THE CLIMAX. 



109 



greater rainfall or less evaporation would continue the development beyond 

 prairie into woodland. The latter then becomes an intrinsic member of the 

 successional sequence as recorded in the series of zones. 



Changes of climate. — A change of climate can not initiate succession except 

 where extreme drouth or frost destroys essentially an entire plant commimity. 

 Practically no such instances are recorded for native vegetation, and such 

 climatic changes as we know can only continue a sere already begun or bring 

 it to a close in a stage earlier than the climax. Indirectly, changes of climate 

 may result in new areas being produced by other agencies as a consequence of 

 increased rainfall. When operating over long periods, they may produce 

 profound changes of the flora, and hence change the whole climax commimity 

 and its development. The effect of climatic oscillations may be seen from year 

 to year in the ecotone between two climatic associations. In short, the ecotone 

 is largely a record of the effects of small variations of climate. If accumulated 

 or allowed to act in one direction, the latter are sufficient to give the advantage 

 to one of the contiguous associations. In the midst of the prairie region, the 

 forest edge of the valleys yields in years of severe drouth, as in 1893-1895, 

 while ia a series of years with unusual rainfall it advances visibly. If similar 

 dry or wet conditions become permanent, the forest would gradually give 

 way before the prairie, or the latter would disappear before the forest. The 

 completeness of the replacement would depend upon the amphtude and the 

 duration of the climatic change. All timberlines, especially alpine ones, show 

 similar movements, and the latter can be recognized in all herbaceous eco- 

 tones, though with less readiness. When the change of climate favors meso- 

 phytic conditions, the existing seres are continued by the addition of one or 

 more stages dominated by higher hfe-forms. In the case of the prairie, the 

 potential climaxes in this case are deciduous forest in the east and scrub and 

 pine woodland in the west. An efficient increase in rainfall might well bring 

 these two together, and result in the prairie climax being replaced by a pine 

 climax in the present plains area and a deciduous forest climax in the prairie 

 area proper. It is far from improbable that something of the sort has happend 

 in the past. Such a contact has actually occurred in the valley of the Niobrara, 

 where Pinus ponder osa reaches its eastern limit just east of the one hundredth 

 meridian, where it is met by Juglans, Ulmus, Tilia, and other members of the 

 deciduous woodland (Bessey, 1887, 1894 : 109, Pound and Clements, 1900 : 

 322). If the swing of climate results in decreased rainfall, the potential 

 climax is found in the areas with a vegetation one stage more xerophytic than 

 the existing climax. These are the crests and ridges on which the present 

 climax has not yet established itself, or the secondary disturbed areas which 

 , are in the subclimax stage. The corresponding communities of Aristida, or 

 of Gutierrezia-Artemisia, would probably become the cUmax vegetation, 

 though certainty is impossible since the present tendency over much of the 

 prairie and plains area is favorable to scrub and woodland. 



Preclimax and postclimax. — The significance of potential climaxes is best 

 seen in the case of mountain ranges which rise directly from the plains. Such 

 are the Front and Rampart Ranges of Colorado. In these, the narrow zones 

 stand out sharply, and the effect of possible changes of climate is demonstrated 

 most clearly by east-and-west canons. On the north exposiu"e of these a meso- 

 phytic association may descend far below its horizontal limit and thus occur 



