Plant Succession

The Ecological Concept

It is important to understand the concept of ecology and how it affects plant succession

 

Ecology is the study of one or more living organisms in their natural environment (habitat) and of the habitat itself. It is derived from two Greek words oecus which means home and logos which means study. Plant ecology thus means the study of a plant or plants in their natural home or habitat. If an individual species is studied in its natural habitat, it is known as autecology while synecology is the term applied when several species living together are studied.

The ecology of natural grazing veld with which the farmer is concerned is defined by Humphrey as a study of the influence of plants in the veld (grazing veld) on animals, on each other and on the area in which the animals live. Veld management is thus applied veld ecology in practice.

Importance of Plant Succession

Different stages of primary succession from bare rock to a full climax community

 

Primary succession refers to the development of plant and animal life in an area without topsoil; the development of biotic communities in a previously uninhabited and barren habitat with little or no soil.

For various reasons, it is important to be familiar with plant succession. The following reasons can be advanced for the necessity of the farmer who has to deal with grazing practices to know plant succession.

The farmer must be able to determine the stage in the plant succession which his veld occupies, e.g., the pioneer stage or a stable climax stage. These stages will be discussed in more detail further down.

The degree of improvement or retrogression must be monitored continually, using as a criterion the decrease or increase of indicator species, the extent of soil protection and the productivity of the veld.

There must be a clear image of the species composition, density, and productivity of the various veld types in different stages of plant succession. This is useful so that long-term planning can be applied.

This knowledge of plant succession must be applied in the veld management programme.

Vegetation and Development of the Soil

Soil is critical to the survival of plant, animal and human life

 

The soil and its processes do not constitute an independent system but are part of a larger ecosystem which embraces both the vegetation and the whole habitat. Just as the vegetation affects the soil, so the soil affects the vegetation. It is known that certain soils can support only certain plants and therefore there is a close relationship between the distribution of vegetation types and soil types.

Vegetation and development of soil can best be illustrated as follows: Suppose that the soil from a big area (1 ha) is removed so that just the parent material and the underlying rock remain. This residual part of the soil, which is mainly rock, will not be able to support the same vegetation as the surrounding well-developed soils. The plants which begin to grow on the bare area can assist in the development of soil by the addition of organic matter over time (100 years or more); a soil profile with different horizons will develop and the vegetation will also change until it eventually resembles the natural vegetation of the surrounding area. In such a case the development of the vegetation will be governed largely by the development of the soil. This is known as edaphically controlled plant succession.

The final stage of soil development and plant succession will be determined by climatic factors.

The Plant Community

Distinctive grass plain plant community

 

Various plants occur in groups with different combinations which form more or less definite communities and can be regarded as distinctive units of vegetation. Examples of such communities are grass plains, bushman grass veld, forests in mountain ravines, and swamp grasses in water courses. A plant community can thus be described as a group of plants which occurs in a more or less stable association having an underlying influence on each other and the environment. Such a group of plants would have similar requirements for growth and survival. Plant communities are thus related to a particular habitat and are the product of environmental factors. It can also be accepted that a plant community is a reliable indicator of environmental conditions provided that one knows the indicator value of the community.

A plant community can thus be defined as any recognisable unit of vegetation.

Plant communities found along the Eerste Rivier in Western Cape

(a) Populus community. Tall Populus x canescens with a herbaceous understorey of Tropaeolum majus and Vinca major, with Olea europea subsp. africana and Kiggelaria africana in the middle shrub layer; (b) Bromus community with Tropaeolum majus and Bromus diandrus in the foreground; (c) Arundo community showing dense thicket of Arundo donax with Tropaeolum majus in the understory and Anredera cordifolia climbing the Arundo stalks; (d) Phragmites community showing dense thicket of Phragmites australis, with Salix mucronata subsp. mucronata and Pennisetum clandestinum present on the left and (e) Salix community showing bushy patches of Salix mucronata subsp. mucronate.

 

The main plants adapted for certain environmental conditions:

Mesophytes:

These include most of the green plants and thus also most of the cultured plants. They are of such a nature that they only grow well under average moisture conditions. They are not adapted to grow under very wet or very dry conditions.

Mesophytic plants have rigid, sturdy, freely branched stems, and fibrous, well-developed root systems– either fibrous roots or long taproots. The leaves of mesophytic plants have a variety of leaf shapes, but they are generally flat, thin, relatively large, and green in colour. During hot weather, the waxy cuticle of the leaf surface protects the leaves by trapping moisture and preventing rapid evaporation. Stomata, small openings on the undersides of the leaves, close in hot or windy weather to prevent evaporation and minimise water loss. Stomata also open to allow the intake of carbon dioxide and close to release oxygen as a waste product.

