Influence of Moisture on Growth and Distribution 125 



aspects of moisture conditions may be taken into account by calculat- 

 ing precipitation-evaporation ratios for the "frostless season," i.e., the 

 growing season for plants between the last spring frost and the first 

 autumn frost. The pattern of the distribution of these ratios in the 

 United States (Fig. 4.15) shows a general agreement with the oc- 

 currence of the major vegetation zones. The natural forest vegeta- 

 tion of the eastern part of the country occurs where the precipitation- 

 evaporation ratios are high. In the Southeast and Northeast, the P-E 

 ratio exceeds 100 per cent; that is, more rain falls during the course 

 of the average frostless season than evaporates. Incidentally, this 

 fact accounts in part for the excessive amount of soil leaching in these 

 regions. In the Middle West precipitation-evaporation ratios be- 

 tween 80 and 100 per cent exist. Values below 100 per cent indicate 

 that the potential rate of water loss is greater than the rate of water 

 supply. The line representing a P-E ratio of 60 per cent runs almost 

 due south from the Dakotas to the southernmost point of Texas. It 

 will be noted that this line agrees closely with the division between 

 the tall-grass prairies and the short-grass rangelands. The 60 per cent 

 line also approximates the position of the division between the pedo- 

 cals and the pedalfers discussed earlier. P-E ratios of less than 20 

 per cent are found in the southwestern states, and their occurrence 

 corresponds to the distribution of the desert vegetation. 



In view of the modifying influence on plant distribution of such 



Modified from Livingston and Shreve, 1921 



Fig. 4.15. The precipitation-evaporation ratio indicated as a percentage for the 

 average frostless season in the United States. 



