Xerosere is defined as a succession which begins in dry habi- tat and reaches a climax.
The habitat, besides being deficient of water, is devoid of organic matter.
The most typical xerosere is the lithosere that starts on a bare rock (Fig 30.15). Different stages involved in a xerosere (lithosere) are described below.
1. Crustose lichen stage:
A bare rock is the substratum for the growth of the pioneers. The rock is exposed to high tem- perature and has very little moisture and organic matter. The only category of plants that can grow here are crustose lichen like Rhizocarpon, Lecanora, Rinodive.
These lichens secrete some acids that cause weathering of the rock. Death of the lichens provides dead organic matter. The soils formed by weathering of the organic matter make the substratum suitabte for the growth of foliose lichens.
2. Foliose lichcn stage:
Foliose lichens like Par-melia, Dermatocarpon appear on the substratum which gradually flourish and replace the crustose types. Presence of foliose lichens not only adds organic matter but accumulates dust particles and thus helps in the creation of hu- mus. Formation of thin layer of soil creates a new habitat where moss can grow any) and the floating plants. Plant debris and deposition of soil reduces the depth of water and the habitat bccomes less suitable for the reed-swamp plants.
Sedge grass stage: This stage which is l.lso called sedge marsh or meadow stage is the result of the formation of marshy soil due to further decrease in water level after the death of preexisting community. In the beginning, plants belonging to the families Graminae and Cyperaccae start growing in the habitat. Examples of plants that belong to this stage arc Carex, Juncus, Mentha, Iris etc.
Growth of these plants causes excessive absorption and transpiration. This, in turn, greatly modifies the habitat and makes it unsuitable for existing plants. Accumulation of plant debris and deposit of soil par- tides along with absorption-transpiration patterns of the plants creates an environment totally unsuitable for the growth of hydrophytes. Gradually mesophytes appear and the sedge vegetation gets replaced.
Creation of a mesophytic habitat allows shrubs and medium sized trees to grow. These plants not only produce more shade but also absorb and transpire large quantities of wa- ter. Shade loving herbs or shrubs may grow under the trees. Examples of plant species belonging to this stage are Acacia, Cassia, and Salix etc.
With the continuous creation of humus due to deposit of plant debris and addition of more soil, a suitable habitat is created not only for the growth of microorganisms, but also huge plants. Creation of such a community is called the climax after which further succession is not possible.
The hydroseric succession takes a very long time (thousands of years) but is responsible for the creation of a forest from a pond. It is a very slow process that cannot be observed.