376 



PAST succession: the ceneosere. 



while it was probably but slight or local in the ranges of New Mexico. The 

 diversity of slope and exposure, moreover, must have led to the frequent 

 occurrence of non-glaciated areas alongside of glacial valleys and cirques. 

 Whether the alpine and montane floras were differentiated from the arctic and 

 boreal before the glacial period or as a result of it is in doubt, but there can be 

 no doubt that the mountain vegetation showed a definite series of zones, 

 which were forced to migrate in accordance with the movements of the ice. 

 Both the zones and their movement, however, were less regular and massive 

 than the continental ones, owing to the influence of mountain topography. 



Seres and coseres. — ^It is obvious that priseres and subseres were in the 

 course of development in the timdra, bog, conifer, and hardwood zones before 

 the first advance of the ice. While those in the tundra would have been over- 

 whelmed by the ice, those of the other zones would have continued their 

 development to end in a preclimax rather than the normal climax, i. e., the 

 scrub seres would have stopped in a tundra preclimax, the conifer seres in 

 bog scrub, and hardwood ones in coniferous forest in the northern part of the 

 zone. A further ice advance would overwhelm the tundra climax of the 

 original scrub zone, and substitute a new tundra climax for the scrub of the 

 original coniferous zone, etc., while a still further advance would destroy this 

 new ttmdra climax and produce another tundra on the scrub which had re- 

 placed the coniferous zone invading the deciduous forest. South of this last 

 tundra would he the new zones of bog-scrub and coniferous forest (fig. 37). 

 This is upon the assumption that the ice advanced gradually enough to permit 

 the development of seres while each zone was occupied by the successive pre- 

 climaxes. This must have been the case, it would seem, for a rapid advance 

 would have overwhelmed the zones before they could be shifted by migration. 



^ Hardwood - Conifers ^ Scrub < Tljndra_ 



/ / / A 



, Hardwood ^ Conifers ^ Scrub . Tundra_ 



Hardwood .Conifers Scrub Tundra 



/ / / / 



Hardwood Conffers Scrub Tundra 



■^ V, "«v V. 



Hardwood Conifers Secdb 



"^ V. -^ 



Hardwood Ocoiffers Scrub /^ Tundra 

 "*«* X 'S, 



Hardwood Conifers Scrub 



Fig. 37. — Diagram showing the successive positions of the zones of the cUsere during a 



glacial-interglacial qrcle. 



At the beginning of the interglacial phase the four climaxes would lie in 

 zones in front of the ice-mass, and seres would be in active development in 

 each. With the melting of the ice, a short tundra sere would develop on the 

 exposed ground-tUl, while the seres of the timdra climax at the edge of the ice 

 would pass into the bog-scrub as the shrubs invaded. The bog-scrub seres 

 would likewise pass into a postclimax of conifers, and the coniferous seres into 

 a postclimax of hardwoods. With the advance of the climax zones in the wake 

 of the ice, each sere would terminate in a new climax, imtil the positions held 



