REVIEWS 455 



The foothills under study are covered generally with very open 

 forest of western yellow pine and pifion-juniper, especially toward the 

 south, but although these are the most conspicuous vegetational units 

 the grasslands are the most wide-spreading and important. Aleso- 

 phytic communities are rare and are limited to small areas, for al- 

 though the rainfall is somewhat greater than on the plains to the east 

 (i8 to 20 inches) and evaporation is not as great, conditions remain 

 rather severe on most sites. 



A classification of habitats has been made based upon their degree 

 of xerophytism, followed by a classification of the plant associations 

 of the region. This classification is based on a number of features. 

 The primary division is based upon the successional stage, whether 

 primary and unstable or established in character ; subsequent divisions 

 are made chiefly on the basis of the site upon which the association is 

 found and the form of the association, whether of trees, shrubs, or 

 herbs. 



The associations recognized are lichen, sumac, and foothills primi- 

 tive grassland association as the first vegetation on new or unfavorable 

 habitats, followed by rock pine (western yellow pine), pifion cedar, 

 Oiierciis cercocarpos, Arctostaphylos, Ceanothus, foothills mixed 

 grassland, bunchgrass, Pseudotsuga, Populiis salix, canyon forest 

 aspen, foothills mixed shrub, Symphoricarpos, and the foothills meso- 

 phytic grassland association. 



Some of these are of particular interest. The rock pine (western 

 yellow pine) association is very wide-spread and is found in a wide 

 variety of sites. Its establishment depends chiefly upon favorable 

 conditions during the critical period of the first few years of life. 

 Small areas of soil deposition burying the seed often give rise to dense 

 young stands. The efifect of fire in this association has tended to re- 

 strict the pine and aid its shrubby associates, but the magnitude of the 

 efifect is difficult to determine. A frequent associate of the pine is 

 Ceanothus fendleri: other conifers are usuallv rare. 



The pinon-cedar association is prominent only toward the south. 

 where, with the increasing aridity, the western yellow pine appears at 

 higher altitudes, leaving the lower foothills to this more drouth-en- 

 during type. .\ closed canopy is never found. There is little vegeta- 

 tion on the spaces between the trees. 



The arctostaphyllos association, frequently considered as an almost 

 constant companion of western yellow pine, is found to go vcr\- nnich 

 higher than the pine and to be tvpically a formation of the montane 

 region rather than the foothills. 



