NATURAL REGENERATION OF DOUGLAS FIR. , 9 
ture were the same at all depths. But the fact that the surface soil 
often dries out, while the soil at a depth of 6 inches remains moist 
on protected slopes and dries on exposed slopes, changes the type 
and gives a decided advantage to the seedling with a deep root 
formed early in its development. In its early root growth the 
western yellow pine has the advantage over Douglas fir, western 
hemlock, and western red cedar, and it is largely on this account 
that the yellow pine forms the dry-slope type in the border zone of 
the Douglas-fir forest region. 
For the same reason Douglas fir is able to establish itself on the 
drier slopes of the Cascades, where the western red cedar and western 
hemlock fail. A south slope covered with Douglas fir and a north 
slope covered with western hemlock, western red cedar, and other 
species does not prove that each of these species is in its most favor- 
able situation, but that these are instances of successful competition 
and establishment. Where two types met on a ridge it was found that 
the south slope was seeded by the species of the north slope, and the 
seedlings of western hemlock and western red cedar were germinating 
along with those of the Douglas fir and western yellow pine in the 
spring. When the area was examined in the fall only seedlings of 
the western yellow pine and Douglas fir were left, because the small 
seedlings of the other species were unable to live through the dry 
period of the summer on account of their shorter roots and conse- 
gent inability to reach the moist layer of soil below the dry surface. 
These conditions are repeated year after year, and still the type 
remains the same.’ It is very noticeable that wherever a ravine or 
spring keeps the south slope moist the north-slope species are found. 
Evaporation, then, is one of the chief factors in the establishment 
of the seedlings, for, while the different slopes often get about the 
same amount of precipitation, there is such a marked difference in 
evaporation that the exposed slopes dry out while the north and 
protected slopes remain moist. 
SOIL. 
The loose volcanic-ash soil found in the Cispus region northwest 
of Mount Adams heats during a forest fire or when exposed to the 
sun’s rays. In this type of soil fire causes complete destruction of 
all vegetable matter and the seed stored in the forest floor is de- 
stroyed. The importance of seed stored in the forest floor in natural 
reforestation is discussed later. The hot, dry soil in these burned 
areas prevents establishment of seedlings after the fire. On the other 
hand, the heavy loam soil of the Willamette Valley and of other 
similar regions protects the forest floor from the heat of fires, and 
the greater moisture-holding capacity of this type of soil aids refor- 
estation. | 
Douglas fir grows best on sandy loam and reaches its greatest size 
on moist, porous, well-drained soil. It is absent from the wet bottom- 
land and marshy places and sphagnum bogs, on which Sitka spruce 
and lodgepole pine grow fairly well. It seldom occurs on light, dry, 
sandy soil or on heavy clay soil, and where it does occur on these soils 
it usually forms a light stand of poorly developed trees. 
™Hofmann, J. V. Seed Vitality as a Factor in Determining Forest Types. The Ames 
Forester, Iowa State College, 1917. 
60634 —24—_2 
