268 MINNESOTA BOTANICAL STUDIES 



table within a few inches of the surface, an extremely dense, and 

 in many places impenetrable, bush is developed by an association of 

 Prunus americana, P. melanocarpa, Ribes aureiim, Crataegus occi- 

 dentalism and Lepargyraea argentea in various degrees of mixture. 

 Thickets of this sort often cover many acres of the harder land upon 

 river flats not occupied by grasses. Along the Niobrara scrubby 

 individuals of Quercus macrocarpa enter into the formation of 

 somewhat similar, though more open, thickets. Rhus radicans is 

 very commonly present as a secondary layer in both types of 

 thickets, and the low much-branched and interlaced crowns of the 

 dominant species are often rendered still more dense by the copious 

 development of the four lianas, Parthenocissus quinquifolia, Vitis 

 vulpina. Clematis ligusticifolia, and Smilax herbacea,. 



It frequently happens in those places where there is a wide 

 interval of comparatively hard land between the stream and the 

 first hills that the above bushland is bordered by a belt of lower, 

 finer, wirelike bushes lying between the taller thickets and the bases 

 of the hills. This belt, most often composed of Symplioricarpos 

 occidentalis, is a pronounced vegetative feature of river flats in 

 many places, especially along the Middle Loup and the Dismal. 

 Rosa arkansana is frequently mingled in great abundance with the 

 "buckbrush." The former species often forms exclusive communi- 

 ties which alternate with similar patches of the latter. These dis- 

 tributional relations are often emphasized upon the dry river flats 

 of the Middle Loup, especially where distinct belts are developed 

 according to the above sequence. 



Peculiar modifications of the thicket are common far back in 

 the hills away from the taller woodland species. The species that 

 most commonly leave the neighboring fringe of woods are Sym- 

 phoricarpos occidentalis and Prunus americana. The former typical 

 border plant occurs in very close stands of individuals about two 

 feet in height in many of the "pockets" and dry valleys. Frequently 

 the floor of a dry valley is covered for many acres by this species 

 alone or in mixture with Rosa arkansana, a regular member of the 

 bunch-grass association. Smaller "pockets" or basins with a com- 

 plete cover of this sort are especially conspicuous interruptions of 

 the usual grassy tone of the uplands. Such stands of bush are often 

 so dense as to exclude nearly every other species. The "buckbrush" 

 has been carried into almost every part of the region and one may 

 come suddenly upon such a "buckbrush pocket" in the bunch-grass 



