EDAPHIC OR SOIL FACTORS: PHYSICAL 



217 



Table 23. — Wilting Coefficients and Ghowth Watek in the Fohest 



Succession Near Lake Michigan, Indiana 



(After Fuller) 



Populus 

 deltoides 

 associa- 

 tion 



Pinus 

 hanksiana 

 associa- 

 tion 



Quercus 

 velutina 

 associa- 

 tion 



Q. hor talis 

 associa- 

 tion 



Fag us 

 grandi- 

 folia-Acer 

 associa- 

 tion 



Wilting coefficient: 



At 7.5 cm 



At 25 cm 



Crowth water, mean per 

 cent 



Evaporation, cubic centi- 

 meters 



Ratio between evaporation 

 and growth water 



13.5 

 9.5 



4.4 



5.3 



1.81 



The "moisture equivalent" is the percentage of moisture remaining 

 in the soil when it has been uniformly wetted and subjected, in a 

 centrifuge, to a force of 1,000 gravities (Briggs and McLane, 1907). 



The contention of many investigators that the wilting of plants, and 

 hence the value of the wilting coefficient, depends not only on the 

 regulation of the water absorption from the soil but also on atmospheric 

 influences (humidity and movement of the au-, etc.) has led Lundegardh 

 to propose a determination of the permanent wilting point at a certain 

 moisture content of the air. It is doubtful, however, if this would 

 make an appreciable difference in the wilting coefficient, since the 

 investigations of Shull (1916) have shown that the wilting coefficient 

 is really a function of the rate of movement of the capillary water in the 

 soil. 



Ground Water. — The level of the water table determines the 

 nature of vegetation, especially in areas where topographic changes are 

 in progress. Artificial changes in the ground-water level, by drainage 

 for the production of arable land, may have fatal effects upon the 

 original vegetation and lead often to accelerated succession. The 

 natural changes involved in the formation of moors or the intrench- 

 ment of streams are often more rapid than silting up of ponds and 

 lakes (cf. Godwin and Tansley, 1929). 



By the lowering of a stream bed the vegetation of the lower flood 

 plain gradually merges into that of the higher, pari passu with the 

 lowering of the ground-water level, until the climax is reached. 



