348 CARNEGIE INSTITUTION OF WASHINGTON. 



were studied in much detail, and its reconstruction carried toward completion 

 by the discovery of many rehct areas along the foothills of the Sierra Nevada. 

 The desert scrub and sagebrush were traversed in their entirety, the former 

 from southern CaHfornia to western Texas and the latter from the Sierras to 

 central Wyoming. The upland areas that stretch from trans-Pecos Texas 

 to the Santa Cruz Valley in Ai'izona are more or less evidently grassland of the 

 desert-plains type, though this fact is often obscured when mesquite. Yucca, 

 or Ephedra is thickly dotted over it, and especially by excessive grazing or by 

 wind erosion. The valleys of the Pecos, Rio Grande, San Pedro, and Santa 

 Cruz, on the other hand, are covered with what appears to be a typical growth 

 of desert scrub, composed chiefly of Larrea, Flourensia, and Prosopis. In all 

 of these, however, grasses have now been discovered to be the true climatic 

 dominants in all areas protected against overgrazing, such as rights of way, 

 cemeteries, etc., the former regularly being covered with the characteristic 

 grasses through scrub in which these are largely or entirely lacking. As 

 ah-eady suggested, cHmax desert scrub is confined to the regions of the Mohave 

 and Colorado deserts and ad j acent Mexico. Similar results have been obtained 

 for the widespread sagebrush of the Great Basin. The chmax area, now 

 known to be confined to Utah, Nevada, and adjacent CaUfornia, was traced in 

 detail during the summer and found to contain grasses only as climatic relics, 

 while the adjoining areas show mixed prairie wherever protection against 

 grazing exists. 



Changes in Grassland, by F. E. Clements and E. S. Clements. 



The investigation of the changes shown by grassland in response to grazing 

 and climate has been carried on actively throughout the year. All the 

 associations of the grassland chmax have had a share in this, though the most 

 striking results have been obtained with the three found in arid chmates, 

 namely, mixed prairie, desert plains, and bunch-grass prairie. This is further 

 due to the presence of a scrub community favored by grazing and in Cali- 

 fornia to the presence of aggressive introduced grasses as well. Significant 

 changes due to chmate are restricted to the Mohave and Colorado Deserts 

 and to Nevada and Utah, and these are considered in the next section, while 

 the modification arising out of grazing and settlement are considered here. 

 The replacement of grasses by others or by scrub in consequence of grazing 

 takes place in accordance with several well-defined principles. The species 

 that is most eaten yields to one that is less eaten, whether this be a native 

 grass, weedy annual, or a shrub. Tall-grasses give way to short-grasses 

 wherever the two are mixed, since they are most handicapped by grazing. 

 Sod-formers are favored in comparison with bunch-grasses because of their 

 method of propagation and consequent hold on the soil, and sod-formers 

 with runners or stolons, such as the buffalo-grass, tend to replace those that 

 lack them, such as grama. Finally, the grasses of the lowlands are more 

 persistent than those of the uplands because they are coarser and more vigor- 

 ous and possess a better water-supply, and for this reason grasses often hold 

 out longer in sandy areas if grazing and blowing are not excessive. 



Because of sufficient rainfall and the exclusion of grazing as such by culti- 

 vation, the subclimax prairies have undergone practically no change. Indeed, 

 the grasses are so vigorous that, once in possession as a result of a dry phase of 



