Cnapter 16 



THE GYMNOSPERM TREES 



There are three principle types of land vegetation — forests, 

 grasslands and deserts. The type predominating in a given region 

 and the variety of the species comprising it, are dependent 

 largely upon the fact that different geographical areas present 

 varying environments which exclude certain kinds of plants and 

 favor others. Two factors which vary widely over large land areas 

 are climate, especially temperature and rainfall, and soils. Tree 

 growth is closely correlated with ample rainfall during the warm 

 growing season, and a winter without too dry air or high winds. 

 Where the soils are favorable, Angiosperm trees make up the 

 forests of warmer temperate portions of the United States, being 

 deciduous in habit in the region between the Mississippi River 

 and the Atlantic coast. In the cooler temperate regions, however, 

 the common forest trees are frequently Gymnosperms of the 

 conifer group. These are at their best on mountain slopes and 

 highlands where the severity of winter living conditions prevents 

 Angiosperm tree growth; they also thrive on sandy and rocky 

 soils where most Angiosperm trees cannot grow. This fact has led 

 to their being widely used in reforestation work, especially on 

 rugged watershed areas where the soils are too poor to support a 

 good growth of broadleaf or Angiosperm trees which require 

 twice to three times as much nutrient materials in the soil. 



Conifers are so dominant in occurrence and economic impor- 

 tance that one often thinks only of coniferous plants when 

 speaking of the Gymnosperms. This group of cone bearing trees, 

 though world wide in its distribution, is best developed in the 

 cooler regions of the north temperate zone, where nearly pure 

 forests of coniferous trees cover widespread areas. The tendency 

 for conifers to grow in nearly pure stands greatly enhances their 

 economic importance to man, since it facilitates the exploitation 



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