Plagioclimax- the plant community that exists when human interaction prevents the climatic climax vegetation being reached.
Climatic climax vegetation- the vegetation that evolves in a climate region if the seral progression is uninterrupted by human activity, tectonic processes, etc. The climatic climax vegetation for the UK is Oak and Ash trees.
Heather moorland is one of the major components of the British Isles, especially in the upland areas, but it was never a major part of the primary succession that followed the retreat of the ice at the end of the last ice age, about 20,000 years ago. It owes its present extent to human interference and the clearance of the upland forests.
This means that the Heather moorland is a plagioclimax. A plagioclimax is when humans stop the climatic climax vegetation being reached.
The woodland was first cleared out to allow crops to be grown and to provide space for grazing, but once the trees were removed there were no deep roots to bring the nutrients to the surface and renew the soil fertility. Instead the heavy rainfall, which would have been intercepted by the trees, was able to leach the nutrients out of the soils. So the upland areas were often colonised by bracken, grasses, scrub woodland and heather.
As long as the moorlands were grazed heavily this mixed moorland vegetation was maintained. Sheep often grazed these areas as they are indiscriminate eaters, which means they would eat anything.
In many areas, such as the North York Moors, parts of the Pennines and large parts of the Eastern Scottish Highlands, there is a deliberate management policy to maintain the land as Heather moorland. This is because the young shoots of the heather provide ideal food for red grouse, which are used as the basis for the very lucrative shooting industries which are present in these areas.
To encourage the growth of new shoots, the old, woody heather plants are burnt off every three or four years. Estate managers burn off sections of their moor in rotation, so that at any time the moor has a variety of different habitats, with some areas of new Heather providing food supplies and areas of older Heather providing good cover for the grouse.
If burning and grazing stops the Heather will grow old and woody. It would then be possible for scrub and woodland to invade. Overgrazing can lead to destruction of the young Heather shoots and invasion by the bracken or by mat grasses.