22 NOKTH AMERICAN FAUNA [No. 55 



SEMIARID TRANSITION ZONE 



The semiarid (better called semihumid) division of the Transi- 

 tion Zone as marked by the yellow-pine forests covers the broad 

 basal slopes east and south of the Cascades and the extensive plateau 

 levels of the Blue Mountain section. Its breadth and altitude vary 

 with the configuration of the land, base level, and slope exposure. 



On the northeast slope- of Mount Hood the Transition Zone 

 reaches from about 1,000 to 3,000 feet in altitude, although but 

 a short distance west of The Dalles (near Mosier) it comes down to 

 the banks of the Columbia River at less than 100 feet above sea level 

 where the cool, moisture-laden winds from the west come up through 

 the river gorge. On the southwest slope of Mount Hood the zone 

 extends from the broad bottom of the Willamette Valley up to 

 about 4,000 feet near Government camp but does not cross over the 

 crest of the range so as to encircle the mountain completely (pi. 

 11, A). 



On Mount Jefferson it reaches up to approximately 3,500 feet on 

 cold slopes and to 4,500 feet on warm southwest slopes, showing a 

 slight rise in altitude with slightly raised base level on both sides 

 of the range (pi. 11, B). 



In the vicinity of the Three Sisters Peaks, with a base level of 

 about 3,000 feet on the east and 1,000 on the west, the zone reaches 

 up to about 4,500 feet on northeast slopes and about 4,800 on south- 

 west slopes, although the zone does not reach to the actual base of 

 the peaks nor cross over the crest of the range north or south of 

 them (pi. 12). 



Mount Thielsen (pi. 13) and Crater Lake are more nearly in the 

 zonal position of the Three Sisters, mainly above the Transition 

 and passing through Canadian and Hudsonian Zones. 



In the Klamath section, with Mount McLoughlin (Pitt) as the 

 highest center, and with base levels of 4,200 feet at Klamath Lake, 

 3,000 feet in the Klamath River Canyon to the south, and 1,400 

 feet in the Medford section of the Rogue River Valley on the west, 

 the Transition Zone extends up to approximately 5,000 feet all 

 around Mount McLoughlin (pi. 14). On the southwest slope it 

 reaches from about 2,500 feet, east of Medford, to 5,000 feet on the 

 west base of the mountains, while on the Klamath Valley side it 

 runs from the level of Klamath Lake up to about the same altitude, 

 5,000 feet, on the northeast slope of the peak. In the Klamath coun- 

 try there is no lower limit of the zone shown on any northeast slope 

 as the Sonoran Zone occupies only the valley bottoms and warm 

 slopes. In the Klamath Canyon directly south of McLoughlin, 

 Transition Zone comes down to 3,000 feet on local cold slopes and to 

 about 4,000 feet on local hot slopes, but this deep, narrow canyon 

 shows all the complications of such types of country. 



In the broad expanse of comparatively low country stretching 

 north from Fort Klamath and the Klamath Marshes to the Paulina 

 Mountains, where Transition Zone would naturally be expected, the 

 plant and animal life show a predominance of Canadian Zone species, 

 due to local influences, as noted under Canadian Zone (p. 25). 



In the Blue Mountain section, Transition Zone on the Columbia 

 River Valley side extends on northeast slopes from about 2,000 to 



