102 The Heath Formation 



WTiile the oak-birch, heath type of woodland frequently 

 shows a complete mixture of the various trees (Plate IV), 

 the birches are frequently the only trees over considerable 

 areas (Plate V). They are often hardly dominant in the 

 strict sense because when in open association they scarcely 

 affect the ground vegetation. This local purity of the birch 

 may be due to felling of the oak (many of these woods 

 having been at one time extensively depleted of oak for 

 charcoal and for ship-building) after which the vacant 

 ground is rapidly colonised by birches. The birches also 

 often successfully colonise open heathland. In both of 

 these ways birch heath (Betuletum ericetosum) origin- 

 ates. 



The beech is also often locally dominant, as in the dry 

 oakwood from which oak-birch heath is derived. When 

 this is the case the ground vegetation is much modified 

 owing to the deep shade cast by the trees. Vaccinium 

 Myrtillus (not flowering) and Leucohryum glaucum are 

 often the only plants persisting under the shade of the 

 beech, and the ground is frequently quite bare. Both 

 this community (which may be called Fagetum ericetosum) 

 and the birch heath are best considered as societies of the 

 oak-birch heath association. 



The oak-birch heath is a very picturesque type of 

 woodland and covers wide areas in south-eastern England; 

 it is specially characteristic of much of the Lower Green- 

 sand fringing the northern and western edges of the 

 Weald, and of the Ashdown sand in the centre of the 

 Weald (Plate Yb). It is also found in Sherwood Forest 

 (Nottinghamshire) and in Delamere Forest (Cheshire), 



