Pool: THE VEGETATION OF THE SANDHILLS OF NEBRASKA 229 



material during- calm days. Even under the most advantageous 

 circumstances this method would be extremely risky. The method 

 would be very laborious because of the close spacing of the trees. 

 Furthermore it is frequently the case that the surrounding grasses 

 are taller than the trees so that the more or less hidden trees are 

 very sure to be injured. Because of such relations as these, and 

 because of the ease with which fire escapes, the method must be 

 regarded as questionable. Grazing injury to the young coniferous 

 trees would probably not be great. The solution of the problem is 

 thus seen to lie in the adoption of some grazing rotation. 



Aside from Andropogon scoparius the most conspicuous plant 

 of the uplands in practically every portion of the region is Yucca 

 glauca. This plant follows rather closely the distribution of the 

 dominant species of the bunch-grass association. Because of its 

 evergreen frutescent habit and its great frequency, Yucca is one of 

 the most noticeable features of the sandhills. Yucca is an especially 

 striking feature of the floral covering during the early days of June 

 when the long racemes or spikes of large whitish flowers may be 

 seen on almost every hill. Typically the species is found over the 

 upper slopes and crests of hills and ridges in the more open sites of 

 the association. Whenever the less conspicuous interstitial species 

 are poorly developed in number and size. Yucca assumes a com- 

 manding importance. 



The Yucca appears to thrive in the more open areas of the 

 association. The south and southwest exposures are as a rule the 

 most xerophytic situations throughout the hills. These sites are the 

 typical habitats of this species where it is not only most frequent but 

 also reaches its greatest abundance. Its sand-collecting and sand- 

 holding ability coupled with its extreme xerophilous anatomy makes 

 possible the persistence of the Yucca long after all other interstitial 

 species have been tramped out and blown away. It frequently rivals 

 Andropogon scoparius in its ability to resist wind erosion and is 

 therefore to be found in numerous places where all of the bunch- 

 grasses and practically everything else have been removed. Fire is 

 the only force of sufficient magnitude, active at present, to eradicate 

 Yucca. Conditions where this species is practically the only plant 

 left on the uplands are produced by overgrazing. When such con- 

 ditions occur over areas where Yucca is especially abundant a very 

 striking effect is produced by the dark green rosettes of many 

 swordlike leaves scattered over the otherwise bare sand. Under 



