Xerosere and Stages of Xerosere - BioQuestOnline stages of xerosere

Xerosere, also known as xerarch succession, is a vegetational succession which develops in xeric environments such as sand dunes, deserts and bare rock surfaces where the initial substrate is poor in water and organic matter, consisting only of minerals in a disintegrated, unweathered state.

The first organisms to colonize this primitive substrate are crustose lichens. Through a series of progressive seral stages, the succession eventually leads to the establishment of a forest, which represents the climax community.

Table of Contents

Xeroseres are typically categorized into two types: Psammosere and Lithosere.

Psammosere: Psammosere refers to succession that occur on newly exposed sand, like sand dunes, and represent a transitional phase in ecological succession.

Lithosere: Lithosere refers to a succession that often takes place on bare rock surfaces exposed by glaciation or erosion by wind and water. These rocky habitats present extreme xeric conditions. With direct sun exposure, rock surfaces can reach very high temperatures, and the smooth, exposed rock lacks any mechanism for retaining water and nutrients. Only plants capable of enduring severe drought conditions can survive in these habitats.

Stages of Xerosere (Lithosere)

Following are the typical seven stages of xerosere (Lithosere) during which pioneer colonizer, crustose lichens are replaced gradually eventually leading to the establishment of climax community.

Pioneer crustose lichen stage
Foliose and Fruticose lichen stage
Xerophytic Moss stage
Herb stage
Shrub stage
Tree stage
Climax stage

Pioneer Crustose Lichen Stage

Bare rocks are formed when glaciers recede or volcanoes erupt. These rocks are gradually eroded by rainwater and wind carrying soil particles.
Rainwater mixes with atmospheric carbon dioxide (CO₂ + H₂O = H₂CO₃, forming carbonic acid) which corrodes the rock surfaces, creating crevices.
Water enters these crevices, freezes, and expands, causing boulders to separate. Through these processes, a thin layer of soil eventually develops on the rock surfaces.
Algal and fungal spores, carried by air from surrounding areas, land on these rocks and establish a symbiotic association, forming crustose lichens, which serve as the first colonizers of bare rocks.
Succession begins when autotrophic organisms start to inhabit the rocks, forming a membranous crust.
Examples of crustose lichens include Rhizocarpon and Lecanora. During dry periods, these pioneer species may appear desiccated but remain alive.

Lichens release excess carbonic acid (formed when CO₂ from respiration combines with water), which breaks down rocky material.
Combined with decaying lichens, this process forms the initial thin layer of soil on the rock surface.
Nitrogen compounds created from atmospheric gases during lightning are then added to the soil by rain.
Over time, the habitat becomes less suitable for the existing plants, causing them to gradually disappear.

Foliose and Fruticose Lichen Stage

The rock surface now becomes populated by foliose lichens, which have leaf-like thalli, and fruticose lichens, which resemble small bushes, such as Dermatocarpon, Parmelia, and Umbilicaria.
These lichens attach to the substrate at only a single point, leaving some soil exposed.
They are capable of absorbing and retaining more water and can accumulate more dust particles.
As they die, their decomposed remains add humus to the soil, enriching it and enhancing moisture retention. Soil continues to accumulate in shallow rock depressions and crevices, thickening the topsoil layer.
These autogenic changes create conditions that support the growth and establishment of mosses.

stages of xerosere — Fig: Stages of xerosere

Xerophytic Moss Stage

Spores of xerophytic mosses, like Polytrichum, Tortula, and Grimmia, settle on the rocks and slowly take over from the lichens.
Their rhizoids extend into the soil within crevices, releasing acids that further erode the rock.
Moss bodies are rich in organic and inorganic compounds, which enrich the soil upon their decomposition, boosting soil fertility.
Growing in clusters, these mosses also capture soil particles from the air, contributing to the accumulation of substrate.

As the environment changes, lichens begin to migrate, allowing herbaceous plants to invade and outcompete the mosses.
These mosses develop rhizoids that penetrate deeply into the rocky soil. The decaying older parts of the mosses form a thick mat on the rock surface, enhancing its water-holding capacity.

Herb Stage

As the soil layer thickens, herbaceous weeds—mostly annuals like asters, evening primroses, and milkweeds—start to colonize the rock.
Their roots penetrate deeply, release acids, and accelerate weathering. The accumulation of leaf litter and decomposing herbs adds humus to the soil.
With soil shading, evaporation decreases, and the temperature rises slightly, shifting the xeric conditions.
This allows biennial and perennial herbs, along with xeric grasses such as Aristida, Festuca, and Poa, to establish themselves.
These changes in climate conditions also encourage bacterial and fungal growth, increasing decomposition activities.

Shrub Stage

Shrub species, like Rhus and Physocarpus, gradually invade the herb and grass mix.
Initially, the spread of shrubs is slow, but once some bushes take hold, birds begin to visit the area, aiding in the dispersal of shrub seeds.
This leads to dense scrub growth, which shades the soil and creates conditions unfavorable for herbs, prompting them to migrate.
Soil formation continues, and moisture content increases.

Tree Stage

Environmental changes create favorable conditions for tree species to colonize.
Tree saplings start growing among the shrubs and gradually establish themselves.
The type of trees that thrive depends on soil characteristics; for instance, oaks establish well in poorly drained soils.
As the trees form a canopy, they shade the area, while shade-tolerant shrubs persist as secondary vegetation.
Leaf litter and decaying roots continue to weather the soil, adding humus and making the habitat even more suitable for tree growth.
Mosses, ferns, and an abundant population of fungi also begin to appear.

Climax Stage

Succession reaches its peak with the formation of a climax community, which is the forest.
Several intermediate stages of tree growth occur before this climax community is fully established.
The type of forest that forms depends on the climate conditions.
The climax forest can be:

Oak-hickory climax forest: In dry habitats, oak and hickory trees form the climax vegetation. This forest type features a single tree stage and is accompanied by shrubs, herbs, ferns, and mosses.

