228 THE STUDY OF PLANT COMMUNITIES • Chapter IX 



holds a postclimax relationship to grassland that has desert as pre- 

 climax. The use of the concept is not restricted to formations as 

 illustrated above since it is just as applicable to associations, even 

 within the same formation. For example, within the deciduous 

 forest formation oak-hickory is preclimax and hemlock-hardwood 

 is postclimax to the beech-maple association. Likewise, oak-hick- 

 ory is preclimax and beech-maple (or hemlock-hardwood) is post- 

 climax to the oak-chestnut association. 



Should the present phase of relatively stable climates be inter- 

 rupted, the climate of any given area would undoubtedly tend to 

 become more like that of one of its contiguous areas and a migra- 

 tion or shift of climax would result. Such a shift occurred during 

 the glacial period when the northern coniferous forest moved 

 southward, and the northern extent of the deciduous forest was 

 proportionately constricted. When the climate ameliorated, the 

 ice receded, and again, the ranges of the climaxes were readjusted. 

 When such shifts occur, remnants of the previous dominants are 

 left behind in locally favorable habitats where they may maintain 

 themselves indefinitely as relicts of a previous climax. These relicts 

 are either preclimax or postclimax depending upon their relation- 

 ship to contiguous climaxes and the direction of the climatic shift. 

 The habitats in which they survive must have edaphic or physio- 

 graphic characteristics that differ so markedly from the average 

 for the region that conditions for growth are similar to those of a 

 contiguous climatic area. Deep valleys or canyons with steep bluffs 

 and contrasting exposures, poorly drained flood plains, bogs, ridges 

 of rock or gravel, areas of deep sand or other peculiar soil condi- 

 tions are specific examples. 



Where there have been shifts of climax, it is apparent that pre- 

 climax and postclimax communities should occupy such habitats. 

 Not all preclimax and postclimax communities, however, need be 

 relicts. Within the general range of a climax, there are bound to be 

 local habitats such as those mentioned above that will continue 

 indefinitely to be somewhat more favorable or less favorable, wet- 

 ter or drier, than the conditions controlled by climate in the region 

 as a whole. As a result, when vegetational development proceeds 

 to a condition of stability on such a site, it will have characteristics 

 of the contiguous more or less favorable climate. Such localized 

 stable communities are likewise postclimax or preclimax for the 



