PIPER— FLORA OF THE STATE OF WASHINGTON. 35 



correspond in conditions, and in a large part in species, with the 

 arctic meadows north of the limit of timber. 



The Upper. Sonoran area in eastern Washington is practically coex- 

 tensive with the distribution of the sagebrush. Agriculturally it 

 is the region where the commercial growing of peaches and water- 

 melons is practicable. 



The Humid Transition, or Pacific area, includes the great forests 

 of red fir in western Washington. Other characteristic trees are the 

 giant cedar, red alder, and Oregon maple. 



The Arid Transition area in eastern Washington includes two 

 subdivisions, a lower one — the bunchgrass prairies, and an upper — 

 the yellow pine forests. 



The Canadian zone has for its most characteristic tree the western 

 . white pine. Here, too, the lodgepole pine, the western larch, and the 

 western hemlock are most abundant and best developed. 



The Hudsonian is the zone of the subalpine fir, the Alaska cedar, 

 Ihe black hemlock, and the white-bark pine. At their extreme limits 

 of altitude all of these become prostrate mats of branches. 



These zones are not separated by level altitudinal lines. This may 

 readily be seen Avhere the Hudsonian and Arctic zones meet. The 

 trees of the former zone extend up the mountain sides much higher 

 on the ridges than in the valleys between. The dividing line is thus 

 a sinuate one. This has been considered by Merriam partly the result 

 of more and less favorable exposure to the sun's rays, and partly to air 

 currents, the warm currents tending to follow up the steep ridges 

 while the cold currents flow down in the valleys. Consequently, cer- 

 tain species ascend highest on the warm ridges, while others descend 

 farthest in the cool valleys. 



Exactly similar conditions, but with the forest line reversed, are 

 seen at the low altitude timber line of the j^ellow pine, where this 

 borders on the bunchgrass prairies. The timber here descends to 

 much lower altitudes along valleys and draws than on the ridges ; or, 

 what is the converse statement of the same thing, the bunchgrass 

 flora ascends highest on the warm slope. 



The most notable examples of the effect of slope exposure alone 

 are perhaps seen on high ridges, or in canyons that extend in a gen- 

 eral east and west direction. For example, Kamiak Butte, a bold 

 peak in ^^Tiitman County, has nearly its entire south exposure cov- 

 . ered by a bunchgrass flora, while the northern slope is densely tim- 

 bered with yellow pine and other coniferous trees. In canyons of 

 low altitude the sunny north wall is often timberless, while its shady 

 south wall is timbered. 



The same fact is generally true in the case of high mountain peaks. 

 The corresponding zones extend relatively higher on the south or 

 warmer side than on the north or colder side. 



