Xerosere

Xerosere is a plant succession which is limited by water availability. It includes the different stages in a xerarch succession of ecological communities originated in extremely dry situation such as sand deserts, sand dunes, salt deserts; rock deserts etc. a xerosere may include lithoseres on rock and pasammoseres on sand.

Stages of Xerosere

Stage 1: Crustose Lichen stage

A bare rock consists of solid surface or very large boulders and there is no place for rooting plants to colonize. The crustose lichens like Licanora, Rhinodina can adhere to the surface of rock and absorb moisture from atmosphere. Therefore these colonize the bare surfaces of rocks fast. The Propagulis of these lichens are brought by air from the surrounding areas. These lichens produce acids which corrode the rock and their thalli collect wind blown soil particles among them that help in formation of a thin film of soil. When these lichens die their thalli decomposed to humus. This promotes soil building and the environment becomes suitable for growth of foliose and fruticose type of lichens.

Stage 2: Foliose and Fruitcose Lichen Stage

Foliose have leaf like thalli while the fruticose lichens are small bushes. They are attached by the substratum at one pint only. Therefore do not cover the soil completely. They can absorb and retain more water and are able to accumulate more dust particles. Their dead remains are decomposed to humus which mixes with soil particles and help building substratum and improving soil moisture contents further. The shallow depreciation in the rocks and crevices become filled with sold and top soil layers increases further. These autogenic changes favor growth and establishment of mosses. This community includes Permelia and Dermatpcarpom etc which have large leaf like thalli.

Stage 3: Moss Stage

The spores of xyrophytic mosses such as Tortula and Grimmia are brought to the rock whether they succeed lichens. They are rhizoids penetrates soil among the crevices secret acids and corrode the rocks. The bodies of mosses are rich in organic and inorganic compounds. When these die they add these compounds, to the soil to increase the fertility of the soil. Since mosses develop in patches they catch soil particles form air and help increasing substratum. The changing environment leads to migration of lichens and help invasion of herbaceous vegetation.

Stage 4: Herb Stage

Herbaceous weeds mostly annuals such as asters and evening primroses invade the rock. Their roots penetrate deep down, secret acids and enhance the process of weathering. Leaf litter and death herbs add humus to the soil. Shading of soil results in decrease in evaporation and there is a slight increase in temperature. As a result in decrease in evaporation and there is a slight increase in temperature. As a result the xeric condition begins to change and biennial and perennial herbs and xeric grasses begin to inhabit. This climatic conditions favor growth of bacterial and fungal populations resulting in increase in decomposition activity.

Stage 5: Shrub Stage:

Herb and grass mixture is invaded by scrub species such as Rhus and others. Early invasion of scrub is slow but once a few bushes have become established birds invade the area and help disperse scrub seeds. This results in dense scrub growth shading the soil and making conditions unfavorable for the growth of herbs which begin to migrate. Te soil formation continues and its moisture contents enhance. The environment becomes mesic or moderately moist.

Stage 6: Tree Stage

Change in environment favors colonization of tree species. The tree saplings begin to grow among the scrubs and establish themselves. The kind of tree species inhabiting the area depends upon the nature of the soil. In poorly drained soils oaks establish themselves. Te trees form canopy and shade the area. Shade loving shrubs continue to grow as secondary vegetation. Leaf litter and decaying roots weather the soil further and add humus to it making the habitat more favorable for growth to trees. Mosses and ferns make their appearance and fungi population grows abundantly.

Stage 7: Forest Stage or climax Stage

The succession culminates in a climax community, the forest. Many intermediate tree stages develop prior to establishment of a climax community. The forest type depends upon climatic conditions.

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