horses will not. In tlie middle of protecting tliickets 

 of briars, hawtliorns. and redcedar snch deciduous 

 trees as elm, ash, tuli])tree, sycamore, and oak come 

 in. After a few years they grow beyond the reach of 

 animals, shade out the briars, hawthorns, and red 

 cedar, and establish a forest dominance. Where left 

 undisturbed by man, tiie succession of native vegeta- 

 tion will thus bring about the elimination of domestic 

 animals from the area and replacement with the biotic 

 climax natural for the region. 



In western areas too dry for deciduous forest, 

 overgrazing reduces the vigor and abundance of the 

 taller climax grasses, and the short grasses that are 

 less easily grazed are favored. Unpalatable herbs, 

 sagebrush, cacti, and mesquite may also replace 

 grasses over extensive areas. Although native animah 

 such as the bison and pronghorn may have heavily 

 grazed the original prairie in locally arid regions, the 

 result was less drastic than that produced by the 

 heavy concentrations of grazing stock on our farms 

 and ranches at the present time. When the most 

 favored vegetation was reduced, native animals com- 

 monly dispersed into other areas so that the carrying 

 capacity of the land was not critically reduced. 



Burns 



Prairie fires, frequently started by lightning or 

 by Indians, were doubtless important in preventing 

 deciduous forest from succeeding grassland in parts 

 of the middlew^est. More lately, fires are started by 

 careless campers or travelers. Fires are especially 

 destructive in coniferous forests, as the clinging dry 

 needles encourage crown as well as ground fires to 

 develop. Many thousands of square miles of forests 

 are burned over annually. 



The extensive pure stands of longleaf pine on the 

 coastal plain of the southeastern states are probably 

 a consequence of ground fires that regularly occurred 

 at intervals of 3 to 10 years before white men came. 

 The terminal bud of the longleaf pine is well pro- 

 tected by a thick covering of green leaves, one of 

 several characteristics that make the species ex- 

 tremely fire resistant (Chapman 1932). Fire de- 

 stroys all seedling hardwood trees as well as other 

 species of conifers. 



When coniferous forest is destroyed by fire, the 

 first trees to invade are usually quaking aspen, paper 

 birch, and sometimes balsam poplar. These forests 

 cover extensive areas in Canada and southward on 

 the Rocky Mountains. Jack pine in the north and 

 lodgepole pine in the western mountains either come 

 in with the deciduous trees or succeed them. The 

 cones of these two trees take several years to open 

 and shed the seeds held within, and may not do so 

 at all unless heated by forest fires. Aspen and pine 

 are eventually replaced by the climax forest. In many 



TABLE 8 6 Breeding bird pairs per 40 hectares (100 acres) in 

 sere developing on abandoned fields, Georgia Piedmont region, 

 averaged fronn two stations in horb-shrub (I, 3 years old), 

 three stations in grass-shrub-tree (IS, 20,25 years old), four 

 stations in pine forest (25, 35, 60, 100 years old), and one 

 station in oak-hickory (over 150 years old) (condensed from 

 Johnston and Odum I9S6). 



Grass- Oak- 



Herb- shrub-tree Pine hickory 

 Bird species grass (forest-edge) forest climax 



Rock, sand, and clay 



117 



