289 



much more frequently blown away by the wind. In view of this, un- 

 less the sides are fixed w-ith Jiinipenis Jiorizontalis or Arctostaphylos, 

 a Juniper us com munis depressa dune is liable to be blown aw^ay, thus 

 forming a break in the line of dunes through which the wind carries 

 sand on to the heath behind them. At the same time, adjoining dunes 

 of Jnniperus horizontalis are undermined until the exposed side be- 

 comes covered with vegetation. 



The junipers are the most efficient dune-builders in this region, 

 but they can build dunes only w'here their westward side is protected 

 from the prevailing wnnds. Normally the junipers are mat- formers 

 in the heath association, which will be considered later, but in the 

 presence of blowing sand they meet the change of condition by be- 

 coming dune-builders. These dunes must be closed associations, since 

 any open place on them would be seized upon by the wind and the re- 

 moval of the dune effected. The vegetation being dense and com- 

 pletely covering the ground, secondary species, with the exception of 

 relics on the crests, do not occur. Of these relics, which were the 

 nuclei about which the dune originated, Calanwvilfa is the most fre- 

 quent, W'ith Prunus puuiila second, and a very few plants of Andro- 

 pogon scoparius and a single one of Cornus stolonifera. 



IvIST of' THE SPECIES OE THE JUNIPERUS DUNES ASSOCIATION 



Dominant Species Relic Species 



Juniperus horizontalis Calamovilfa longifolia 



Jiinipenis conimiinis depressa Prunus piunila 



Arctostaphylos iiva-ursi Andropogou scoparius 



Cornus stolonifera 



Miscellaneous Dunes 



In addition to the associations given above, which occupy about 

 97 per cent, of the dune areas, there are isolated dunes, each one of 

 which is characterized by a rather definite association of plants. In 

 each case the plants are more typical of other associations, but they 

 grow within the range of blo^^•ing sand and consequently dunes may 

 be formed around them. 



THE POPULUS-SALIX DUNE ASSOCIATION 



But two well-marked examples of this dune association, which has 

 been described from Presque Isle by Jennings (1909), occur in the 

 region. In both cases the dunes are low and are formed on the east- 



