Successional Stages of Xerosere
Succession of plant that begins from any dry area and ultimately reaches to a climax is termed as xerosere. Initiation of xerosere usually begins from a bare rock surface. The rock surface is normally very smooth and gets heated with bright sunlight.
Lithosere is a xerosere that occurs on a bare rock surface. This habitat lacks water and organic matter. The substratum consists of minerals in disintegrated unweathered state. The pioneer community is formed by crustose lichens while the forest forms the climax community. Following are the various stages of a lithosere.
Crustose lichen stage:
Growth of plants on smooth surface or a bare rock is difficult. It is because it does not contain water and sufficient nourishment. Moreover the rocks get very much heated with sun. Here, the habitat is a bare rock that has no moisture, no organic substance or mineral particles. Only the lichens are able to grow under these conditions. Crustose lichen species such as Rhizocarpon, Lecanora, Lecidea,Usnea, Rinodina etc.,adapt to such a condition. They form the pioneer vegetation of the xerosere.
The lichen body consists of both algae and fungus, which live on symbiosis. The algae partner of the lichen prepares food and supplies to the fungus while the fungus forms a thin base on which the alga grows. These lichens grow profusely especially during the rainy season. These plants dry up and stick to the rocks just like a bark.
These lichens release some acid which slowly corrode the rocks producing a few mineral particles. Water on the under surface of these plants reacts with carbon dioxide and produce carbonic acid. This acid has the capacity to dissolve the rocks. The dissolved rocks gradually accumulate and form a type of soil. Formation of more amount of soil creates a new condition where some new type of plants can grow.
The death of lichens produces organic matter. Such a habitat with mineral particles and some amount of organic matter is unsuitable for crustose lichens to grow.
Foliose lichen stage:
As some amount of soil deposits on the rock surface, foliose lichen species such as Cladonia, Parmelia, Dermatocarpon etc., begin to grow there. They gradually spread above the crustose lichens. When the foliose lichens become dense the crustose lichens do not get any light and ultimately die.
The decay materials form too much humus in the soil and as a result the soil retains more amount of water. High water content of the soil helps the lichens to produce more amount of carbonic acid which converts the rock into more and more amount of soil. Thus with the increase in soil and humus content over the rock surface the habitat is changed.
The habitat modified by the foliose lichens is unsuitable for itself but suitable for mosses to grow. Accumulation of patches of soil on rock surface creates the first condition for the growth of mosses. Some of the mosses such as Torula, Polytrichum, Grimmia, etc., occupy the place.
They grow rapidly and compete with the foliose lichens. The stems of the mosses come up above the lichen thalli. Gradually the moss plants get tangled with their foliages and form a carpet like covering. As a result, foliose lichens gradually perish. Their decay materials form humus and finally add to the soil content. At this stage the depth of the soil becomes nearly 2-3 centimetres.
The soil formed at the moss stage becomes suitable for the growth of some xerophytic herbaceous plants. Accumulation of humus and leaching results in more water retention and increase in soil mineral content. The dry conditions slowly begin to change the substratum. This now becomes suitable for shallow rooted grasses such as Aristida, Festuca, Poa etc.
These herbs are very short lived and mostly annual. Roots of these herbs penetrate into the rock and break it. They also suppress the growth of moss plants. Since the herbs are annual, they die every year. As a result they add to the soil content as well as the fertility of the soil. In this way the condition gradually become suitable for the growth of biennial and perennial herbs as well as shrubs.
Towards the end of the herbaceous stage the amount of bacteria, fungi and other microorganisms increases in the soil. These organisms prepare a lot of humus from the herbaceous plants and the soil gets enriched with fertility. Then gradually shrubs such as Phoenix, Cactus, Cycas etc., migrate into the place. The shrubs are usually taller than herbs and the herbs cannot compete with the shrubs for light, air, space etc. The herbs gradually die and add to the humus of the soil. Gradually the water retaining capacity of the soil also increases.
Forest stage or Climax stage:
At the beginning of this stage some xerophytic plants grow here and there. Gradually these plants create better condition of the soil and bigger plants come to existence. The weathering rocks increase the amount of soil particles. The death and decay of shrubs also add a large amount of humus to the soil.
The climatic condition changes. Moisture content of the atmosphere and soil increases. This substratum favours the growth of trees. The habitat is now mesic and thus suitable for mesophytic flora. As a result finally more number of mesophytic plants grow there. Once the forest of mesophytic plant is established the succession stops and the vegetation reaches to the climax.