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.
A special feature of heaths is the well-marked continuity of the association of such dominant plants as Furze, Broom, Ling, Heath, Whortleberry, etc. This is partly due to the uniformity of the conditions. The heath is developed upon a particular kind of soil, hence it is natural that certain plant types, as Ling, etc., are characteristic of such a soil. Heath is a stable association, and as such not liable to invasion by or migration into other types.
Where Ericaceous types cover wide areas, other plants cannot grow, or become dwarfed. If the formation is open, however, grass heath develops, and the grass type becomes equally prevalent or even entirely dominant.