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.
One of the chief characteristics of the heath is the predominance of the plant which gives its name to this type of formation. Such plants as Ling, Bell Heather, Cross-leaved Heath, Ciliate Heath, Cornish Heath, Mediterranean Heath, all have the characteristic Heath habit. They are all adapted to resist the effects of drought, to which heaths, especially on stony or gravelly soils, are much subject.
In some cases Ling itself is the principal plant; on damper soils the Cross-leaved Heath may take its place, or the Whortleberry, which is also adapted to dry-soil conditions. The dominance of the Heath plants excludes all but a few plants that in the main have the grass habit, and so can exist side by side with the Heaths, and maintain their struggle successfully where others would be crowded out. Where there are a good number of other species present, the heath merges into a grass heath. One reason for the dominance of the heath plants is the periodic burning of Ling and Heath each spring to encourage the growth of new shoots, largely done where grouse are preserved. This destroys the other plants. A few, however, recover, even before the Ling, as Whortleberry, Sheep's Fescue, Matweed, Cotton Grass, etc.