NATURAL AREAS AND REGIONS 



183 



Wherever this western mesophytic 

 coniferous forest borders on the prairie 

 it is bounded by the Garry or Oregon oak 

 (Quercus garryana), in the northern part 

 of the state and by the black oak (Quer- 

 cus calif ornica) , madrona and manzanita 

 in the southern portion. 



The Willamette valley is the largest 

 of the important valley sections in 

 western Oregon. It differs from the 

 Umpqua and Rogue river valleys chiefly 

 in its size and the more northerly loca- 

 tion. It is therefore a cooler valley. 

 A study of the floras of these valleys 

 shows that the southern floras extend 

 northward into the Rogue and Umpqua 

 valleys, while the northern plants may 

 be found southward into these sec- 

 tions. Intercepting transverse mountain 

 divides serve in part as climatic barriers 

 to the northward and southward migra- 

 tion of plants. These intercepting 

 ranges are neither high enough nor com- 

 plete enough to serve as effective bar- 

 riers in themselves. Their true signifi- 

 cance seems to be in the alteration of 

 the temperature and moisture condi- 

 tions, which in turn controls the charac- 

 ter of the vegetation. The numerous 

 smaller valleys take on the character of 

 the larger valleys. The Coos Bay, and 

 Tillamook sections are more humid, 

 whereas the Hood River Valley which 

 drains into the Columbia on the north 

 is more like the Willamette Valley. 



The floor of the Rogue river valley was 

 originally a sagebrush desert surrounded 

 by grassland. The prairie may be seen 

 on the higher ground and lower slopes. 

 Between the hill-prairie and the forest 

 is a zone of chaparral. There may be a 

 question regarding the occurrence of 

 typical chaparral, in Oregon. The name 

 Siskiyou chaparral may be used to 

 distinguish the brush lands of South- 

 western Oregon from chaparral of Cali- 

 fornia and elsewhere in the Southwest. 

 However, at present, there is much 

 ground occupied by manzanita (Arcto- 

 staphylos spp.), antelope bush (Purshia 

 tridentata} , young madrona, scrub oaks, 

 and young yellow pine. 



In the Umpqua valley and corre- 



sponding situations the black oak is 

 intermixed with the Garry or Oregon 

 oak and replaces it to some extent on the 

 exposed hill slopes. Alder, broadleaf 

 maple, service berry, chinquapin (Cas- 

 tanopsis chrysophylla) , Oregon or Coos 

 Bay myrtle (Umbellularia calif ornica) , 

 and black cottonwood are characteristic 

 hardwoods. The Oregon or Coos Bay 

 myrtle is found more abundantly in the 

 coast sections of southern Oregon. 



There are two types of prairie in the 

 Willamette, Umpqua, and other north- 

 ern valleys in western Oregon hill 

 prairie and valley prairie. These are 

 not always distinct. The hill slopes 

 are drier early in the season. At present 

 weedy bromes and fescues are the 

 dominant hill prairie grasses. The 

 grasses of the valley are almost all 

 introduced. One of the original grasses 

 (Deschampsia caespitosa) may still be 

 seen along the roadside places. 



Eastern Oregon. The eastern section 

 is a high plateau that rises rapidly from 

 the Columbia river southward to 4000 and 

 5000 ft. in altitude on the interior high 

 desert. Deep canyons are cut through 

 the plateau and in the mountains. 

 Rising above the plateaus are numerous 

 buttes and mountain ranges varying in 

 altitude from a few hundred feet above 

 the plateau to over 9000 ft., above sea 

 level in the Blue Mountains. 



All of the area lies in the Cool Zone of 

 Livingston and Shreve, 3 so that the 

 character of the flora is determined by 

 this and the low average rainfall, which 

 is especially true during the growing 

 season. Local differences are affected 

 chiefly by slope exposure and altitude. 



The eastern plant communities may 

 be given as follows: 



Sagebrush desert, or (Great Basin Micro- 

 phyll desert) 



Bunch grass prairie, or grassland 



Juniper forest, or (Xerophytic Conif- 

 erous forest) 



Yellow pine forest, or (Xerophytic 

 Coniferous forest) 



Mesophytic coniferous forest 



'Livingston, B. E., and Shreve, Forrest. "The 

 distribution of vegetation in the United States, as 

 related to climate conditions." Carnegie Inst. 

 Publ.284; 



