224 Forestry Quarterly. 



flexilis), are found as' scattered trees or in small groups, so rare 

 that they long escaped notice. 



White spruce (Picea canadensis) occurs in a few solid bodies 

 on the slopes of the highest hills. Elsewhere it is largely con- 

 fined to canyon bottoms and steep north slopes in mixture with 

 other species. 



Red cedar (Juniperus sp.j is scattered about the foothills, 

 chiefly in the Wyoming section. The form in the Hills seems 

 intermediate between the eastern and the western Red cedar. 



There are a large number of hardwoods, all of minor impor- 

 tance. The list comprises such species as : elm (Ulmus Ameri- 

 cana), hackberry (Celtis occidentalis), Bur oak (Qiierciis macro- 

 carpa), boxelder (Acer negundo), Paper birch (Betula papyri- 

 fera), Red ash (Fraxinus pennsylvanica) , Trembling aspen 

 (Populus tremuloides), cottonwood (Populus deltoides), and 

 others'. There are several other species but they are too insig- 

 nificant to mention. 



Bur oak is the only hardwood reaching any size. Occasionally 

 in stream bottoms in the foothills it becomes large enough for 

 railroad ties. 



All the hardwoods, with the exception of aspen and paper 

 birch, and in a few sections Bur oak, are confined to the stream 

 bottoms or the vicinity of water. They are more numerous 

 and better developed in the foothills and at lower elevations. 

 Aspen and birch often form temporary types after fire or clear- 

 ings'. 



It will be seen that the species in the Black Hills fall into four 

 groups according to the forest regions where they are most com- 

 mon. These are : Rocky Mountain species ; Northern Forest 

 species ; Eastern Forest species ; and Transcontinental species. 

 Most of the last group might be classed also in the third. 



The pines belong in the first group. The spruce and Paper 

 birch in the second. The other hardwoods in the third. Trans- 

 continental trees such as aspen and boxelder are undoubtedly of 

 eastern origin. 



How have these representatives of different forest regions 

 found their way to this prairie island — the Black Hills? 



There seem to have been three migration lines. From the 

 north, the east and the west. 



At one time, the climate of the west was much more humid 