Examples of mesophytic plants include herbs, crops and deciduous trees.

 

 

Hydrophytes:

These plants are habitants of very wet habitats, and they have to cope with an excess of water. Usually, they are water plants of which the roots are anchored in the soil, whilst the leaves float on the water surface or stems and leaves that stand out above the water, e.g., waterlilies. Some plants grow entirely submerged. Hydrophytes have adapted with the following adaptations to survive in these water-drenched areas.

Cuticle: The majority of floating hydrophytes have a thin, waxy cuticle that allows for good gas exchange while preventing excessive transpiration. Fully submerged plants lack a thin waxy cuticle but may have a thin cuticle on occasion.

Air sacs: One of the distinguishing characteristics of a hydrophyte is the presence of huge air cavities around the spongy and palisade mesophyll cells. It facilitates gaseous exchange, maintains water balance, and aids hydrophyte flotation.

Roots: Because the roots of floating and partially submerged hydrophytes are diminished, the water can be taken up directly by the leaves. Thus, hydrophytes have roots that can change into fibrous or spongy structures to offer anchoring and stability to the plant body rather than absorbing oxygen.

Leaves: Fully submerged leaves have thin leaves that increase the surface area to volume ratio, boosting the rate of water and mineral penetration into plant cells. The leaves of a few hydrophytes become broader and flattened to float.

Examples of Hydrophyte plants include water lilies and plants which grow in marshes or along ponds or other bodies of water.

 

 

Halophytes:

These plants usually grow in soils with an exceptionally high salt concentration. Physiologically, such soils are regarded as dry despite the abundance of water e.g., salt marshes and sea lagoons. These plants have a very high cell sap concentration in the root cells. They normally occur in salty marshes and mangroves. Mangrove trees (see mangrove swamp), thrift (Armeria), sea lavender (Limonium), and rice grass (Spartina) are examples of halophytes.

 

Examples of halophyte plants: 

 

Xerophytes:

These inhabit dry regions where there is intense competition for water. They have a high drought resistance. Usually, they are herbs or dwarf shrubs. They can be divided into two large groups:

Non-fleshy xerophytes; and Fleshy xerophytes (succulents):

• Various types of non-fleshy xerophytes are found, e.g.,

Thin-leaved xerophytes possess thin, soft leaves that wilt quickly if they are detached from the plant e.g. Lucerne. In this case, the root system is adapted to resist drought conditions.

Lucerne grass

 

Thirst plants or sclerophyllous xerophytes can survive more arid conditions than the thin-leaved types. Mostly, they possess small, stiff, and leathery leaves e.g., the macchia types and Karoo bush species. The aerial parts are also much reduced in comparison to the root system. Their absorption power is high: that is, they possess a highly concentrated cell sap solution in the root cells. Many modifications are found e.g., the phylloclade of asparagus. Phylloclades are modifications of the stem to do the work of leaves.

The King Protea is an example of a sclerophyllous xerophyte in the Western Cape which falls under the macchia plant types

 

Pseudo xerophytes. In desert and semi-desert regions plants are found resembling mesophytes. They are the so-called ephemeral plants. Ephemeral plants are plants with a short life cycle. If for example, a shower of rain falls, then seeds germinate immediately, and the young plants develop so quickly that within 3 – 4 weeks flowers and ripe seeds are produced. These plants survive drought using seeds. Because they are practically entirely mesophytic, they cannot be regarded as true xerophytes, but are also known as drought-evading plants.

Namakwaland is known for its burst of colourful flowers in the desert shortly after a rainshower

 

• Fleshy xerophytes (succulents) are easily distinguished on account of their unusual, fleshy appearance. Large water reserves are stored in the leaves and stem. Transpiration is slow. Modifications are usually small leaf surfaces e.g., the thorns of cacti, which are modified leaves. Another modification is the peculiar leaf arrangement of e.g., aloes, where fleshy leaf rosettes are present. In other cases, the stomata are few and sunken. The leaves have a thick cuticle. Many hairs are sometimes present.

Examples of fleshy xerophyte plants which can withstand dry conditions due to their water-storing abilities

Dynamics of Plant Growth

The vegetative layer is continuously changing and growing along with the different environmental conditions and factors

 

A feature of vegetation is that it is not static and is always changing according to environmental conditions. Under optimum conditions, the vegetation will more or less stabilise until a disturbance takes place. Such disturbance can be the result of climatic changes, particularly rainfall, or it can be an artificially induced change brought about by man management. Thus, a stable plant community can be forced to change. The speed and perceptibility of these changes are dependent on the various factors which affect the plant community and enforce change.

Since the plant communities, which originally occurred in an area when the white man began farming in South Africa, are generally regarded as the best from the aspect of grazing, the management of such areas should be such that the plant communities do not change.