150 CLIMAX FORMATIONS OF WESTERN NORTH AMERICA. 



Agropyrum and Stipa consociations meet in southern Oregon and northern 

 California, though here fire, grazing, and the invasion of ruderal grasses have 

 almost completely destroyed the native grassland. The Stipa consociations 

 seem formerly to have dominated the interior valley from Bakersfield to 

 Mount Shasta and from the foothills of the Sierra Nevada and Cascade 

 Mountains, through and over much of the Coast Range. The successive 

 invasions of European weedy grasses, the extensive cultivation of the land, 

 and the repeated burnings which favored chaparral at the expense of grassland, 

 have operated to practically eUminate the original grasses. A special search 

 has been made for relict patches of Stipa during the past three years, with 

 the result that ^uch areas have been found more or less continuously or at 

 frequent intervals from La Jolla and San Diego northward to Sisson and 

 Weed. Further information as to the original extent of the Stipa grassland 

 has been obtained from collections, ranges, the statements of early settlers, 

 and the accounts of earHer collectors and explorers. 



The bunch-grass prairie passes so gradually into the mixed prairie in central 

 Montana that no line can be drawn between them. This is readily understood 

 when it is known that Stipa coniata, Koeleria cristata, and Agropyrum glaucum 

 occur in both, and that a large number of the societies are identical. The 

 change is marked chiefly by the appearance and increasing importance of 

 Bouteloua, and the transfer of the major dominance from Agropyrum spicatum 

 to Stipa comata and Agropyrum glaucum. As already mentioned, there is no 

 connection between the bunch-grass prairies, and the short-grass and desert 

 plains in the south. The Colorado and Mohave Deserts have proved an 

 efifective barrier, which was probably in existence before the Pleistocene. It 

 thus appears probable that the bunch-grass prairies were derived from the 

 northeast and spread southward along the Pacific Coast. 



CONSOCIATIONS. 

 Agroptrttm spicatum. Stipa setigera. 



PoA tenuifoua. Stipa eminens. 



Festuca ovina. Stipa comata. 



Koeleria cristata. Elymus sitanion. 



The two most important dominants are Agropyrum spicatum and Stipa 

 setigera. The first is the major and often the exclusive dominant throughout 

 the Palouse, southward into Oregon and California and eastward into Idaho 

 and Montana. The second is, or rather was, the great dominant throughout 

 Cahfornia, and it extends well into Oregon. The others are all secondary to 

 these in importance. Festuca is the only other one which frequently makes 

 pure stands, and there is some question as to its true relationship^ It seems 

 to attain the maximum development at higher elevations, as is true also in the 

 Rocky Mountains, and to have recently made its way into the bunch-grass 

 prairies. However this may prove to be, it is impossible to ignore it as a 

 dominant member of the latter (Weaver, 1917 : 42). In California, Stipa 

 eminens stands next in importance to S. setigera. It is usually mixed with the 

 latter, but may constitute a pure community. Elymus sitanion has been 

 found in pure stands also, but as a rule it is mixed with Stipa setigera or 

 Agropyrum spicatum. Poa tenuifolia is a fairly constant associate of Agro- 

 pyrum and Festuca, but is never a pure dominant. This appears to be the 

 rule also for Koeleria and Stipa comata. They may be expected throughout 



