Development of Vegetation and Succession

We have met the phenomenon succession several times by now. Since fossil pollen occurs in a rather good state and sufficient amounts especially since pleistocene, the development of the vegetation after the ice age was relatively easy to reconstruct. 14C analysis helped in many cases to draw up a reliable time-scale. The results allowed not only to draw conclusions regarding the development of moors and woods after the ice had molten, but also such regarding the beginning and the degree of deforesting of Central European woods, since the percentage of herbal pollen rose steadily in contrast to that of trees.

The vegetation after the ice age according to the pollen diagram drawn at Süderlügum, Germany. The calibration of the time-scale occurred by analysing the ¹⁴C-content of the samples. The calibration marks are given as horizontal lines. The diagram shows that the tree species were subject to a succession. At first, Pinus and Betula dominated, then they were followed by Corylus, Quercus, Alnus, and Fraxinus in more recent times. Two herbal species are shown in the right part of the picture: Caprinus and Plantago lanceolata (according to K. KUBITZKI and K.O. MÜNNICH, 1960).

The concept of succession is based on the assumption that a habitat is changed by its vegetation and that this change again causes another change of vegetation, even if all other factors, especially the climate, stay the same. It is therefore distinguished between primary succession like the colonisation of rocks by lichen, mosses and then herbs, shrubs, and finally trees, and a secondary succession , i.e. the reestablishment of a vegetation that had been destroyed by either human or natural influences (lightning, for example).

The phenomenon competition causes the fact that species are often limited to only a part of the habitats they could principally settle in. This fact has led to the definition of the concept of ecological niches. An ecological niche is not only a spatial or timely section of the environment, but the totality of the abiotic and biotic factors used by a population of plants or animals. The concept of niches does, too, explain why species with similar requirements cannot coexist (complete competitors cannot coexist!). We have already discussed coexistence and illustrated it with the results of an experiment involving two clover species. Both species, Trifolium repens and Trifolium fragiferum, coexist only with decreased performances. The reason is a timely shift of the growth phase of both species as well as different growth forms. Trifolium repens develops earlier and has therefore a denser foliage at an earlier time, while Trifolium fragiferum has longer stems and is thus able to grow higher than the first species inhibiting its further growth. Coexistence in the same habitat at the same time has four requirements:

1. Different nutritive conditions (for example leguminoses with their ability to supply themselves with nitrogen and all other plants on the other hand).

2. Different causes of dying (different sensitivities against feeding upon, for example).

3. Different sensitivities against toxins (see the example mentioned at the beginning).

4. Sensitivities against a certain factor during different stages of development.

Successions can consequently be described by three, partly already mentioned criteria:

1. Succession is a well-organised process during which a system develops while its composition of species and thus the system itself change. The development itself is directed.

2. The changes caused by organisms have an effect on the physical environment. The succession is caused by the organisms themselves. The physical environment determines only the pattern and the velocity of the changes.

3. Succession ends with stable ecosystems that are characterised by a maximal biomass (a high degree of usage of information) and a high percentage of interactions between species.

The early stages of development are called pioneer stages, the last is called climax. Successions can also be regarded as the evolutions of ecosystems. A driving out of species occurs necessarily since all species together ( = the ecosystem) change the physical environment and thus take away the existence of certain species while offering a new existence for others. The process of driving out occurs until a new composition of species has developed that is in balance with both the biotic and the abiotic environment. This changes the flow of energy of the system drastically. Energy is increasingly required only to maintain the system, but not to produce additional biomass.

If you call the gross primary production of a system R and the respiration of all system elements P, then the ratio P/R > 1 means autotrophy, while P/R < 1 means heterotrophy. As long as P > R, biomass and dead organic material accumulate, the rate of increase drops steadily and decreases finally drastically. In other words: During a succession, the character of a system changes from autotrophic to heterotrophic. Woods with a high percentage of wood plants are typical climax societies. This seems to be paradox as it is known that wood plants are more original angiosperms than herbs. The paradox is solved when condidering that a climax society can only exist under continuous abiotic environment factors (mostly climate). It is well-known that such conditions exist hardly ever. Climatic changes and the conditions in most parts of the world (too dry, too moist, too cold) do not allow woods, and this is the chance of herbs. Enormous areas of wood have been cleared by humans. And still does it look as if the yearly production of biomass would be rising. The evolution of herbs is thus a successful and opportunistic strategy of plants to survive in a variable environment and to maintain their genetic information. Selection starts, as is known, with the individual, not the whole society.