CLIMAX COMMUNITIES : PRESENT DISTRIBUTION 291 



extremely high summer temperatures are characteristic. Through- 

 out the formation, late summer dry spells with high temperatures 

 and drying winds are the rule, but, if there is sufficient moisture 

 for the grasses during the spring growing period and summer ma- 

 turation, such extremes affect them but little because of their long 

 period of dormancy. The hot season with limited precipitation is 

 probably of great importance in maintaining grassland climax 

 against the advance of forest. 



The increasingly severe moisture conditions from east to west 

 are accompanied by changes in the dominant species whose com- 

 binations are distinguishable as associations of the formation. Three 

 major regions are recognizable either by climate or vegetation, or 

 both. Their limits, climate, and vegetation have been summarized 

 and the important regional and local studies of grassland have been 

 listed in a concise presentation by the late Dr. J. R. Carpenter. 52 

 This condensation of grassland information could well be used as 

 the starting point for any consideration of the nature and distribu- 

 tion of grassland. The great number of classifications attempted 

 for grassland communities and the disagreements as to major dom- 

 inants and most important species implied by the terminology sug- 

 gest the complexity of the formation. Probably, too, there is a 

 suggestion of much more variation regionally than might at first 

 be supposed. Of necessity, we are restricted here to a simple pres- 

 entation. On this basis, the discussion will deal with only three 

 major associations, which may be termed Tall Grass Prairie, Mixed 

 Prairie, and Short Grass Plains. Some authorities recognize as many 

 as seven associations, 57 and, even then, most of these can be di- 

 vided into several faciations. Furthermore, a detailed discussion 

 must recognize within each faciation the usually distinct upland, 

 slope, and lowland variations. 



Tall Grass Prairie— Sometimes called "true prairie" this associa- 

 tion borders the deciduous forest, receives the most rainfall, has 

 the greatest north-south diversity and the greatest number of 

 major dominants of the association. Bunch grasses are the con- 

 spicuous species, for many of them grow in excess of six feet tall, 

 but sod-forming species are also dominants. Because of the gen- 

 erally favorable climatic and soil conditions, most of the area is 

 cultivated and little of the original vegetation remains today. 



