[Chap. LIIl THE VEGETATION OF NORTH AMERICA 749 



glacier that now covers Greenland. The trees of the forest survived about 

 the edges of the several successive ice masses; and as the masses melted, 

 the forest spread into its present transcontinental region. 



The tundra plants survived the glacial periods on the northern islands, 

 on transitory areas not occupied by ice, and near the coast, just as they 

 today live on the fringes of land encircling the Greenland ice. The poor 

 drainage, shallow soils, and frequent ponds and lakes are also the results 

 of glacial erosion and deposition. Irregularities result from the unequal 

 deposition of rock material in the ice at the final melting, and from the 

 meltine of buried, detached ice masses after the retreat of the main 

 masses of the glaciers. 



Tundra and boreal forest are examples of climatic plant formations. 

 Each is a large area of nearly homogeneous vegetation limited in its 

 occurrence by a group of characteristic climatic conditions. The tundra 

 is covered by a mixed population of perennial herbs and low shrubs, 

 which vary in proportion from one locality to another; but organized 

 communities are indistinct. The boreal forest, on the other hand, has 

 well-defined communities of specific tree, shrub, and herbaceous layers, 

 with mosses and lichens in all layers. There is also more diversity of 

 communities on sand, on clay, and on rock exposures; on alkaline and 

 acid soils; and on well-drained soils in contrast to wet, poorly aerated 

 soils. 



On burned-over lands there is abundant evidence of recent succession 

 from bare soil to annual and perennial weeds, to aspen thickets, or to 

 alder thickets on wetter soils, followed by jack pine or balsam poplar, 

 and finallv to white spruce and balsam fir. Bogs and muskeg, first cov- 

 ered by sphagnum and other mosses, are invaded by black spruce and 

 tamarack and in less acid situations by arbor vitae; and in the most 

 mature communities white spruce eventually becomes dominant. The 

 youngest flood plains have sedge, rush, and grass communities, soon fol- 

 lowed bv willow and alder thickets. These in turn may be succeeded by 

 balsam poplar communities, and followed by mixtures of balsam poplar 

 and white spruce; eventually white spruce becomes dominant. 



White spruce may thus predominate in the final stage in the develop- 

 ment of vegetation which started with a variety of different communities 

 on such diverse habitats as flood plains, sand plains, bogs, and muskeg. 

 For this reason white spruce is regarded as a climax tree; and since a 

 comparable sequence occurs in similar habitats from Alaska to Labrador, 



