48 ECOLOGICAL RELATIONS OF THE FLORA. 



The plants averaged about 26 to the sq. m. With even spacing 

 this means one plant to every 400 sq. cm., a very sparse vegetation. 

 Dune. Heath Formation: This formation is typical all along the 

 shores of the Great Lakes. There is shown also great similarity to the 

 corresponding formations on sand}^ tracts inland (Warming 1909, p. 

 272). Such areas make up a large part of the northern half of lower 

 Michigan. 



Wherever clearings have been made, or where the land has been re- 

 claimed from the lake, the jack pine association is the dominant one 

 in the open area, but it is almost always preceded by a heath formation. 

 (Plate IV a.) 



In the heath formation the plants grow on very fine leached sand. 

 The waterline is low — Kedzie (1888) gives it as from 15 to 30 feet below 

 the surface of the soil for the jack pine barrens and at Sand Point it 

 is determined by the height of the ridges above the lake level (average 

 about 10 to 15 feet). Humus is practically absent, the dune sand 

 showing up one inch below the surface. In this sort of location the 

 leaves and stems of previous years are slow to decay, which partly 

 accounts for the barrenness of the soil. The plants are subjected to 

 strong light, and the temperature extremes very nearly approach the 

 rigorous ones of the beach. 



The most typical heath conditions were found on the landward 

 side of the outermost dune. 



In the dune heath formation two well-defined associations appeared, 

 the Poa and the Arctostaphylos. The former was' a closed one con- 

 sisting of a firm sod broken only in places by seedlings of jack pine 

 and choke cherry. 



The Arctostaphylos association was different. It did not conceal 

 the sand but it collected the dead leaves forming a thick dry mat. With 

 it occurred the blueberry — dwarfed and stunted — and a great army 

 of other species. The reindeer moss was plentiful with these two 

 associations. 

 Detail of the Arctostaphylos Association. 

 Dominant Species: 



Arctostaphylos uva-ursi. 

 Vaccinium pennsylvanicum, 

 Sub-Dominant Species: 

 Pteris aquilina. 

 Lycopodium complanatum. 

 Panicum xanthophysum. 

 Myrica asplenifolia. 

 Comandra umbellata. 

 Oxalis stricta. 



