a 
NATURAL REGENERATION OF DOUGLAS FIR. D1 
risk from early spring to late fall, but in the open area the danger 
period is confined to a time in the spring before the new vegetation 
covers the ground and to a period in the fall after the vegetation 
has dried up or has been killed by frost. 
The inflammability of any material is in direct relation to the 
amount of moisture it contains, and the amount of moisture in the 
material is largely regulated by the condition of the air. Evapora- 
tion depends pr rimarily upon the relative humidity of the air. Wind 
velocity, temperature, vapor pressure, and barometric pressure are 
influencing but secondary factors. 
The fire risk may be about equal in a young Douglas-fir stand 
with snags and débris and in an open stand with ‘dry grass or 
weeds, so far as starting a fire is concerned, although the rate of 
spread and difficulty of control make the former a much ereater haz- 
ard. The snags and débris not only spread the fire rapidly, but also 
develop such a high degree of heat that all the young growth is 
quickly killed and dried to the point of inflammability. The snags 
also form a dangerous risk throughout the season. In an open Doug- 
las-fir stand a fire will spread rapidly if fanned by a wind, but the 
surrounding air is not heated so much, and consequently only a part 
of the young growth is killed and little of it is burned. The season of 
high risk is shorter, because it is confined to the periods when the 
grass and weeds are dry. Some of the overmature Douglas fir 
stands contain a large percentage of dead and decaying trees, which 
are a detriment to the stand from a commercial standpoint and 
in addition make a dangerous fire hazard. During the dry season 
_shags are easily ignited and burn so furiously that they spread fire 
rapidly and cause the flames to climb into the crowns of nearby 
trees. In a mature stand free of snags a crown fire is not so likely 
to develop from a surface fire as in a stand containing snags, and 
the fire is easier to control because*it is on the surface only. 
The needles, being the most inflammable, constitute the principal 
fuel for the spread of a crown fire. Douglas fir needles will ignite 
when they contain moisture equal to about 35 per cent of their dry 
weight. Consequently, any amount of moisture greater than 35 per 
cent must be evaporated before the needles will burn. They ignite 
at_a temperature of about 650° F. and burn quickly. 
The water content of Douglas fir needles varies greatly through the 
season and is, no doubt, the underlying factor in the behavior of 
crown fires in mature forests and in the spread of fire in young 
stands. During the season of 1921 the percentage of water content 
of needles in mature Douglas fir was as follows: 95.9 per cent in 
early May, 77.3 per cent in early July, and 100.4 per cent in late 
September. Coincidently, the respective percentages of water con- 
tent of the needles of well-stocked young growth 13 years old_were 
120.8, 99.0, and 115.3. This means that a fire, in order to burn Doug- 
las fir needles and to spread, must evaporate the following amounts 
of water, based on the dry weight of the needles: 
Q 2 
Early May. Early July. pase oes 
ff0 Bh oaks RAB Wee MLE) ogl) 0\e) ete > Snipa ee ea tap Ue TREE Se OPT Oe Vel en 
65.4 
In youngicromeh A058 8 Es) ee a Bae 3 
Per cent. Per cent. Per cent. 
60.9 42.3 
85. 8 64.0 80. 
