Southeastern Washington and Adjacent Idaho. 21 



community instead of a climax one. Like the society it is a case 

 of subdominance within a dominance. 



The following table shows the relation of climax and serai or 

 development units to the formation, as well as the relation of 

 the units of each series to each other and the correspondence of 

 units in each series. 



FORMATION 



Climax Units: Serai Units: 



Association Associes 



Consociation Consocies 



Society Socies 



EVAPORATION AND SOIL MOISTURE IN RELATION TO 

 SUCCESSION 



Without question, the water relations of the habitat are the 

 most important cause of succession. This is especially true of 

 grass land communities, and even in forests where light is a con- 

 trolling factor, it is well known that the water relation has a 

 marked effect upon the tolerance of forest trees. As Fuller (4, 

 5) has pointed out, in a study of water conditions, two phases 

 of the subject are of importance. 



They are the direct source and amount of the supply and the region 

 and cause of the loss. The latter is a climatic, the former largely an 

 edaphic problem, for it is evident that plants derive their moisture from 

 the soil and lose it into the air, and for the quantitative solution of these 

 problems it is necessary to measure the power of the air to extract water 

 from the plant; in other words, the evaporating power of the air, and the 

 amount of moisture in the soil available for the use of plants. 



Fuller (4, 5) and others have pointed out that the evaporation 

 rates in the lower aerial stratum and the range of water content 

 in the upper subterranean stratum are the most critical ones. 



Within this (aerial) stratum develop all the seedlings, and upon their 

 death or survival depends the character of the succeeding vegetation. 

 Therefore, if the vegetation of an association so affects the evaporation 

 rate of this stratum that it permits the survival of seedlings of more 

 mesophytic species, it is evident that a more mesophytic association will' 

 develop, and succession will be accomplished. 



Measurements of the water content of the soil and the evap- 



21 



