28 The Botanical Survey of Nebraska 



ward. Even the dominant red oak and shell-bark hickory lose 

 their importance near the mouth of the Platte and are replaced by 

 the bur oak-elm-walnut type. 



In contrast to the open forest canopy with its development of 

 layers of vegetation as found at the Salt creek station, the forest 

 types at Peru in general and in the better developed red oak- 

 hickory associes or hnden consocies in particular, present a very 

 distinct difference. Here the trees stand closer together with tall 

 straight trunks and rather narrow compact tops. The crowns 

 normally touch each other thus giving continuous shade beneath. 

 Not only are the dominants better developed, reaching greater 

 heights and much greater trunk diameters than the dominants 

 along Salt creek at Lincoln, but where the latter occur as sec- 

 ondary species in the forest about Peru they respond to the more 

 favorable environmental conditions by a much more sturdy 

 growth. 



Owing to low light intensities the secondary layer of small trees 

 and shrubs is never well-developed in this woodland. Asimina 

 triloba, Etionymus atropurpureus, Rhamnus lanceolata, Staphy- 

 lea trifoliata, Cornus asperifolia with Cercis canadensis are the 

 most important species. They occur frequently but seldom very 

 abundantly. In the more open bur oak-black oak forests, and 

 especially on the forest borders, they find greater expression and 

 together with certain other species frequently are components of 

 thickets or extensive bushland. However, the chief components 

 of the latter are usually Corylus americana, Xanthoxyhim amer- 

 icanum (both of which are less xerophytic than the following and 

 consequently do not reach the Lincoln station) Rhus glabra, 

 Symphoricarpos symphoricarpos, Primus americana and Prunus 

 virginiana. 



Thus the woodland proper, whether occupying the lowlands 

 and steep bluffs along the Missouri river or following one of its 

 many tributaries far into the prairie region, but always with a 

 decreasing number of both species and individuals, is usually 

 separated from the grassland by a bordering fringe or zone of 

 chaparral. This may be composed almost wholly of a single 



34 



