COMMUNITY ORGANIZATION: STRATIFICATION 



475 



three common pasture grasses (Kentucky 

 bluegrass, Bermuda grass, and orchard 

 grass), Brown (1943) found that a high 

 soil temperature (100° F.) was much more 

 harmful than a high air temperature. Sim- 

 ilarly, when grasses were exposed to eight 

 weeks of continuous air temperature of 

 100' F. and 70° F. soil temperature, the 

 plants remained nonnal in appearance, and 

 grew. 



More obvious results are observed in an 

 unusually dry period. The North American 

 prairie has three well-defined types of 

 grassland communities: tall grass, mixed 

 grass, and short grass (Clements and Shel- 

 ford, 1939; Carpenter, 1940a). The tall 

 grass type has a relatively continuous stand 

 of dominant grasses, such as Andropogon, 

 forming the herbaceous stratum matrLx, 

 with an understory of a few discontinuous, 

 low-growing grasses. The mixed grass type 

 has a more marked substratification in 

 which the herbaceous stratum has a domi- 

 nant tall grass and a dominant short grass 

 stratum. 



Weaver (1943) studied the tall grass 

 prairie of eastern Nebraska and central 

 Kansas during an unusually dry climatic 

 cycle extending over seven years (1934 to 

 1941). In 1941 the tall grass prairie had 

 been replaced by mixed grass prairie, with 

 changes in the taxonomic character of the 

 herbaceous stratum as well as a shift from 

 one to two layers of dominant grass species 

 in this stratum. Such a profound change 

 in the structure of the top stratum was 

 brought about by drought over an area 

 100 to 150 miles wide. This appears to be 

 an outstanding example of the influence 

 of drought on grassland, and with the 

 changes not attributable to man's direct ef- 

 fect upon the vegetation or his indirect 

 effect through grazing cattle. 



Temperature and precipitation also act 

 together to change grassland communi- 

 ties. There appears to be a good corre- 

 lation with weather and the abundance 

 of such important insects as chinch bugs. 

 Shelf ord and Flint (1943) found these 

 insects to be sufficiently abundant to 

 cause crop damage in grassland maintained 

 by man in the upper Mississippi valley in 

 periods when temperature was above and 

 rainfall below normal. 



Human influences upon the grassland 

 commimity are obvious and important. It 



is especially well shown through their ef- 

 fects upon the structure of the several 

 strata. In addition to obliteration of the 

 community by human dwellings, man's in- 

 fluence runs the gamut from chance pas- 

 ture in vacant lots to the complete rebuild- 

 ing of grassland, with cultivated plants 

 forming the equivalent of native tall grass, 

 and beef or dairy cattle the equivalents of 

 prongbuck and bison. 



Such rebuilding of grassland by man 

 amounts to maintaining substituted equiva- 

 lents that can be consumed or profitably 

 manipulated, on areas that would other- 

 wise support a competitive grassland com- 

 munity. 



Another type of grassland develops in an 

 area that would otherwise support dif- 

 ferent communities— for example, forests. 

 The cutting-off of the forests in the cor- 

 ridor of states from New York to Indiana 

 has produced an artificial steppe, now be- 

 ing actively invaded by some of the steppe 

 fauna. Another example is aflForded by 

 some 126 million acres of western United 

 States that are in range service. Within 

 forests (Campbell, 1940), the problems of 

 cattle grazing versus timber growing, vir- 

 tually side by side, have not yet been fully 

 harmonized. Forest grazing is a definite 

 part of Louisiana's cattle-raising industr)^ 

 In a survey of 118 farms where cattle were 

 grazed in forests, more than half of the 

 herds grazed in small open glades or mead- 

 ows between trees for the entire year 

 (Campbell and Rhodes, 1944). During this 

 period the cattle obtained 69 per cent of 

 their food from these grassland inclusions. 



Overgrazing is an uneconomic and bio- 

 logically inept human influence upon grass- 

 land. One immediate result is the deteriora- 

 tion of the basic food supply for the graz- 

 ing population. When either overcultiva- 

 tion or overgrazing, or both, are practiced, 

 the floor of the community is eroded away. 

 This is followed by erosion of the surface 

 layers of the subterranean stratum imtil the 

 entire grassland community is destroyed 

 and its regrowth prevented until the soil 

 cover can again be formed. 



A by-product is the destructive dust 

 storm (Clements and Chaney, 1936). 



Often the results of overgrazing are in- 

 direct, and are sometimes beneficial to 

 native grassland species. In eastern Texas 

 the jackrabbit (Lepiis californiciis merri- 



