12 years old containing 
8.000 trees per acre, 3.900 gallons per acre are evaporated. Of the 
two types of young stands the well stocked are less inflammable than 
the very densely stocked. The difference in water content of the 
needles in different stands is due to the rate of growth of the tree 
and the proportion of new needles to old. Mature stands grow very 
slowly and retain the needles in whole or in part for 6 or 7 years, 
with the result that the proportion of new needles to the older ones is 
small. Older needles contain more resinous deposits and mineral 
matter and less moisture than new ones and are, therefore, more in- 
flammable. In a dense young stand the top shoots grow rapidly, but 
the side branches are crowded and make little growth. This leaves a 
high proportion of new needles, but the total needle growth is less 
than in a young stand in which each tree has growing space enough 
to allow all of the side branches to develop new needles. Dead twigs 
on the stems are very inflammable and will carry a fire through a 
dense young stand which would not burn if only green twigs and 
needles were present. 
Clean, weil-stocked young stands are the lowest fire risk. The de- 
gree of hazard in these stands depends upon the density of the stand; 
the denser the stand the greater the inflammability. It is an en- 
couraging fact in the management of Douglas fir that the most de- 
sirable forest—the well-stocked young stand—is at the same time the 
lowest fire risk. 
FIRE PROTECTION. 
The greatest factor in forest protection is the prevention of fire. 
An analysis of the fire risks is the greatest aid in prevention, because 
it makes possible the most effective use of the protective force. Fire 
protection may be effectively accomplished through the efforts of 
individual owners, although it almost invariably proves to be ex- 
pensive. At present a large degree of protection is afforded in the 
