104 The Heath Formation 



stunted), and this association easily passes over into pure 

 heath \ 



Very occasionally in the south of England the heath 

 association is developed on fine sand or even on clay, but 

 the reasons for this have not been fully investigated^ 

 In the north and west of Britain heath is developed over 

 a great variety of siliceous rocks, all of which however 

 give a light sandy or gravelly soil. According to Hardy 

 the association is not developed at an altitude greater 

 than 2000 feet (c. 600 m.)'. 



The association, in its pure form, is typically treeless, 

 the dwarf shrub Bricaceee, generally with linear "ericoid" 

 {Erica spp.) or leathery leaves {Vaccinium spp., Arcto- 

 staphylos), being the dominant plant-forms. Vaccinium 

 Myrtillus with deciduous leaves and assimilating stems is 

 an exception. The ling (Calluna vulgaris) is by far the 

 most widespread and abundant species, frequently covering 

 wide areas as the sole dominant (Plate VI b). With it are 

 often associated other Ericaceous dwarf shrubs, of which 

 Erica cinerea (in the drier areas) and Vaccinium Myrtillus 

 may be dominant over considerable tracts. 



Under the dense shade of Galluna very few species 

 can exist. Dwarfed plants of Vaccinium Myrtillus and 

 Viola Riviniana, which do not flower, mosses such as 

 species of Polytrichum and Hypnuni, and lichens such 

 as Gladonia, are commonly found forming a subordinate 

 layer of vegetation, and where the mantle of ling closely 

 covers the ground, very little else occurs. Where the 

 Callunetum is not closed, however, especially in damper 



1 Beech however seems to flourish quite well above chert which is 

 developed near the surface, and this may sometimes account for the local 

 dominance of beech in the oak-birch heath association on these beds. 



^ In some cases it may be due to stagnation of the soil water and the 

 consequent formation of acid humus. In others it appears to be correlated 

 (Bayner, Jones and Tayleur, New Phyt. Vol. x., in the press) with excess 

 of magnesium in the soil. 



3 Hardy, 1905, p. 107. 



