CHAPTER XI 



THE DISTRIBUTION OF CLIMAX COMMUNITIES 

 SHIFTS OF CLIMAXES WITH TIME 



The present distribution and limits of climax communities are 

 not necessarily static, nor have they been in the past. Looked at in 

 terms of geological time, changes of climate must be recognized 

 that were so extreme that vegetation must likewise have changed 

 radically. Within relatively recent geological time, glaciation of 

 northern North America obviously must have produced such 

 changes in climate that disruption of then existing lines of vegeta- 

 tional distribution were inevitable. Advance of the ice southward 

 resulted in constriction of vegetational zones and retreat of species 

 and growth forms as the climate changed. With the recession of 

 the ice, there was again a northward advance of species and a re- 

 adjustment of plant communities as the glaciated area was reoc- 

 cupied by vegetation. Probably there were several minor advances 

 and retreats of vegetation correlated with the shifting ice fronts 

 and the similarly varying climate. 



Within historical time, there have been major shifts of climate 

 producing conditions that may have had serious effects on vege- 

 tation. There is evidence that early Norsemen who colonized 

 Greenland were able to carry on a primitive sort of agriculture on 



lands alone the southern coast. Between the twelfth and the four- 

 th 



teenth centuries the climate there deteriorated rapidly so that 

 summers became shorter and colder, the soil remained frozen, and 

 the colonists disappeared. Today, as for some time past, the reced- 

 ing glaciers in Greenland indicate an increasingly favorable cli- 

 mate. Receding glaciers in Alaska have been similarly interpreted. 76 

 In recent years, conifer forest has been advancing into the tundra 

 in Alaska. 110 Periodically, prairie vegetation is invaded for some 

 distance by forest, and although drought often eliminates such ad- 

 vances, they may be permanent or, at least, appear so. 



That climates have changed over long periods of time cannot 

 be questioned, and that slow change continues today in certain 



300 



