The Origin of the Desert Climax and Climate 135 



westward along river valleys; U. americana reaches the Black 

 Hills and Rocky Mountains, and fulva, western Kansas; Celtis 

 occidentcdis attains western Washington in the north, and sev- 

 eral species of willow are found westward to the Pacific Slope. 



It is hardly necessary to emphasize the fact that climax relicts 

 west of the deciduous border have survived because of edaphic 

 compensation in terms of stream valleys, altitude, slope-exposure, 

 or soil and that, in consequence, they indicate water relations 

 higher than that of the circumjacent climax but lower than that 

 of their own climax to the east. Such species, in conjunction with 

 serai dominants, permit the plausible reconstruction of a post- 

 climax along streams not much different from that of the Ni- 

 obrara, Platte, and Republican in Nebraska and of the rivers of 

 central Texas today. The rainfall of the enclosing prairie climax 

 fluctuates about 25 inches in the north and 30 in the south, 

 the effective values being much the same as a result of the evapo- 

 ration gradient. 



In considering as indexes the dominants that still confront 

 the prairie climax, it is significant that three-fourths of them 

 find their western limit in one of the three parallel valleys of 

 central Texas, the Trinity, Brazos, and Colorado, while at the 

 north the majority still persist in the valley of the Missouri along 

 southern Nebraska and adjacent Kansas. This belt falls for the 

 most part between the isohyets of 30 and 40 inches with that of 

 35 as the median line; however, the full climax of oak-hickory 

 finds its boundary at approximately 40 inches. If this relation be 

 applied to the region of the present desert, it leads to the con- 

 clusion that such a forest was in complete or major occupation 

 at a rainfall of this amount, and that it had already withdrawn 

 to valleys and other compensatory sites when the precipitation 

 had dropped to 35 inches. On the basis of Chaney's conclusions 



