NATURAL REGENERATION OF DOUGLAS FIR. > Q 
Douglas fir during its early years helps to maintain it. Because it 
demands an abundance of overhead light, it produces the tallest and 
straightest stems in dense pure stands or in mixture with the more 
shade-enduring species. 
On the sites of poorer quality, especially in open stands, the lateral 
branches are persistent and form hard, dense wood. ‘These branches 
persist for 20 or 30 years after all foliage has died, and are embedded 
in a large section of the trunk. In localities where the growth is 
more rapid the lateral branches contain much softer wood and are 
not so persistent after the foliage has died. 
On good sites the favorable soil and abundant moisture enable 
Douglas-fir seedlings to endure more shade than on poorer sites. The 
same conditions also favor increased survival and growth of the 
shade-enduring western red cedar and western hemlock, with the re- 
sult that they often crowd out the Douglas fir. 
TEMPERATURE. 
Douglas fir is apparently adapted to severe climatic conditions in 
the Rocky Mountain region and on the east slopes of the Cascade 
Mountains of Oregon and Washington. However, the fast-growing 
Pacific-slope form of the species does not bear exposure to severe 
cold. In winter the cold, dry east winds sometimes kill the trees out- 
right and often kill the ‘growing tips, especially on the east side of 
the trees. Such conditions are particularly i injurious to young trees 
and either retard growth or kill the seedlings. 
Throughout the range of Douglas fir the seedlings are often killed 
on hot, exposed slopes through injury by heat to the cambium ring 
at the surface of the ground. It has been found that a temperature 
of 144° F. at the surface of the soil kills the cambium and causes 
girdling of the seedlings. This injury often affects seedlings or 
transplants in the nursery and has been described as “ stem girdle.” 
The cambium of older Douglas firs separates from the sapwood when 
it is heated above 160° F., and occasionally a scar results. If the 
temperature is raised to 200° F., the cambium becomes discolored and 
1s permanently injured. 
The length of the growing season in the Douglas fir region is 
variable, and the seedlings have apparently not become adapted to 
this variability. Often late spring or early fall frosts cause exten- 
sive injury to young growth. If frosts occur after growth has 
started or before the buds mature, the buds—particularly the termi- 
nal buds—suffer, height growth is checked or completely stopped for 
one or more seasons, and the bushy seedlings, so common up to 4 or 
5 years old, are formed. If the terminal buds are killed by frost, 
the lateral or adventitious buds develop, and it may be three or more 
years before a leading shoot is formed. The actual killing of young 
_ growth by frost is not common, but the death of seedlings as the 
result of heaving by frost is often extensive. The principal dis- 
_ advantage resulting to the tree, from its inability to withstand frost, 
is the loss of its place among its competitors. Western hemlock and 
_ western red cedar are often found uninjured by frost that has killed 
| the immature buds of Douglas fir. When in mixture with Douglas 
fir these species take advantage of the retardation of Douglas fir 
_and overtop it, thus eliminating it from the stands. 
