Ecosystems 7 - Plant Succession Theories

Plant Succession Theories

The classical plant succession theory suggests that once the climatic climax

vegetation is reached then the community remains in a steady state – is this likely to be

the case?

MonoClimax Theory

• The monoclimax theory was an invention of the American ecologist F.E. Clements (e.g. 1916). This states that every region has only one climax community, toward which all communities are evolving and that, given sufficient time and freedom from interference, a climax vegetation of the same general type will be produced irrespective of the earlier site conditions. Clements believed that it was climate, and climate alone that determined the final "forest-type".

Is such a monoclimax succession towards a climatic climax community inevitable?

The classical plant succession theory suggests that once the climatic climax

vegetation is reached then the community remains in a steady state – is this likely to be

the case?

Polyclimax Theory

• The polyclimax theory arose as as an obvious reaction. A.G. Tansley (1939) was an early proponent. There are many different climax communities that can be recognised in a particular region. These climaxes are controlled by soil moisture, mineral ions, activity of animals, topography, and other factors.The difference is the time factor applied in measuring relative stability. In the view of the supporters of the monoclimax theory, given enough time, a single climatic climax community would be achieved, eventually overcoming the edaphic and other climaxes.

Climax Vegetation

Plagioclimax subclimax (after deforestation, ploughing, burning)

Secondary succession

Primary succession

New inorganic (non-vegetated surface)

Biotic subclimax

Edaphic (soil) subclimax

THE POLYCLIMAX THEORY

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Primary succession

natural interruptions in primary succession

Retrogressive succession due to disturbance (natural or human)

Secondary succession

This begs the question, should we consider time on an ecological

or geological scale?

However we consider time, is the climate stable?

Climax Pattern Hypothesis• R.H. Whittaker (1953) proposed a variation of the polyclimax

idea, the climax pattern hypothesis. This emphasises that a natural community is adapted to the whole pattern of environmental factors in which it exists. In addition to climatic factors, e.g. wind, there are a whole range of edaphic factors, fire, and biotic factors, including grazing, disease and mutualistic and competitive interactions.

• Whereas the monoclimax theory allows one climatic climax in a region and the polyclimax theory allows several climaxes in the same area, the climax pattern hypothesis allows a continuity of climax types, varying gradually along environmental gradients and not neatly packaged into discrete climax types.

• The climax is recognised as a steady-state community in dynamic balance with the environment in all its gradients. Rather than referring to a climatic climax, the correct term is prevailing climaxes and these are determined by climate, soil, topography and biotic factors as well as fire, wind, salt spray and other influences, including "chance".