42 The Botanical Survey of Nebraska 



The transition to the mixed hickory-oak community is rather 

 gradual. The community is marked by the presence of a number 

 of large hickories, by the increasing abundance of red oak in the 

 lower portion and by the appearance of the ironwood, — trees 

 entirely absent from the preceding community. However, bur 

 and especially yellow oak occur throughout, the former disap- 

 pearing entirely and the latter being much less abundant in the 

 red oak community. 



The red oak community is characterized not only by the abun- 

 dance of the dominant, but also by the dropping out of the less 

 mesophytic and less tolerant yellow, bur, and black oaks on the 

 one hand and the increasing abundance of ironwood on the other. 

 The red oak gives way lower down the slope to a well developed 

 linden-ironwood community. Here only an occasional oak is to 

 be found. These are invariably red oak. A few trees of black 

 walnut, swamp hickory and a few other species (not found in 

 this transect) complete the list. 



The preceding examples of the community arrangement of tree 

 dominants are typical for the forest in this portion of the forma- 

 tion. Grassland is replaced, usually through a chaparral stage, 

 by forest of the more xerophilous trees, — Qtiercus macrocarpa 

 and Q. acuminata followed by Q. velutina. In more mesophytic 

 situations Q. rubra and Hicoria ovata dominate, while a com- 

 munity of Tilia americana wiih. Ostrya virginiana hold possession 

 of the more moist and richer slopes and ravines. 



Three primary habitat factors are concerned in this tree dis- 

 tribution. They are soil moisture, atmospheric humidity and 

 light. The pioneer tree associes invading situations quite too 

 extreme for their more exacting successors, react upon the habitat 

 in such a manner as to make it more moist as regards both soil 

 and air. These reactions, likewise favorable to the early tree oc- 

 cupants, make possible the invasion of the area by more tolerant 

 species. For a time the area is covered with a mictium of the 

 members of the two associes. But gradually, because of the un- 

 favorable light reaction, the species of the earlier woody com- 

 munity largely or entirely disappear. Change of dominants in an 

 area is usually characterized by striking changes in the shrubby 



48 



