PHYSIOGRAPHIC FACTORS 



185 



justs itself to changes in the soil and permits growth under a 

 variety of conditions. Some can tolerate only small amounts of salt 

 and do better in its absence, while a few others absolutely require 



Fig. 86. Margin of a saline flat in the Smoke Creek Desert, Nev. The 

 shrub at the margin is the relatively salt-tolerant greasewood (Sarcobatus 

 vermiculatus). Extending farther into the playa is salt grass (Distichlis 

 stricta), which is more tolerant but soon also fades out until nothing grows 

 over most of the area— Photo by W. D. Billings. 



salt to survive, some even requiring a fairly high concentration. 

 The extreme in salinity is illustrated by portions of the Great Salt 

 Lake area in Utah where salt concentrations are so great that no 

 vascular plants can grow. 



TOPOGRAPHY 

 Although topography affects vegetation indirectly by modify- 

 ing other factors of the environment, it has nevertheless a signifi- 

 cant influence upon all plant communities. If an area is so level that 

 topographic variations are practically nonexistent, then, other fac- 

 tors being equal, uniform vegetation may be anticipated through- 



