338 TURESSON: SLOPE EXPOSURE AND PSEUDOTSUGA 
to the behavior of Pseudotsuga taxifolia, popularly known as 
Douglas spruce. The range of this conifer is extensive. It is 
found most luxuriant in the Vancotiver strip, reaching perhaps its 
highest development in the Puget Sound region (see also Sud- 
worth, 9). This region, falling within the limits of Merriam’s 
Humid Transition area (5), is notable for its moist climate favoring 
a luxuriant growth of dense forests, almost exclusively composed 
of Douglas spruce, which here assumes gigantic dimensions. Less 
conspicuous and never attaining the development it reaches in 
the Vancouver strip, it reappears in the extreme eastern part of 
Washington and in northern Idaho where the climatic conditions 
approach those of the coast region. The main distribution of the 
tree in this region seems to coincide with the Canadian Zone of 
Merriam. 
Abundance of moisture in atmosphere and soil seems to be of 
vital importance to the growth of Douglas spruce. It is therefore 
surprising to find this conifer occupying the drier parts of eastern 
Washington, Merriam’s Arid Transition area. Realizing the 
scanty rainfall—about 18 inches in Spokane—it may well be 
questioned how the tree is able to endure the extremities to which 
Humid Transition plants are submitted in this arid region. The 
problem loses some of its complexity when it is found that Douglas 
spruce in this region always occupies the shady northern slope of 
hills and ridges, and is entirely absent on the southern, thus afford- 
ing a most notable example of the effect of slope exposure. No- 
where is this more beautifully shown than on the banks of Spokane 
River in the vicinity of Spokane. The banks, composed of glacial 
detritus, sometimes attain a height of 50 meters, and the vegetation 
of the sunny south-facing bank is in striking contrast with that 
of the more shady north-facing bank. This latter is occupied by a 
pure growth of Pseudotsuga taxifolia. A small belt of Populus 
trichocarpa, Alnus tenuifolia, Cornus stolonifera and Salix Scoulert- 
ana fringes the edge of the river. The following herbs are common 
in this belt: 
Artemisia ludoviciana* Aster foliaceus frondeus 
Aster Douglasii Aster laevis Geyert 
eer ey ee ee 
*In questions of nomenclature Piper’s Flora of the State of Washington (6) 
is followed. 
