TURESSON: SLOPE EXPOSURE AND PSEUDOTSUGA 343 
fluences, and how it determines, the kind of vegetation present. 
Coming back to Pseudotsuga taxifolia the evidences have showed 
that exposure is the regulating factor in the distribution of the 
tree in this region, the northern slopes and ridges being the only 
localities which offer the needed humidity in soil and atmosphere. 
Not only around Spokane but in all more or less arid regions can 
this be observed. Piper (6) cites the example of Kamiak Butte, 
a granitic cone in the arid region of eastern Washington projecting 
about 500 meters above the surrounding basalt, having its entire 
south exposure covered by a bunchgrass flora, while the northern 
slope is densely timbered with yellow pine and other coniferous 
trees (including Pseudotsuga taxifolia). Even in regions with a 
higher amount of annual precipitation, as in the extreme eastern 
part of the State and in northern Idaho, we find Pseudotsuga by 
far better developed on the northern exposure, though lacking in 
the more moist situations, not being able to compete success- 
fully with Thuja plicata, which here becomes dominant. At a 
higher altitude, mainly in the Canadian zone, it again becomes the 
dominant tree on the northern slopes, while Pinus contorta often 
becomes the most conspicuous on the southern exposure. Watson 
(12) describes a similar case from north central New Mexico, in 
which region Pseudotsuga taxifolia covers the north-facing slopes 
above 8,000 feet (the Canadian Zone) and extends down in the 
narrow canyons to about 70009 feet. 
Turning now our attention to the distribution of Pseudotsuga 
taxifolia in the dry regions of western Washington, we find it 
growing in the San Juan Islands. These islands, situated between 
the Strait of Juan de Fuca and the Strait of Georgia, are re- 
markable for their dry climate and for the many Arid Transition 
plants which there have been able to get a foothold, since the 
climatic conditions resemble those in eastern Washington. The 
nature of the climatic conditions may best be illustrated by men- 
tioning that the only cactus which occurs in the Vancouver strip, 
Opuntia polyacantha borealis, is confined to one of these islands. 
Other truly Arid Transition plants are also found, for instance 
Sieversia ciliata, Polemonium micranthum, Lupinus microcarpus 
and Juniperus scopulorum. The problem is a somewhat puzzling 
One, and Piper (6, p. 44) remarks: “Only one physical factor 
