Figure 3-3. A late summer aspect of an almost pure stand 

 of tobosa grass {Hilaria mutica) in Graham County, Arizona, 

 showing good response to summer rains. Density, height, 

 herbage cover, and weight indicate excellent cover of the 

 grass. Savanna type with scattered mesquite trees in the 

 background; overgrazing, however, alters it to woodland. 

 (U. S. Forest Service.) 



quirement. A long growing season with ample soil moisture 

 throughout is conducive to the formation of a community rich in 

 species because great opportunity is provided for their growth at 

 different times. The prairie, rich in habitat resources, therefore, 

 can support a large number of species, in contrast to the habitats 

 of shortgrass vegetation, which are deficient, especially in soil 

 moisture. 



Phenological events are usually well coordinated with environ- 

 mental conditions in a community in the final stage of succession 

 (Figure 3-3). Such a community uses so fully the resources of the 

 habitat that it may be considered closed or saturated, so that other 

 species cannot invade; in fact, many of the species within it can- 

 not survive unless their main periods of growth, blooming, and 

 other activities, do not coincide. Even then, competition within 



Ammalytic: CHaracteristics o£ time CoMnmnimMmity • 89 



