Influence of Moisture on Growth and Distribution 123 



Differential ability to withstand drought will also play its role in 

 controlhng geographical distribution. If the soil moisture drops be- 

 low the permanent wilting coefficient, most annuals and other tender 

 plants will die within a matter of hours or days unless the soil water 

 is renewed. On the other hand, succulents and plants that can 

 aestivate are able to withstand varying periods of drought extending 

 into years with some species. Thus the duration of drought periods 

 is of crucial importance in determining whether a region may be in- 

 habited by different plant types. 



Local differences in the moisture factor were clearly shown to in- 

 fluence the species composition of the forest vegetation in a study 

 made in Indiana (Potzger, 1939). A series of observations in a line 

 running from north to south over a ridge revealed the fact that soil 

 moisture was 20 per cent higher on the north side of the ridge than 

 on the south side. Moreover, the rate of evaporation was more than 

 twice as great at stations on the south slope than at those on the north 

 slope. Quadrat counts of the number of trees on the two sides of the 

 ridge gave the tabulated results. Although other ecological factors 



undoubtedly played some part, moisture conditions were chiefly re- 

 sponsible for the very pronounced differences in the occurrence of 

 the various species on the two sides of the ridge. 



Another example of the control of distribution by the moisture fac- 

 tor, and one on a much larger scale, is found in the regional occurrence 

 of the natural vegetation in the United States. A glance at Fig. 4.13 

 will reveal the fact that in the central part of the country, leaving 

 out of account the irregularities due to the mountains, the zones of 

 principal vegetation types tend to run north and south. In the eastern 

 part of the continent the typical natural vegetation is forest (in areas 

 where man has not interfered). West of the forest zone is a belt of 

 tall-grass prairie, and beyond that, running from Montana and North 

 Dakota down to Texas is a zone of short-grass rangeland. Still farther 

 westward lies the desert belt. These fundamental zones are indicated 

 schematically in Figure 4.14. 



This zonation is obviously not controlled primarily by temperature 

 since the temperature belts are seen to run generally east and west. 



