This section is from the book "British Wild Flowers - In Their Natural Haunts Vol5-6", by A. R. Horwood. Also available from Amazon: A British Wild Flowers In Their Natural Haunts.
Though heaths most often are open treeless tracts, they may be wooded, or have a scrub association. Much of the heathland in the south-east of England in fact is wooded, and elsewhere wood and heath are found to alternate.
The type of wood associated with heath conditions is the dry Oak Wood, and a scrub may also develop made up of Sloe, Gorse, Hawthorn, Rose, Bramble, etc. The heath flora occurs in open parts of the dry Oak woods, with Whortleberry, Ling, and Heath Hair Grass.
Oak and Birch are the prevalent trees, but sometimes Beech or Pine, associated with Bracken and Ling, or White Beam, may be dominant. There is generally a good deal of light, so that the ground flora may be rich and made up of large associations, as Holcus mollis, Bluebell, Wood Sorrel, Wood Anemone.
The ground flora in the dry Oak wood consists of heath plants, in addition to members of the true dry Oak wood formations, many Grasses, and plants of the grass heath. There may elsewhere be a Birch heath characterized by a dominance of Birch with Bracken.