CHAPTER XXI 



THE DISTRIBUTION OF PLANT COMMUNITIES IN 

 THE UNITED STATES 



The present and past distribution of plants over the surface of the 

 earth and the causes thereof constitute the science of phytogeography 

 or plant geography. This is a science that is closely related to plant 

 ecology and it is, therefore, important that a student of ecology 

 should know something of it. It is a large subject in itself, however, 

 and space does not permit us to treat of it fully here. All that will be 

 attempted in this chapter, therefore, is a brief and necessarily incom- 

 plete account of the present distribution of plant communities in 

 the continental United States. 



143. Deciduous Forest Communities.— The deciduous forests of 

 the north temperate zone occur in regions where the annual rainfall 

 is between 25 and 60 inches with more than half of it falling during 

 the warm months. In the United States the deciduous forest com- 

 munities of varying floristic composition occupy nearly all of the 

 area east of the Mississippi River and north of central Louisiana, 

 Alabama and Georgia and the coastal areas of North and South 

 Carolina, with the exception of the northern part of Maine, the 

 higher parts of New York, New Hampshire and Vermont, the north- 

 ern parts of Michigan and Wisconsin and the central part of Illinois. 

 West of the Mississippi these forests occupy most of southeastern 

 Missouri, nearly all of Arkansas and much of the eastern parts of 

 Oklahoma and Texas. Long tongues of deciduous forests also extend 

 along all of the tributaries of the Mississippi that flow from the west 

 and northwest. 



All of the deciduous forests are considered as belonging to the 

 same formation but there are at least three types of climax associa- 

 tions due to the differences in climate in different parts of the area 

 occupied by the formation. The beech-maple forest is typical of 

 nuich of central Wisconsin and Michigan, northern Pennsylvania, 

 central and southern New York and central and southern New Eng- 

 land. The two most important dominants are the hard maple 

 Acer saccharum, and the beech, Fagus grandifolia. These two 

 trees are the most tolerant of shade of all deciduous trees of this 

 formation. The yellow birch, Betula lutea, and the linden, Tilia 

 ( 238 ) 



