264 MINNESOTA BOTANICAL STUDIES 



little, and as a consequence there are few springs for the develop- 

 ment and upkeep of streams. 



The broadleaf forest formation reaches its maximum expres- 

 sion for the sandhills along these streams which emerge from the 

 northern boundary of the region from central Cherry County east- 

 ward. The same general type of vegetation prevails as a frequent, 

 although often disconnected, characteristic of the lower portions of 

 the Loup and Dismal rivers farther southward. Pound and Clem- 

 ents have indicated (57) that this formation, which they called 

 "The Springbranch Canyon Formation," represents certain modi- 

 fications of the woodlands along the Missouri River with which it is 

 directly connected by means of the courses of the sandhill rivers all 

 of which eventually pour their waters into the Missouri. The 

 above writers regarded this formation as a modification of "the bur 

 oak-elm-walnut formation" of the Missouri River woodlands. The 

 modification is mainly one of simplification. 



Notable differences occur in the composition of the woodlands 

 as we pass from the Missouri up the Niobrara and enter an increas- 

 ingly drier region. Some of the characteristic tree specie's of the 

 east, such as Ulmns americana and Querciis macrocarpa, decrease in 

 abundance, and the growth of other woody plants becomes consider- 

 ably dwarfed. This tendency toward nanism is a striking charac- 

 ter of a number of woody species as^we pass farther and farther 

 westward. The increasingly scrubby form of the bur oak, Quercus 

 macrocarpa, has caused the application of the common name "scrub 

 oak" to this species. The green ash, Fraxinus lanceolata, also 

 becomes remarkably dwarfed as compared with its growth farther 

 eastward. Many species become dwarfed and infrequent along the 

 lower 100 miles of the courses of the Niobrara and Loup, while 

 other trees become less abundant until finally, when the sandhills 

 are reached, the dominant species are quite dififerent from those that 

 control the Missouri River woodlands, and the forest is greatly 

 reduced. The species that are most conspicuous in this formation 

 in the sandhills are Tilia americana, Ostrya virginiana, Fraxinus 

 lanceolata, and Jimiperus virginiana. These trees are of sufficient 

 importance in the formation in many localities as to produce a well- 

 defined association. 



THE LINDEN-CED.^R-IRONWOOD-ASH ASSOCIATION 



By far the greater portion of the woodland formation is domi- 



