PLANT SUCCESSION 227 



forests are maintained indefinitely by the constantly recurring 

 fires to which the pines are resistant and which keep down hard- 

 woods. 



Disclimax.— When disturbance is such that true climax becomes 

 modified or largely replaced by new species, the result is an ap- 

 parent climax, called disclimax. The disturbance is usually pro- 

 duced by man or his animals and the introduction of species that, 

 under the existing conditions, become the dominants over wide 

 areas. The prickly pear cactus thus has formed a disclimax over 

 wide areas in Australia. A grass, Bromus teetotum, forms a discli- 

 max in much of the Great Basin where, because it burns readily, it 

 facilitates fires, which reduce dominance of desert shrubs and in- 

 crease the area of grass. The short grasses of the Great Plains were 

 long considered as climax but now are generally considered as 

 disclimax resulting from grazing and drought, which have prac- 

 tically eleminated the midgrass climax. The ravages of chestnut 

 blight illustrate how disclimax may result from disease. Oak-chest- 

 nut climax is today an oak disclimax. 



Postclimax and Vr e climax. —Ps. climatic area is normally bor- 

 dered, on the one hand, by one that is drier and warmer and, on the 

 other, by one that is moister and cooler. The contiguous climates 

 are, therefore, either less favorable or more favorable to plant 

 growth. As a result, each has its own climax, distinct in species and, 

 often, in growth form. On a large scale, this is apparent in latitudinal 

 zonation from the tropics to the arctic. Often it is noticeable in the 

 climaxes along a line from oceanic or maritime climate to the in- 

 terior of a continent. It is most conspicuous on mountains where 

 altitude produces a zonation of climates and climaxes. Each of the 

 climatic areas in such a sequence has a bordering climate with a 

 more favorable water balance, usually on the north, toward the 

 coast, or at higher altitudes; while the climate to the south, toward 

 the interior, or at lower altitudes, usually is less favorable. 



For any particular climax the contiguous climax produced by a 

 more favorable climate, usually cooler and moister, is termed post- 

 climax, and the one produced by less favorable conditions, usually 

 drier and hotter, is termed preclimax. To illustrate on a broad basis, 

 deciduous forest climax has grassland as preclimax and northern 

 conifer forest as postclimax. At the same time, deciduous forest 



