Ill 



catns and Paniciiin zirgatiiiit. These, as well as the appearance of 

 the goldenrod, indicate that the zone is migrating up hill over the 

 partially stabilized sand. The rounded contour of the willow thicket 

 with the youngest plants at the edge, show that it is also enlarging 

 and occupying successively higher levels. This movement is corre- 

 lated with the development of the retentive layer of humus under 

 the thicket, but the mo\-ement of the Solidago, already six feet (2 m.) 

 above the willows, depends more upon the g'eneral vegetati\e activ- 

 ity of the plant itself. 



These two associations, Sulidago and Sali.v, may then be expected 

 to appear in any deep blowout. Naturally the deeper ones only can 

 support the willow, which recjuires a larger supply of moisture. 

 This zonal relation of willow and goldenrod is by no means local, 

 but may be observed in many localities in the eastern states. The 

 absence of the goldenrod zone around the willows in the first depres- 

 sion described is merely one of those chance instances of distribu- 

 tion for which no explanation can be given. Possibly the presence 

 of Jiincus aciiniiuatiis indicates the iirst stage in its formation. 



It is evident that with the establishment of the dense growth of 

 Solidago the movement of the sand must cease, and it may be that 

 it does not appear until the sand has first become static. In either 

 case, if its depth is not sufticient to reach moist layers of sand the 

 willow can not develop and the blowout will be occupied by Solidago 

 alone. On the other hand, if the conditions are suitable for the 

 growth of willows, the Solidago association can develop simulta- 

 neously around it. The willows therefore, requiring the deeper 

 excavation, can not follow the goldenrod, but must appear before or 

 with it. When both are estalilished, the former becomes dominant 

 because of its greater control of the physical conditions and tends 

 to succeed the latter. 



This condition of aft'airs is peculiar in two respects. First, the 

 development of the dominant Sali.v association can not follow in 

 time that of the minor Solidago association. Second, the general 

 movement of the zones is centrifugal, extending progressively fur- 

 ther up the sides of the blowout, and the direction of succession is 

 apparently toward a hydrophytic climax. 



It must not be presumed that in this case a hydrophytic climax 

 will appear. It is probable that the water-retaining humus does not 

 become thick enough to hold standing water, and it is still more 

 probable that a new movement of sand from the west may overwhelm 

 the whole association. As in another case already mentioned, there 

 is at present no association in the region that could possibly be 



