100 The Heath Formation 



conditions^ it may often be interpreted as the first step 

 in the degeneration of the dry oakwood association and 

 its replacement by the heath formation. Thus heath 

 plants are frequently found invading the edge of an 

 oakwood whose centre still shows the typical dry oak- 

 wood association. When the entire ground vegetation 

 is of the heath type it is doubtful if the oaks can per- 

 petuate themselves indefinitely from seedlings, and the 

 wood is then doomed. 



Oak-birch heath association (Quercetum ericetosum). 

 The initial association of the heath formation in its 

 transition from dry oakwood is a very characteristic 

 partially open and generally mixed wood of oak, birch, 

 and in south-eastern England frequently beech, in which 

 the oaks are gradually losing their dominance, and typical 

 heath plants form the ground vegetation. Except where 

 the beech is locally dominant, this kind of woodland 

 allows far more light to reach the ground than does 

 the tjrpical oakwood. The foliage of the birches casts 

 a lighter shade, and the trees are often separated by 

 considerable intervals which are occupied by the heath 

 association. Occasionally areas are met with in which 

 the plants of the ground vegetation of the oakwood still 

 maintain themselves, e.g. Holcus mollis, Scilla non^scripta, 

 Anemone nemorosa, Oxalis Acetosella, etc. This variation 

 in facies is characteristic of transitional (intermediate) 

 associations in general, and particularly of associations 

 that form the first stage of a succession iu which a 

 new formation is supplanting one already occupying the 

 ground. 



Another feature of t^ie oak-birch heath association is 

 its frequent invasion by the Scots pine (Pinus sylvestris), 

 which may sow itself freely from neighbouring planta- 

 tions (Plate IV a), and frequently perpetuates itself in 

 the mixed woodland, whose openness is favourable for 

 such colonisation.. 



