108 STABILIZATION AND CLIMAX. 



such final dominants as Picea or Fagus are prevented from spreading through- 

 out a natural region. Thus Pinus and Qu&-cus have formed or still form sub- 

 climaxes in areas ultimately to be occupied by beech or spruce. In the valleys 

 of the Missouri Eiver and its tributaries in Nebraska, as elsewhere along the 

 western margin of the Mississippi Basin, the forest is in a subclimax stage com- 

 posed of Quercus, Hicmia, and Juglans. Further westward, the valley woodland 

 is a subclimax formed by a still earlier stage composed of Populus and Salix. 



Subclimaxes due wholly or partly to the activities of man are numerous. 

 Conspicuous causes are burning, clearing, and grazing. These produce sub- 

 climaxes in a particular area by disturbance and destruction of the com- 

 munity. This results in subclimaxes in adjacent areas in consequence of 

 destruction of the source of migrules. Grassland areas are produced the world 

 over as a result of burning and grazing combined, and they persist just as long 

 as burning recurs. Woodland is frequently reduced to scrub by fire, and the 

 scrub often persists wherever repeated fires occur. Even when fires cease with 

 the settlement of a region, grassland and scrub subclimaxes persist for a long 

 time because of the more or less complete removal of the forest. The clearing 

 of the forest in connection with Imnbering or cultivation may result in more 

 or less permanent scrub. When clearing is followed by fire or grazing or by 

 both, as is often the case, the scrub may be entirely replaced by grassland, 

 which remains as a subclimax as long as the causes are effective; or it may 

 persist almost indefinitely in consequence of the removal of natvu'al forest and 

 scrub from the region. In the case of silviculture! activities, it is evident 

 that any forest stage may be fixed as a subclimax, or that a new climax may 

 be produced artificially by the planting of exotics. Similar modifications are 

 possible in the treatment of natiu-al grassland. The final climax in a grass- 

 land region, such as that of the Great Plains, may be inhibited by fire or 

 grazing. The area may remain for a long time in a grass subclimax, such as 

 the Aristida consocies, or it may show an undershrub climax of Gutierrezia 

 and Artemisia (plate 31, a, b). 



Potential climaxes. — As has been stated previously, zones of vegetation 

 indicate the changes of vegetation possible in consequence of a change of 

 climate. This is fairly evident in the case of zones which correspond to marked 

 differences in latitude or altitude, but it is equally true of other great zones, 

 such as the prairie, plains, and interior basin of North America. These are 

 aU responses of vegetation to a progressive change in the controlling factors, 

 as is true of the more striking zonation of ponds, streams, islands, etc. The 

 regional zones are produced by the cimiulative change of climatic factors in 

 one direction, while the local zones are due to the gradual change of water- 

 content, often in consequence of reaction. The latter are independent of 

 chmate to the extent that they exist beside each other, but they are only 

 records of a development which comes increasingly under climatic control 

 with every step away from the original extreme of soil conditions. The zones 

 of a prairie lake are the result of the reaction control, or what might be called 

 the habitat control, of succession, but the paramount part of climate in the 

 development is shown not merely by its setting the usual climax Unfit, but by 

 the fact that it can fix an earher or later limit. Normally, the stages of inva- 

 sion end with the outermost zone, since this is the climax in which the new 

 area for development has been set, but a change of climate in the direction of 



