6 BULLETIN 1200, U. S. DEPARTMENT OF AGRICULTURE. 
MOISTURE. 
Although young Douglas fir is found distributed on all sites 
throughout the region, its successful establishment and growth are 
largely controled by moisture. The poor growth along the Colum- 
bia River gorge, as compared with the adjoining side valleys, may be 
attributed to the drying east winds that sweep down the gorge. 
These winds cause excessive transpiration, which makes the buds 
mature early and consequently shortens the growing season. 
Site studies have been made in different localities, and the soil 
moisture correlated with the establishment and survival of Douglas- 
fir seedlings. The ability to extend its root system 6 or 8 inches deep 
during the early part of its first growing season is an important fac- 
tor in perpetuating Douglas fir. When it is in competition with such 
other species as western red cedar and western hemlock, which pro- 
duce shallow-rooted seedlings, the Douglas fir often is able to sur- 
vive where the other species. fail. 
Records of soil moisture taken in 1919 on the flat river bottoms 
and south slopes in the Cispus burn, near the Tower Rock Ranger 
Station north of Mount Adams, demonstrated the ability of the - 
Douglas fir to resist the adverse conditions of severe sites. On the | 
south slope the moisture in the surface soil reached.a minimum of 
0.18 per cent in July and did not go above 0.85 per cent in August. 
At a depth of 6 inches the south slope soil contained 6.55 per cent of 
moisture in July and 5.50 per cent in August. The wilting co- 
efficient of 2-year-old Douglas fir seedlings was found to be 1.25 per 
cent for this soil. These data show conclusively that seedlings could 
not live in the surface layer of the soil, because-the available moisture 
was below their requirement. At a depth of 6 inches, however, there 
was sufficient moisture for growth throughout the season. In the flat 
river valley, where the soil is a silt loam, the surface soil contained 
0.19 per cent of moisture in July and 0.09 per cent in August. At a 
depth of 6 inches the soil contained 11.45 per cent of moisture in July 
and 9.382 per cent in August. Obviously, then, seedlings that have 
root systems which penetrate to a 6-inch depth before July of their 
first season may become established in this region. 
These extremes of soil moisture are readily explained by the rec- 
ords of soil temperature and evaporation. The surface soil on the 
south slope reached a maximum temperature of 128° F. in July and 
135° F. in August. From July 15 to October 1 evaporation records 
with the Forest Service evaporimeter showed an evaporation of 1,070 
cubic centimeters on the south slope as compared with 690 cubic 
centimeters on the flat; that is, more than one and one-half times 
as much on the slope as on the fiat. With such severe conditions of 
temperature and moisture only those Douglas-fir seedlings that are 
protected by shrubs or annual plants survive the first one or two dry 
seasons and become established. 
After a forest canopy is formed and duff accumulates on the 
ground the site becomes more favorable; western red cedar and 
western hemlock are then able, gradually, to gain a foothold in the 
5 Hofmann, J. V. The Establishment of a Douglas Fir Forest. Ecology, vol. 1, No. 1, 
January, 1920. 
