succession

Succession

    Vegetation succession can be described as the development of vegetation through a number of stages until a climax community has been established. There are two types of vegetation succession, primary and secondary succession. Primary succession is succession on newly exposed surfaces that have never been colonised by vegetation before, while secondary succession describes succession on sites that previously held vegetation which was wholly or partially destroyed. One of the first people to put forward a substantial theory on vegetation succession was Frederick Clements. His theory was advanced in 1902 when there was little research on the topic. Many of his assertions are now being challenged by examples around the world but his theory is a good place to start in order to understand vegetation succession.

Clement's Model of Succession (facilitation model)

    Clement's theory stated that there are five stages in vegetation succession. These are nudation, migration, ecesis, reaction and stabilisation. his model is called the Facilitation model of vegetation succession.

1/   Nudation
Nudation describes the creation of a bare suface. This surface will be colonised by the plants that are able to arrive. The plants must be able to tolerate the conditions that they find when they arrive at the new site. This means that the plants that are most suitable for the area being colonised will be the ones that survive. If the area that is being colonised is bare rock for example, lithophytes (lithos = stone) will be the colonisers of that area. Lithophytes are mosses and lichens that are able to survive on bare rock.

2/   Migration
The species that are able to move to the area arrive and begin colonisation.

3/   Ecesis (establishment)
The species that arrive establish themselves. Once the lichens and mosses arrive (we are still with the lithophytes), they begin to modify the environment. The old mosses die, new ones grow and roots are sunk deeper so the rock begins to gain enough topsoil for other species to move in.

4/   Reaction
There is competition between the plants that have established themselves and the new arrivals. Soon larger plants that are able to utilise the now available topsoil overshadow the lithophytes until these new plants are now the primary life form in the area. Thus Clement's model states that each generation of pioneers modifies the environment until it is more suitable for another species than for themselves. This is known as facilitation.

5/   Stabilisation
Stabilisation coincides with the climax vegetation (the last and largest plants to succeed). The climax vegetation are now the final species that will dominate the landscape barring major catastrophe. Clement
gives examples such as the coniferous forest in North America. Of course in the tropics many different types of vegetation dominate the landscape but Clement has probably never been to the tropics.Other parts of his facilitation model have been challenged and there are now other models of succession.
Vegetation occurs in different environments such as on rock or sand or in water.

     Succession can be arrested by different factors that inhibit the progression to a climax community. This arrested development leads to what is called a subclimax which is not a true climax. Clement believed that once the arresting factor is remeved, then the vegetation succession will continue on towards climax. If the arresting factor exists for a very long time, then vegetation might develop that is totally unlike any other stages in a normal vegetation succession. The factor has deflected the succession from it's path. A good example is in Britain where, after centuries of sheep farming the landscape is dominated by sheep fescue and red fescue (grasses developed because of sheep grazing). This is called a plagioclimax (man controlled). The fescues are a subsere which persists until the sheep are removed. Clements also theorised that vegetation composition depended on overhead climate. That is, as long as the climate is stable, all other factors are modified until they are unimportant. He thought that eventually all similar climates will have similar vegetation. This supports his monoclimax theory where the climax community will be dominated by similar species due to the climate.

Criticisms

     The monoclimax theory has been challenged by geographers who say that climate cannot be the single most important factor as there are many areas of the world where different types of vegetation flourish under the same climate. It has been argued that different stable communities develop because of the relative importance of other factors. For example, edaphic climaxes are influenced mainly by the soil type as opposed to the climate. In Guyana, different vegetation types exist in the same climate zone but on different soil types. This and evidence from other tropical regions gave rise to the polyclimax theory where there is more than one important factor that determines the climax vegetation.
     Clement stated that early colonisers change the environment until it is more suitable for the competition than for the colonisers. The colonisers do change the environment by producing shade, humus addition and mechanical weathering, but is this the reason why the colonisers are replaced by different vegetation? Another theory is that both the early and the late species arrive at the same time but the early species grow quickly and have a short life span. The early species are intolerant of shade and are efficient at photosynthesis. When these fast growing vegetation reaches the end of their cycle and begin to die, the slow growing vegetation gains a foot hold. the slow growing vegetation dominate because they fast growing vegetation are intolerant of shade and so the seedlings cannot grow beneath the slow growing vegetation's shade. The quick growing vegetation then begin to thin out and the slow growing vegetation thus dominate.