Grassland presently occurs on all the continents, 

 and at one time covered 42 per cent of the earth's 

 surface. Grasslands everywhere possess marked simi- 

 larities in points of climate, physiognomy, and ani- 

 mal mores. In Russia this community is termed the 

 steppe, in Hungary, the puszta, in South Africa, the 

 veld, and in South America, the pampas (Carpenter 

 1940). In North America the tall, dense grasslands 

 with their rich fertile soils in the eastern portion of 

 the biome are called prairie; in the West, the short 

 grasses and shallow soil characterize the plains. 



In North America, grasslands extend from north- 

 ern Saskatchewan and Alberta and central British 

 Columbia to central Mexico, and from Indiana to 

 California. The eastern portion is a huge expanse, 

 continuous except for forest strips in the river valleys, 

 but the continuity of the western portion is broken 

 by the many mountain ranges. In general, the ter- 

 rain is flat or rolling, green in summer and brown in 

 autumn and winter (Weaver and Clements 1938, 

 Clements and Shelford 1939). 



CLIMATE 



25 



Geographic 



Distribution of 



Communities : 



Grassland Biome 



Precipitation in North America may be as 

 high as 100 cm (40 in.) per year adjacent to decidu- 

 ous forest, but trees cannot spread into the grassland 

 because of high rates of evaporation, late summer and 

 autumn droughts that are particularly severe and 

 prolonged during some years, and intermittent fires 

 that kill seedling trees but from which grasses quickly 

 recover. Rainfall decreases and becomes more irreg- 

 ular and evaporation increases in a gradient from 

 East to West or Southwest, because of the general 

 pattern of air circulation over the continent (Kincer 

 1923, Borchert 1950). An isohyet of 2-3 cm (5-8 

 in.) precipitation separates grassland from desert. 



Few species of grasses can tolerate the entire 

 range of precipitation, and differences in the moisture 

 requirements of species is the main reason for the 

 subdivision of the biome into its various plant asso- 

 ciations. There is little snow even in northern por- 

 tions of the grassland, but winds are normally heavy, 

 and there occasionally are severe winter blizzards. 



Temperature is not as critical a factor as mois- 

 ture, as is evident in the great North-South distribu- 

 tion of grasslands. Temperature, however, helps to 

 separate temperate grasslands from tropical grass- 

 lands. In the North, mean monthly winter temper- 

 atures drop to — 15°C (5°F), while summer temper- 

 atures in the South may exceed 32°C (90°F). There 

 are great seasonal and daily ranges of temperature. 

 The frostless period in the North may be only 100 

 days long, but in the South frost rarely or never 

 occurs. In the California and bunch grass prairies. 



324 



