Pool : THE VEGETATION OF THE SANDHILLS OF NEBRASKA 267 



which closely fringes the stream. The hackberry and ash are espe- 

 cially common in this form. 



At the present time along the Dismal the red cedar is repro- 

 ducing and extending its range to a noticeable degree. The distri- 

 bution of the species is characteristically high up on the canyon or 

 bluff sides and in such sites, where there are "breaks" or young 

 canyons in the firmer argillaceous soil, the cedar is abundant and 

 appears to be leading its way out over the surface of these rough 

 places with considerable rapidity. Red cedars are much rarer along 

 the Loup rivers where they now occur singly or at most in scattered 

 groups of few individuals in sites very like those where the species 

 is much more abundant along the Dismal. 



A shrubby layer is poorly developed beneath the trees of this 

 association. Sambucus canadensis, Cornus amomum, Ribes gracile, 

 Rhus radicans, and Symphoricarpos occidentalis are the commonest 

 species of the shrubby layer and some of these species are very 

 infrequent. For instance Cornus amomum, which forms thickets 

 beneath the trees of the springbranches of the north, has not been 

 found farther south. 



The characteristic shrubby growth of the hills is seen in the 

 form of extra-formational thickets along the larger streams. Rhus 

 glabra is a constant member of such thicket associations in many 

 parts of the hills. The species is found in great abundance along 

 the Middle Loup and the Dismal beneath the more open forest cover 

 as well as in the form of highly developed marginal thickets. The 

 sumac exhibits the same extra-woodland aspect here that is so 

 typical of its distribution over the bluffs of the Missouri where it is 

 the almost ever-present forerunner of the forest. Extending from 

 beneath the scanty woodlands of the Dismal and Loup rivers nu- 

 merous species of shrubs are to be found disposed in the form of 

 dense marginal thickets. From such sites these species spread to a 

 wide range of sites over the bluffs and far back into the hills. 

 These woody species are differentiated into two groups dependent 

 primarily upon soil-moisture. Across the lower portions of the 

 canyons or river valleys where the sandy soil is quite moist such 

 species as Amorpha fruticosa and Salix longifolia form areas of 

 bush-land from 6 to 10 feet in height and many acres in extent. 

 Species that are common in similar to drier sites are Cornus stoloni- 

 fera, Sambucus canadensis, Salix amygdaloides, and Xanthoxylum 

 americanum. Where the soil is somewhat drier, but \\\ih the water 



