290 TH E STUDY OF PLANT COMMUNITIES • Chapter X 



and 6,000 feet. Precipitation varies with altitude from three to 

 twelve inches and falls largely in summer. Temperatures are some- 

 what lower than in the Sonoran Desert, and frosts are not un- 

 common. 



Under these conditions, the communities are not as complex as 

 those of the Sonoran Desert or as simple as those of the Great 

 Basin, but there is much regional variation. Shrubs and semishrubs 

 predominate with a great variety of inconspicuous stem succulents 

 in association. Ocotillo (Fonquieria splendens), which is found 

 throughout the area, creosote bush (Larrea tridentata), and mes- 

 quite (Prosopis juliflora) are the only three species common in 

 the Sonoran Desert that are also important and widespread here. 

 A number of species are conspicuous because of size or unusual 

 form. Yucca, Nolina, and Dasylirion are large semisucculents. 

 Agave and Hechtia are particularly abundant leaf succulents. 

 Leafless, green-stemmed trees, columnar cacti, and Dasylirion 

 longissimwn with its six-foot, linear leaves, are examples of locally 

 important species of striking appearance. 



Grassland Formation.— Grasses are climax dominants over all 

 the vast area extending from southern Saskatchewan and Alberta 

 to eastern Texas, and from Indiana and the western margin of 

 deciduous forest westward to the woodland zone of the Rockies. 

 Separated from this major area are the Palouse region of Washing- 

 ton and the grasslands of the great valley of California. The for- 

 mation has the greatest extent of any in North America and con- 

 sequently grows under a great diversity of conditions. This is 

 possible because of the growth form of the species, their long pe- 

 riod of dormancy, and the fact that their moisture requirements 

 are critical only in spring and early summer. 



The eastern transition to forest is marked by an annual precipi- 

 tation of thirty to forty inches from Texas to Indiana and twenty 

 or twenty-five inches farther north. A high proportion of this 

 precipitation falls as spring rain, but westward, as the total de- 

 creases to about ten inches near the Rockies, the proportion fall- 

 ing in spring and summer also decreases. Temperatures are equally 

 variable. In the north, the growing season is cool and short, and 

 subzero temperatures occur for long periods in winter. In the 

 southern part of the range, frosts may be almost unknown, and 



