246 THE STUDY OF PLANT COMMUNITIES • Chapter X 



Grassland as precipitation : evaporation ratios become less favor- 

 able. The irregular line of transition runs northward from eastern 

 Texas with thirty-five inches of precipitation, to central Minne- 

 sota where precipitation falls to twenty-five inches. 



The great extent of the deciduous forest includes soils and to- 

 pography of diverse nature and origin. The northern portion was 



FIG. 118. Sugar maples (160-200 years old) in beech-maple forest associa- 

 tion, Pennsylvania— U. S. Forest Service. 



glaciated. There are mountains in the east. The great valleys of 

 the Mississippi and Ohio Rivers are included as are the Piedmont 

 Plateau and coastal plain of the Atlantic and Gulf coasts. Any and 

 all kinds of topography as well as soil types are, therefore, repre- 

 sented. 



Climate is temperate with distinct summer and winter, and all 

 parts are subject to frost, one of the few environmental factors 

 that applies throughout. Precipitation varies from sixty inches in 

 the southern mountains to less than thirty inches northwestward, 

 but it is everywhere fairly well distributed throughout the year. 

 The ratio to evaporation is most favorable in the north, the east, 



