$8 BULLETIN 1495, U. 8. DEPARTMENT OF AGRICULTURE 
FUELS AND BEHAVIOR OF FIRE 
The nine major types in this region are recognizable as well by the 
character of associated herbaceous and woody plants as by the prin- 
cipal key tree species. AI of this plant life, together with the accu- 
mulation of partially decayed vegetable matter in the form of mulch, 
duff, and humus, determine the amount and character of the fuel. 
. Within the commercial timber belt the densest stands are found 
near the upper edge, where precipitation is greatest, in the fir types; 
and the most open forest is found at the lower edge, where precipi- 
tation is least, in the western yellow pine type. In heavy stands, the 
annual fall of needles and twigs soon becomes compacted into a dense 
layer of duff, poorly aerated and moisture-retaining. Close stands 
of timber, moreover, prevent the growth of annual plants, brush, and 
tree reproduction, so that the duff remains the principal fuel. In 
the more open western yellow pine stands the duff is less compact, 
dries out more rapidly, and is better aerated. Because of the general 
openness of this forest, the subordinate vegetation of annuals and 
brush is relatively abundant and in late summer, when the plants 
dry out and burn use becomes an important element in the 
spread of fire. Young tree growth also may be present to add to 
the fuel. Hence, whereas fires generally creep slowly in the fir type, 
where duff is the chief understory fuel, in the pine type they spread 
rapidly through the considerable understory of vegetation. The 
form in which the fuel occurs is often more important than total 
quantity in controlling spread and severity of fire. 
The mixed conifer type is usually an all-aged forest, with a con- 
siderably heavier volume per acre than is generally found in the 
western yellow pine type. The forest forms an irregular canopy, 
pierced by innumerable snags, which are a serious source of trouble in 
the control of fires. This type is richer in secondary vegetation than 
the western yellow pine or fir types, and openings are rapidly occupied 
by brush species, these forming the nuclei of future brush fields. 
Disintegration of fallen leaves and humus proceeds almost as rapidly 
as in the western yellow pine type, but on the whole the mixed conifer 
type has a slightly higher fuel content than has the western yellow 
pine type. Surface and ground fires are characteristic of this type, 
and crown fires are uncommon. 
The amount and kind of fuel present in the woodland type varies 
ereatly. In the drier situations the oaks, the principal species, occur 
as isolated trees, with widespreading crowns. This form of woodland 
type merges with the grassland type throughout its range; the ground 
is carpeted with grasses and weeds in profusion, and these form the 
principal fuel. Fires, as in the grassland type, spread with great 
rapidity over the surface, though rarely running up to the tree crowns. 
Where precipitation is heavier, the woodland type forms a close 
canopy of many hardwood species and the ground is heavily covered 
with a layer of dried and decayed leaves, and lacks any understory of 
erasses and weeds. Fires in the denser stands eat their way slowly 
through this semicompacted mass of organic material, flarmg up 
occasionally through the trees under increased wind velocity. 
In the brush fields, where fires have been excluded, new coniferous 
stands are making a start and a heavy layer of humus and duff forms 
under the closed canopy of the brush. The brush fields have the 
