Pool: THE VEGETATION OF THE SANDHILLS OF NEBRASKA 205 



dune reclamation or those in possession of the highest degree of 

 adaptation to a dune environment, and those that are secondary 

 invaders that come in only when the severity of environmental 

 forces has abated to a certain degree. Finally there are species 

 that are very late in establishing themselves upon the dunes, which 

 species we may characterize as possessed of a low degree of dune- 

 tolerance. Then there are many whose absolute inability to invade 

 the dunes results in the development of the quality that might be 

 termed dune-avoidance. 



The conditions surrounding the plant-life of a dune area are 

 usually extremely severe irrespective of whether the area is found 

 within a humid or arid climatic subdivision of the earth's surface. 

 In all such places these conditions bring about a series of desert or 

 semi-desert phenomena. On the wind-swept ridges and slopes we 

 find species possessed of varying degrees of wind-toleration. Where 

 wind denudation is absent or has been greatly reduced we find less 

 wind-tolerant species, and in protected basins or valleys, we find 

 certain shade-loving and wind-avoiding species. Wind, then, is by 

 far the most important and constant climatic factor characteristic 

 of dune regions. The sand dunes, and the dune complex as well, 

 are commonly oriented in a definite manner with reference to this 

 force. 



The ecological nature of sand combined with the telling in- 

 fluence of wind in the production of environmental conditions for 

 plant life has been described by many writers, notably Cowles (30), 

 Gerhardt (36), Cockayne (26), Hitchcock (41), Olsson-Sefifer 

 (51), Cornish (29), Warming (71), and others. It is unnecessary 

 in this place to enter into a lengthy discussion of this matter. 



The real destructive action of the wind in these sandhills is not 

 ever-present. Many days are as calm and free from sand move- 

 ment as one could wish, but at other times, and for periods of- 

 several consecutive days, the wind velocity is high and soil dis- 

 semination is excessive. In other words excessive wind erosion 

 and destruction are not a constant character of any dune area. 

 For this reason figures that exhibit the total wind velocity for a 

 given period are misleading. The graphs herewith included will 

 nevertheless indicate something of the relation of this factor to the, 

 development of a plant cover upon different situations. The graphs 

 indicate the total wind velocity by twelve-hour day and night periods 

 for four habitats through an interval of twenty-one days. During 



