Pool: THE VEGETATION OF THE SANDHILLS OF NEBRASKA 273 



24 feet in height. The river lies about 400 feet below the tops of 

 the surrounding hills which cap the rocks of the Loup Fork series at 

 this point. At numerous places along the river the bunch-grass 

 association with its very sandy soil approaches the brink of the 

 canyon so closely that the sand sometimes rolls into the stream. 

 The broadleaf woodland is well developed in the lower portions 

 of the canyon where there is a sufficient width of the floor beside 

 the stream to afford footing for the trees that control the associa- 

 tion. Sometimes a tall pine is seen among such trees, but typically 

 the conifer is seen only in extremely open distribution over the 

 upper canyon slopes and bold ridges. The individual trees vary 

 from 10 to 30 feet in height and, because of their isolation, they 

 are commonly round-topped and branched to near the ground. The 

 red cedar, /. virginiana, is also common here as widely scattered 

 individuals. A number of trees of this species were seen that were 

 as large as the largest pines. 



Here, as in the typical broadleaf canyons, the common species 

 of low shrubs are conspicuous. Thickets of Prunus americana and 

 P. melanocarpa are found at many points in the canyon where the 

 walls are not too steep. Rhus glabra is a common species of the 

 canyon rim. Rhus trilobata is also frequent along these canyons 

 where its characteristic low communities of 1 to 10 feet in diameter 

 are common upon outcrops and new talus slopes. Amelanchier alni- 

 folia is an additional member of the shrub layer, along with Rihes 

 aureum, both of which are found in rock crevices toward the upper 

 portions of the slopes. Herbaceous species of Pine Ridge flora are 

 more or less frequent here. The commonest of these are Arenaria 

 hookeri, Paronychia jamesii, Orophaca caespitosa, Eriogonum 

 Uavum, and Campanula rotundifolia. The latter species is confined 

 to grassy slopes or the slight shade afforded by the open tree growth ; 

 the others are typical mat or rosette plants that are prominent 

 species of rocky substrata. 



Along the upper limits of this formation as it is seen in the 

 sandhills the trees are widely scattered over bunch-grass or other 

 grassy land with a sandy soil. Out of the canyon proper there 

 are very few spots where the trees occur in sufficient density to 

 shade out the grasses completely, and in general it appears that the 

 invasion of the surrounding grassland by the pines is an extremely 

 slow process, and it may be doubted if such invasion will ever 

 progress far under the conditions existing at the present time. 



