CLIMAX COMMUNITIES : SHIFTS WITH TIME 



313 



nants of the former vegetation. Such relicts may be found in 

 fence rows, along railroad right-of-ways, in old cemeteries, and 

 in any areas long undisturbed and may yield much information 

 about the past. The deliberately protected areas, such as game and 

 wildlife preserves, natural areas, Indian reservations, and national 

 parks, offer still more possibilities because of their extent, fre- 

 quently included virgin areas, and relative permanence. 



FlG. 169. Postclimax community of ponderosa pine occurring as an iso- 

 lated island in sagebrush desert wherever the special local soil conditions exist 

 in Nevada. Often disjunct from nearest ponderosa pine forest of the Sierra 

 by fifty miles or more .— Photo by W. D. Billings. 22 



Relicts related to climatic change are most abundant in the 

 transitions from one climax to another but may likewise be found 

 well within the general range of a climax, provided the local con- 

 ditions are present that maintain the necessary compensating fac- 

 tors. Usually the local conditions are a result of topography, 

 which, through its effects on precipitation, drainage, tempera- 

 ture, and air movements, permits the relict to survive. The result- 

 ing relict communities are the postclimaxes and preclimaxes pre- 

 viously discussed in detail. With an understanding of the concept 

 of postclimax and preclimax, their presence greatly simplifies the 

 interpretation of shifts of climate and climax in the past. The pres- 

 ent condition of the relict, if free from disturbance, may furnish 



