most of tlu' rainfall comes in the winter montlis, and 

 in southern California some grasses start their 

 growth in late autumn and come into bloom in De- 

 cember, although others wait until spring. 



VEGETATION 



Grassland owes its characteristics to the 

 perennial grasses that constitute the dominant climax 

 vegetation. Annual grasses are largely confined to 

 serai stages. These climax grasses may be tall ( 1.5-3 

 m). )(;((/ (0.5-1.3 m), or short (less than 0.5 m) and 

 grow in bunches or as sod. Forbs occur mixed with 

 the grasses, and variation in the time of blooming of 

 these broad-leaved and mostly perennial herbs as well 

 as of the grasses gives the grassland a variety of as- 

 pects like the forest (\\'eaver 1954, Weaver and 

 Albertson 1956). The prairie has beauty, character, 

 and a history all its own (Craig 190Sa. Weaver 

 1944). 



Grasses grow (juickly after the onset of warm and 

 rainy weather and are adapted for long quiescent pe- 

 riods of dryness and cold. The leaves or tops of the 

 grasses die down during unfavorable seasons, but 

 underground buds regenerate new growth during the 

 next favorable period, even if this be delayed for 

 some years. After dry-season fires in tropical cli- 

 mates, new shoots sprout from perennial grass bases. 

 These provide a sparse forage for grazing animals, a 

 forage that, e.xcept for the fires, would not be present 

 until the rainy season (\'esey-Fitzgerald 1960). The 

 grasses and forbs are deeply and extensively rooted, 

 except in arid climates, where a hardpan occurs near 

 the surface (Fig. 11-4). Competition is primarily for 

 the limited water supply and only secondarily for 

 light. Since grasses grow from the base of the leaf, 

 they can tolerate considerable grazing by large her- 

 bivorous animals, and this is an important factor in 

 their dominating the prairie. 



Several of these, the most important genera of 

 grasses in the North American grasslands, occur also 

 in other parts of the world (Clements and Shelf ord 

 1939) : 



Mostly tall and Mostly short grasses 



mid grasses 

 Andropogon — blue stem Aristida — triple-awned 

 Agropyron — wheat grass grass 



Elymiis — wild rye Bouteloua — grama grass 



Festitca — fescue Biichloe — buffalo grass 



Koeleria — June grass 

 Panic II lu — panic grass 

 Poa — blue grass 

 Sporobolus — drop-seed 

 Stipa — needle grass 



I'l.ANT A.^.SOCIATIO.NS OK 

 \OHril AMKHICA 



Stipa-Sporobolus association (true prai- 

 rie): Mostly tail and mid grasses in a long strip 

 extending nortii and south in the eastern more humid 

 part of the biome next to deciduous forest. Much of 

 this prairie was marshy and poorly drained before 

 white man came (Hewes 1951). Oak-hickory forests 

 occur as scattered groves in better drained areas on 

 hills, in sandy areas, and along streams making a 

 savanna. The coastal prairies of Texas constitute a 

 faciation of this community. 



Stipa-Bouteloua association (mixed prairie) : 

 Mid grasses confined to tiie moister low areas; short 

 grasses, to drier hillslopcs. 



Bouteloua-Buchloe association (short grass 

 plains) : On Great Plains east of Rocky Mountains. 

 The climate here is so dry that mid grasses are incon- 

 spicuous except during wet years. Pronghorn and 

 bison in former days consumed mid grasses as fast 

 as they appeared, in preference to the short grasses, 

 and hence should be considered co-dominants along 

 with the grasses (Larson 1940). 



Agropyron-Festuca association (bunch grass 

 prairie) : In nortiiern half of the Great Basin and into 

 British Columbia, mostly isolated from rest of biome 

 by mountains and desert. Precipitation comes chiefly 

 as snow and rain during winter months. Dominant 

 species mostly mid grasses which grow as bunch 

 grasses. Overgrazing by domestic animals has per- 

 mitted the less palatable sagebrush and related spe- 

 cies to spread widely and give character to the land- 

 scape. 



Stipa-Poa association (California prairie) : Lo- 

 cated in central valley of California almost completely 

 isolated from rest of grassland, dominants are mostly 

 mid and bunch grasses. This is a region of winter 

 rains. Much of area is now cultivated or overgrazed 

 and contains many weedy annuals and exotic species. 



Aristida-Bouteloua association (desert plains) : 

 Most arid of grasslands, composed mostly of short 

 and bunch grasses. It occurs from southeastern 

 Texas to southern Arizona and extends well down 

 into Mexico. Because of overgrazing and control of 

 fire, desert and tropical shrubs, such as mesquite, 

 creosote bush, Opuntia cactus, are conspicuous 

 throughout the association. The sugary pods of mes- 

 quite are eagerly eaten by cattle although the bony 

 seeds resist digestion and are dispersed widely. 



Grassland biome 325 



