Climatic climax and plagioclimax vegetation

Overview

Climatic climax vegetation and plagioclimax vegetation.

What you need to know

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Questions from this subtopic

Each one with the model answer underneath.

1. 1

   What is climatic climax vegetation?

   Model answer

   Climatic climax vegetation is the final, stable, self-perpetuating plant community that a succession reaches when it is in equilibrium with the climate of the region. No further replacement of one community by another takes place: the dominant species regenerate within their own community, so it maintains itself indefinitely unless it is disturbed. It represents the largest and most complex vegetation that the climate of that area can support, with the greatest biomass, the deepest soil, the most fully developed vertical structure and the most complex food webs.

2. 2

   What is the climatic climax vegetation of most of the British Isles?

   Model answer

   Deciduous broadleaf woodland, dominated by oak over most lowland areas, with ash on more calcareous soils, beech on well-drained chalk and limestone in the south, and birch and pine at higher altitudes and further north. Very little of it survives, because almost all of the natural woodland was cleared for agriculture, fuel and timber over the past few thousand years, so the moorland, heath, hedgerow and grassland that people think of as the natural British landscape is in fact vegetation that has been prevented from returning to woodland.

3. 3

   What is plagioclimax vegetation?

   Model answer

   Plagioclimax vegetation is a community whose succession has been deflected or arrested by continuing human activity, so that it is held permanently at a stage short of the climatic climax. It is not a natural endpoint: it is maintained only for as long as the human interference continues, and it is stable only in that sense. If the activity stops, the arresting factor is removed and succession resumes towards the climatic climax.

4. 4

   Explain the difference between climatic climax and plagioclimax vegetation.

   Model answer

   Climatic climax vegetation is the natural endpoint of succession, determined by the climate of the region and self-sustaining without any human involvement; in most of the British Isles it is deciduous oak woodland. Plagioclimax vegetation is the artificial endpoint produced when human activity - grazing, burning, mowing, felling or draining - repeatedly interrupts the succession and prevents it from reaching that natural endpoint, so the community is held at an earlier seral stage such as heather moorland or grassland. The key distinction is which one is natural: the climatic climax is what the climate would produce if people left the site alone, and the plagioclimax is what people produce by not leaving it alone. Students frequently state this the wrong way round, so it is worth fixing clearly: climate makes the climatic climax, and human interference makes the plagioclimax.

5. 5

   Give examples of plagioclimax vegetation and explain how each is maintained.

   Model answer

   Heather moorland is maintained by rotational burning and by sheep and grouse grazing, which kill tree and shrub seedlings while heather regenerates from its base. Chalk downland grassland is maintained by continuous sheep or rabbit grazing, which prevents scrub and woodland from establishing and allows a rich short-turf flora to persist. A mown lawn, a playing field or a road verge is held at the grass stage by regular cutting. Coppiced woodland is held in a permanently youthful state by cutting the trees back to the stump on a cycle. Deforested and heavily grazed upland pasture is prevented from reverting to woodland by stock. In every case succession is held short of its climax by a repeated disturbance, almost always human management, though grazing animals such as rabbits arrest it in exactly the same way, rather than by the climate.

6. 6

   What happens to a plagioclimax community if the management stops?

   Model answer

   Succession resumes towards the climatic climax. On an abandoned heather moor, for example, the heather is no longer burned or grazed, so it grows leggy and is invaded by coarse grasses and bracken; gorse and bramble follow, then birch, rowan and pine seedlings which are no longer eaten off; and within decades the site develops into scrub and then woodland. The change is a secondary succession, so it is comparatively rapid because soil, nutrients and a seed source already exist. This is precisely why moorland and downland must be actively and continuously managed if they are to be conserved as they are.

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