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.
From the frequent association of certain tree associations with heaths, it is probable that in some cases, at any rate, the heath is a degenerate type of woodland, as where Oak, Birch, and Pine occur, and form a scrub with heath plants. But in many cases, especially on coarse sandstones and sands, the heath was itself originally developed.
In any case, when a heath occurs it transforms the soil characteristics, for a layer of peat is formed, which at once affords a home for certain types of plants, and excludes others that cannot grow in humus. It has been found on the Continent that the cause of the degeneration of woodland into heath is due to the leaching out of the sand from the soil. It is probable that in the same way the leaching out of humus from a soil originally rich in this has been responsible for the disappearance from certain districts of such humus-loving plants as Dog's Mercury and Lords and Ladies. In place of the mild humus of a woodland required by such plants, an acid humus is formed suitable for heath types. The consolidation of sandy, gravelly soil by humus acids similarly affects the growth of trees, and renders the persistence of woodland impossible and the formation of heaths possible.