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.
There is so little variation in the height of the dominant heath plants that the whole flora might be considered to range between a few inches and 2 ft. in height on the average. But in some places the Ling itself is abnormally developed, whilst elsewhere it is equally dwarf and prostrate.
The chief factor regulating the height of the heath plants is the wind. The heath formation is open, as a whole, and exposed to strong air-currents, hence, as on the sea-coast, the plants adapt themselves to this in their stunted growth. Furze and Broom first, then Ling and the Heaths may be said to form the next highest type of association. There are plants of intermediate height that struggle upwards amidst the closely branched Ling or Furze, as Heath Bedstraw and Grassy Stitchwort, whilst, as a rule, the Milkwort and the Tormentil, both grass-heath types also, are trailers that form a third or lowest zone.
The trees are stunted, and the Birch has on the Birch heath often a more erect habit with reduction of the branches. The Creeping Willow is a dwarfed type also, stunted or bent over owing to the wind.