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Southern Limit . Essentially northern species whose 

 southernmost distribution extends into the Bay region: e.g., 

 black ash ( Fraxinus nigra ) . 



Southern Outlier . Populations in the Bay region that are 

 disjunct from the southern continuous populations to the 

 north: e.g., balsam poplar ( Populus balsamifera ) . 



East e rn Outlier . Species whose distribution is primarily 

 midwestern extend eastward as disjunct outliers: e.g. chinkapin 

 oak ( Quercus muehlenbergii ) , shumard oak (Quercus shumardi) , and bur 

 oak ( Quercus macrocarpa ) . 



Coastal Plain Outlier . Upland species characteristic of the 

 Appalachians are occasionally found in small colonies deep in the 

 coastal plain over a hundred miles from the nearest upland popula- 

 tion; e.g., white pine ( Pinus strobus ), hemlock ( Tsuga canadensis) , 

 and rhododendron ( Rhododendron maximum ) . 



Regardless of their nature, these populations are of far 

 greater importance than as mere geographical curiosities. Any 

 organisms living on the edge of its range is operating at the limit 

 of its adaptation to its environment as well, and it may be parti- 

 cularly sensitive to environmental stresses with which it can cope 

 in the center of the range. If we are to understand the ecological 

 amplitude of any species, it must be studied under extreme conditions 

 as well as optimal ones. For this reason, a few acres of scraggly 

 hemlocks on the eastern shore may be worth a hundred acres on the 

 Blue Ridge. These range phenomena have been located as precisely 



