2 44 THE STUDY OF PLANT COMMUNITIES * Chapter X 



mixture or in alternating stands. The transition to grassland in the 

 Middle West is marked by aspen (Populus tremuloides) llb in a 

 band some fifty miles wide. In spite of fluctuations produced by 

 fire, grazing, and drought, the trees persist and, in some instances, 

 seem to have advanced into the grassland. In the west, along the 

 Rockies, the subalpine Abies lasiocarpa is associated with Pice a 



FlG. 116. Interior of red spruce-Fraser fir forest in the southern Appala- 

 chians. Compare with Fig. 113.— U. S. Forest Service. 



glauca, and northward in Alaska there is a merging with the 

 northwestern coastal forest. 



Appalachian Extension — -On the higher mountains of the Ap- 

 palachian system, the northern conifer forest extends as far south 

 as the Great Smoky Aiountains of North Carolina. The growth 

 form and associated species are in every way similar to the main 

 body of the formation, but, from New Brunswick southward into 

 New England, red spruce (Picea rubens) tends increasingly to re- 

 place white spruce. Still farther south, Fraser fir (Abies fraseri) 

 takes the place of balsam fir so that the dominants in the southern 

 Appalachians are ecologically equivalent to those elsewhere in the 

 formation but are taxonomically distinct. It seems reasonable to 

 consider the Appalachian extension as a distinct association whose 

 limits are marked by Picea rubens. A northern and southern facia- 