Beech-hemlock climax forest: These climax forests develop in mesic climates, with beech and hemlock as the dominant tree species. Several intermediate tree stages exist, and the understory includes herbs, ferns, and mosses.

Beech-maple climax forest: Found in mesic climates in the Northeastern United States, these climax forests are dominated by American beech and sugar maple.

FREQUENTLY ASKED QUESTIONS

Define Xerosere or Xerarch Succession?

What are 7 successional stages of Xerosere?

1. Pioneer crustose lichen stage
2. Foliose and Fruticose lichen stage
3. Xerophytic Moss stage
4. Herb stage
5. Shrub stage
6. Tree stage
7. Climax stage

Discuss Pioneer Crustose Lichen Stage of Xerosere.

Bare rocks are formed when glaciers recede or volcanoes erupt. These rocks are gradually eroded by rainwater and wind carrying soil particles. Rainwater mixes with atmospheric carbon dioxide (CO₂ + H₂O = H₂CO₃, forming carbonic acid) which corrodes the rock surfaces, creating crevices. Water enters these crevices, freezes, and expands, causing boulders to separate. Through these processes, a thin layer of soil eventually develops on the rock surfaces.

Algal and fungal spores, carried by air from surrounding areas, land on these rocks and establish a symbiotic association, forming crustose lichens, which serve as the first colonizers of bare rocks. Succession begins when autotrophic organisms start to inhabit the rocks, forming a membranous crust.

Examples of crustose lichens include Rhizocarpon and Lecanora. During dry periods, these pioneer species may appear desiccated but remain alive. Lichens release excess carbonic acid (formed when CO₂ from respiration combines with water), which breaks down rocky material. Combined with decaying lichens, this process forms the initial thin layer of soil on the rock surface. Nitrogen compounds created from atmospheric gases during lightning are then added to the soil by rain. Over time, the habitat becomes less suitable for the existing plants, causing them to gradually disappear.

What happen during Foliose and Fruticose Lichen Stage in Xerosere?

The rock surface now becomes populated by foliose lichens, which have leaf-like thalli, and fruticose lichens, which resemble small bushes, such as Dermatocarpon, Parmelia, and Umbilicaria. These lichens attach to the substrate at only a single point, leaving some soil exposed.

They are capable of absorbing and retaining more water and can accumulate more dust particles. As they die, their decomposed remains add humus to the soil, enriching it and enhancing moisture retention. Soil continues to accumulate in shallow rock depressions and crevices, thickening the topsoil layer. These autogenic changes create conditions that support the growth and establishment of mosses.

Explain Xerophytic Moss Stage of Xerosere.

Spores of xerophytic mosses, like Polytrichum, Tortula, and Grimmia, settle on the rocks and slowly take over from the lichens. Their rhizoids extend into the soil within crevices, releasing acids that further erode the rock. Moss bodies are rich in organic and inorganic compounds, which enrich the soil upon their decomposition, boosting soil fertility.

Growing in clusters, these mosses also capture soil particles from the air, contributing to the accumulation of substrate. As the environment changes, lichens begin to migrate, allowing herbaceous plants to invade and outcompete the mosses. These mosses develop rhizoids that penetrate deeply into the rocky soil. The decaying older parts of the mosses form a thick mat on the rock surface, enhancing its water-holding capacity.

Herb Stage of Xerosere Succession.

As the soil layer thickens, herbaceous weeds—mostly annuals like asters, evening primroses, and milkweeds—start to colonize the rock. Their roots penetrate deeply, release acids, and accelerate weathering. The accumulation of leaf litter and decomposing herbs adds humus to the soil.

With soil shading, evaporation decreases, and the temperature rises slightly, shifting the xeric conditions. This allows biennial and perennial herbs, along with xeric grasses such as Aristida, Festuca, and Poa, to establish themselves. These changes in climate conditions also encourage bacterial and fungal growth, increasing decomposition activities.

Shrub Stage of Xerosere Succession.

Shrub species, like Rhus and Physocarpus, gradually invade the herb and grass mix.
Initially, the spread of shrubs is slow, but once some bushes take hold, birds begin to visit the area, aiding in the dispersal of shrub seeds.

This leads to dense scrub growth, which shades the soil and creates conditions unfavorable for herbs, prompting them to migrate. Soil formation continues, and moisture content increases.

Tree Stage of Xerosere Succession.

Environmental changes create favorable conditions for tree species to colonize.
Tree saplings start growing among the shrubs and gradually establish themselves. The type of trees that thrive depends on soil characteristics; for instance, oaks establish well in poorly drained soils.

As the trees form a canopy, they shade the area, while shade-tolerant shrubs persist as secondary vegetation. Leaf litter and decaying roots continue to weather the soil, adding humus and making the habitat even more suitable for tree growth. Mosses, ferns, and an abundant population of fungi also begin to appear.

Climax Stage of Xerosere Succession.

Succession reaches its peak with the formation of a climax community, which is the forest.
Several intermediate stages of tree growth occur before this climax community is fully established. The type of forest that forms depends on the climate conditions.
The climax forest can be:
Oak-hickory climax forest: In dry habitats, oak and hickory trees form the climax vegetation. This forest type features a single tree stage and is accompanied by shrubs, herbs, ferns, and mosses.
Beech-hemlock climax forest: These climax forests develop in mesic climates, with beech and hemlock as the dominant tree species. Several intermediate tree stages exist, and the understory includes herbs, ferns, and mosses.
Beech-maple climax forest: Found in mesic climates in the Northeastern United States, these climax forests are dominated by American beech and sugar maple.