NATURAL REGENERATION OF DOUGLAS FIR. 43 
viving than other species. If seedlings of any other species had 
become established on the area they would be destroyed by the next 
fire, and the species would be eliminated from the area before the 
next generation reached seed-producing age. 
The lodgepole pine produces seed at a younger age than its asso- 
ciates, and its cones are persistent. In eastern Oregon the lodgepole 
pine holds the same relative position in succession that the knobcone 
pine holds in the southwestern part of Oregon. For these reasons 
lodgepole pine sometimes holds potential western yellow pine sites, 
or is a successful competitor with Douglas fir and the other north- 
slope species. 
In some localities the brush has become so dense that it is almost 
a permanent type, although forest invasion, even under these con- 
ditions, still continues. Successive fires are of no assistance in re- 
establishing the forest on these brush areas. On the contrary, they 
are detrimental to the return of favorable conditions for forest 
growth. If the brush is left on an area the moisture conditions are 
improved, and the accumulation of litter increases the fertility of 
the soil and the moisture of the surface soil. Erosion, which is so 
general in this region because of the heavy showers, is retarded by a 
brush cover. 
The summer drought prevents the establishment of forests on 
severe sites in the open; but seedlings are able to become established | 
under the protection of the brush, and in this way, even though it 
is a slow one, the brush fields are essential to the natural establish- 
ment of forests. The brush is not suppressed by the forest until 
the trees are 30 to 40 years of age. The effect of the brush cover 
upon the improvement of soil conditions is shown in Table 19. 
TABLE 19.—EHffect of fire on moisture and humus content of soil. 
Percentage of moisture | Percentage of humus 
vateol content. content. 
ee pure Aspect. ga ET a eae fs Soa TET Ea Condicions|¢ of vegeta- 
on the | 5 
6-inch | 1-foot 6-inch | 1-foot 
Spek Surface. genth. | depth. SUCe- depth. | depth. 
| | ; 
1| 1915 | SE., 30 per 0. 25 5.4 4.1 2.1 4.9 3.6 | Few sprouts; some an- 
| cent. | nuals. 
2| 1915 | SW., 30 per -3 2.75 3.95 PAth 3.8 3.4 Do. 
| lweecent. 
3} 1915 | N., 10 per . 87 3.0 4.5 24.6 2.0 3.4 Do. 
cent. 
4 TSS62 Ne doe oie 6.2 4250 Md. Bb e1ORSeh y 8) Sil 4.6 | Heavy brush. 
5 | 1914 | SW., 40 per 519) 5. 65 6.9 0.2 552) 5.6 | Light brush. 
| en Cenite 
1 Areas examined in 1916. 
° The brush on this area was killed by the fire of 1915 but was not consumed. 
Table 19 also shows the direct relation between fires and the humus 
and moisture in the soil. The brush and young growth were killed 
in area 3, but not consumed Py the fire, which showed that the fire 
was not so hot as on areas 1 and 2, where a clean burn occurred. 
The abatement of the fire on area 3 is reflected in the greater per- 
centage of humus and a consequently greater percentage of soil 
moisture in the surface layers. The accumulation of humus due to 
