Pool: THE VEGETATION OF THE SANDHILLS OF NEBRASKA 301 



found within the sandhill region. With a little higher and drier soil 

 the association would probably be transformed either into the bunch- 

 grass association if the soil is especially sandy, or, if the soil is 

 harder and less sandy, into the spear-grass association. The change 

 to the grama-buffalo-grass or wire-grass transition associations is 

 also possible in certain portions of the hills, although the relatively 

 mesophytic association appears to be the more probable culminating 

 point on the harder bottom land throughout the region. 



THE WILLOW THICKET ASSOCIATION 



The hay meadows and wet meadows are often strikingly modi- 

 fied by the presence of a chaparral type of vegetation in which Salix 

 longifolia is the dominant plant. Extensive tracts of wet meadow 

 and lower stretches of hay meadows are broken by the presence of 

 round-topped bushy plants varying in height from 6 to 10 feet. 

 Sometimes the willows are often disposed in the form of a belt lying 

 along the outer margin of the marsh formation, thus occupying a 

 position with which the rush-sedge wet meadow is correlated. This 

 belted appearance is a most pronounced feature of such meadows 

 when viewed from some vantage point in the uplands. The belts 

 frequently become discontinuous and then the association takes the 

 form of more or less isolated patches or islands dotted over the dry- 

 ing meadow. 



The much-branched, bushy form of the controlling species with 

 its relatively high tolerance results in the production of dense 

 crowns which produce shade too deep for most of the lower wet 

 meadow herbs. A few species, such as Campanula aparinoides, 

 Galium triUorum, G. triUdum, and Scutellaria galericulata, succeed 

 in populating the less open intervals between the willow clumps. 

 Amorpha fruticosa, a woody species of about the same height as the 

 willow, is occasionally seen in mixture with the willow, but seldom, 

 if ever, becomes sufficiently abundant to dominate the scrub. This 

 species is much less tolerant than the willow and hence it usually 

 occurs in the open portion of the association and as scattered indi- 

 viduals along its margin. Relicts of the marsh formation are 

 commonly present among the willows as scattered tufts or single 

 individuals, while in the drier situations, where the willows become 

 open, many species of the hay meadows invade and completely dom- 

 inate the interspaces beyond the shade of the willows. 



This asociation is doubtless an extension and a modification of 



