266 MINNESOTA BOTANICAL STUDIES 



The dominant species are developed to about the same height, 

 a tendency which results in the production of a rather uniform layer. 

 The only exceptions to this rule are seen in certain habitats where 

 Ostrya becomes very tolerant and develops as a decidedly pro- 

 nounced under story, and again in the broad and more open parts 

 of the canyons where considerable alluvium has accumulated and 

 where the uniformity of height growth is broken by the presence of 

 a few very large and tall specimens of Populus sargentii, Juglans 

 nigra, Ulmus americana, or Tilia americana. These species fre- 

 quently reach a development that compares well with that seen in 

 the maximum broadleaf woodlands upon the alluvial deposits near 

 the Missouri River. 



Mingled with the dominant species are to be found a few other 

 tree species in almost all of the canyons. For instance Celtis occi- 

 dentalis, Ulmus puhescens, and Gymnocladus dioica are common 

 and reach about the same height as the uniform forest of control- 

 ling species. Acer negundo is a frequent additional species that 

 in some places reaches an abundance almost equalling that of some 

 of the dominants. 



Along the Dismal and Loup rivers this association does not reach 

 as highly developed a state as that characteristic of the Niobrara 

 and its springbranch tributaries. This is probably to be expected 

 since these points are much farther from the Missouri woodlands. 

 The ironwood and the linden are much less frequent here and when 

 found are never in the abundance that is typical for the northern 

 streams. At present Fraxinus lanceolata is by far the most char- 

 acteristic and abundant member of the dominant species of this asso- 

 ciation as it occurs along the Dismal and the Loup rivers. These 

 streams follow a much broader valley for the most of their course 

 than that which marks the springbranches in the north. The steep- 

 sided canyon with rocky walls is the exception rather than the rule 

 along the Dismal and Loups which follow an extremely meandering 

 course through the hills. Fires and the axe have had a marked 

 effect upon the woods of these rivers so that at present they exhibit 

 a distinctly patchy appearance. At one time the number of red 

 cedar and ash trees along these streams was much greater than at 

 present as is told by the stump remnants. Along the rivers the 

 woodlands are usually found occupying poorly defined benches in 

 the form of long narrow belts or patches of a few acres in extent. 

 In many places the woods have been reduced to a very narrow belt 



