CHAPTER 2 



RELATION OF PLANT GOI'IMUNITIES 

 TO MOISTURE SUPPLY 



The relation of plant communities to moisture supply is 

 one of the outstanding characteristics of the growth of natural 

 vegetation. While individual species are largely restricted to 

 favorable physical environments, the principal condition that 

 governs the distribution of vegetative groups is the amount of 

 available moisture. Each species responds to individual water 

 conditions for its most favorable growth and its widest distri- 

 bution. As expressed by Shantz (25), "One of the most success- 

 ful correlations yet attempted is that between plant associations 



and the water content of the soil. This correlation has 



been accepted and modified by leading ecologists and has proved 

 one of the most useful generalizations in the study of vegetation.'' 



Temperatures, moisture, and the chemical and physical 

 properties of the soil are contributing factors in the distribu- 

 tion of natural vegetation. However, the quantity of water avail- 

 able for plant use and the effect of plant gro'/rth on supply, are 

 of great interest to the hydrologist. Soil texture and salinity, 

 as well as moisture content, have been correlated with distribu- 

 tion of native growth as indicators of the adaptability of un- 

 cropped land to agricultural possibilities; but to those interest- 

 ed in water supplies, rather than soil, the consumptive use of 

 water by natural vegetation and the residual water available for 

 recovery are of greater economic importance than other character- 

 istics. 



Bowman (6) has said: "It is found that each species of 

 plant requires its own specific water supply for most favorable 

 condition of growth and that the quantity of water in the soil 

 has a greater influence than any other condition on the distribu- 

 tion of plant species." In the absence of ground water, then, the 



1/ Numbers in parentheses refer to Literature Cited, p. 147. 



