188 CLIMAX FORMATIONS OF WESTERN NORTH AMERICA. 



topography of the Bad Lands and pine ridges of western Nebraska and the 

 Dakotas, and it reaches westward into the valleys of the foothills in Colorado 

 and New Mexico. A similar subclimax occurs in central Texas, where the 

 oak forest meets the prairies. On the Edwards Plateau the dwarf form of the 

 live-oak, Quercus virens, mingles with the shin-oak, Quercus undulata, of the 

 Rocky Mountains to form what is probably a climax community, closely 

 related to the Cercocarpus-Quercus association, if not to be regarded as a part 

 of it (plate 43). 



The dominant species are typically shrubs for the most part, but several 

 important ones are trees which become dwarfed in the more xerophytic condi- 

 tions of the prairies and plains. This is the case with the bur-oak (Quercus 

 macrocarpa) , live-oak (Q. virens), ash (Fraxinus viridis), plum (Prunus ameri- 

 cana), hawthorn (Crataegus coccinea), hackberry (Celtis occidentalis) , box- 

 elder (Acer negundo), elm (Ulmus americana), and linden (Tilia americana). 

 A large number of the trees which reach the western edge of the deciduous 

 forest exhibit the same tendency, but they extend little beyond the limits of 

 the forest proper. The majority of the dominants are bushes or bushy shrubs 

 from 3 to 10 feet high. They resemble those of the climax in producing root- 

 sprouts readily and consequently in taking rapid and complete possession 

 where forest is cleared or subclimax grassland is overgrazed. 



Extent and contacts. — Subclimax chaparral appears along the western bor- 

 der of the deciduous forest and through valleys in the prairies from Manitoba 

 and Saskatchewan to northern Mexico. It extends westward to the Rocky 

 Mountains from Montana to Texas, and comes into repeated contact with 

 mountain chaparral in the upper valleys of the North and South Platte, the 

 Arkansas, Canadian, and Pecos Rivers. Throughout the eastern edge of this 

 area, it marks the ecotone between the forest and grassland. It is naturally here 

 that it finds its best expression, in accordance with the fact that the dominants 

 are either trees of the- forest, or shrubs and bushes which constitute a lower 

 layer or play the r61e of serai dominants. The subclimax occurs generally 

 throughout the grassland formation in valleys and sandhills where the water- 

 content is above the normal. It is best developed in the eastern portion of the 

 prairies and decreases steadily toward the west, persisting only in the larger 

 valleys, on buttes, or in sandhills. It is everywhere surrounded by grassland, 

 except where it comes in contact with mountain chaparral, or with the pine 

 or aspen community in the Black Hills or other outlying montane regions. 



DOMINANTS. 



