Q.Name the pioneer species among bryophytes, lichens, and ferns in xeric (xerarch) succession.

Plant (ecological) succession is the gradual, directional, sequential process by which one plant community in an area is progressively replaced by another over time, eventually leading to a stable, self-perpetuating climax community. It proceeds through a recognisable series of seral stages, each of which alters the local environment — chiefly by building up soil, organic matter, and shelter — in ways that allow the next, usually more complex, community to establish itself.

Primary versus secondary succession

Primary succession begins on a bare, essentially lifeless substratum that has never before supported vegetation and that has no soil to begin with — bare exposed rock, for instance, or a newly formed pond. With no soil, no seed bank, and no organic matter already present, primary succession must build everything from scratch, which makes it a slow process.

Secondary succession occurs on a site that previously carried vegetation and still retains soil, but whose existing plant cover has been removed or disturbed by an event such as fire, deforestation, or the abandonment of farmland. Because soil, some organic matter, and often a bank of dormant seeds or root fragments already exist at the outset, secondary succession skips the slow soil-building early stages that primary succession must go through — which is exactly why the climax stage is reached much faster in secondary succession than in primary succession.

Xerarch succession

Xerarch succession is primary succession that begins in a dry habitat, such as bare rock. Its pioneer species — the very first colonisers of the bare surface — are lichens, which are able to grow directly on rock and, through the organic acids they produce, gradually weather it down, beginning the slow process of soil formation. Once a thin soil layer has accumulated, bryophytes (mosses) establish themselves, trapping further dust and organic matter and deepening the soil. Ferns colonise next as conditions improve, followed by herbaceous plants, then shrubs, and finally trees, which together form the stable climax forest community. Two well-known examples of xerarch succession are succession on bare rock surfaces (a lithosere) and succession on shifting sand dunes (a psammosere) — both starting from a bare, soil-free mineral surface and following the same general lichen-to-forest sequence.

Succession is thus not a single event but a chain of cause and effect: each seral community modifies its own habitat just enough to permit the arrival of the next, until a community is reached that can maintain and reproduce itself indefinitely without being displaced further — the climax.