agricultural production guidelines
m
Veld in KwaZulu-Natal
Co-ordinated Extension KwaZulu-Natal Veld 7.4 1999
GRASS AND BUSH
C R Hurt
ARC - Range and Forage Institute, Cedara
INTRODUCTION
The savanna regions of KwaZulu-Natal form an important part of extensive beef and game ranching systems in the province. Correct management of the vegetation is therefore essential in order to ensure the long-term productivity of these areas. Much of the savanna in KwaZulu-Natal can be classified as sweetveld (i.e. the herbaceous layer retains its acceptability and nutritive value after the grass plants have reached maturity). Savanna ecosystems are extremely resilient, and are able to recover rapidly following stress periods (e.g. drought, heavy grazing), provided that a major loss of top-soil has not occurred. Savanna functioning is dependent, to a large extent, on a balance which exists between the bush component and the herbaceous layer, and, depending on objectives, management is able to shift the balance in favour of either the bush or grass component. In the savanna areas of KwaZulu-Natal, where cattle and game ranching enterprises are common, it is essential that a healthy grass cover is maintained between trees and shrubs. If poor grass cover exists, animal production declines rapidly. Poor grass cover results from overgrazing, which, in turn, encourages the woody plants to increase in density and become more competitive, thereby causing an even greater decline in the grass production. In this Production Guideline the balance and interactions between grass and woody plants are examined, and a theory is provided for the management of the vegetation in order maximise animal production. Many of the concepts described in Production Guidelines 7.2, 7.3, 7.5, 7.6 and 7.7 in this series have relevance to the bush/grass balance, and should therefore be referred to.
Why does grass production decrease?
Overgrazing or continual heavy stocking causes particularly the palatable grass plants to lose vigour as a result of the repeated removal of leaves (the plant's ‘energy factory’) and the constant drain on its nutrient reserves. When the plant is not able to replenish these reserves by producing new leaves during rest periods to carry out photosynthesis, it will eventually die, and its place will be taken by other grasses which are either better adapted to heavy grazing, or which are relatively less palatable.As the desirable plants become weaker and die off, the number of roots present in the upper layers of soil decreases and this reduces the competitive ability of the grasses. Bare areas between grass plants also become larger as the grasses become smaller or die. This causes a decline in the effective rainfall in the area as a result of increased runoff, and gives rise to a situation usually blamed on ‘droughts’. These conditions are ideal for woody plants to establish themselves, and eventually take over areas where grass previously predominated.
How does grass keep the bush in check?
Grasses and mature woody plants use different areas in the soil profile to obtain moisture and nutrients. Grass roots are mainly found in the upper soil layers, while woody plants are able to extend their roots to utilize moisture and nutrients at deeper levels, as well as in the topsoil. It is easy to see that once a woody plant has extended its roots to the deeper horizons (i.e. once they have become established), the grass roots will not compete with them for water and nutrients.However, when the seed of a woody plant germinates, it has to compete with the grasses for moisture and nutrients in the upper soil layers until it can extend its roots to soil layers below those utilized by the grasses. At this stage the seedling is most vulnerable, since the root system of the grass plant is more extensive than that of the woody seedling. If the grass layer is in a healthy, vigorous condition, a mass of roots will be present, and will offer strong competition to the young woody plant. A period of stress (e.g. mid-summer drought) will therefore often result in the woody seedling dying. If a healthy, competitive grass layer is maintained, it will only be during periods of above-average rainfall that the woody plants are able to establish themselves.
On the other hand, if the grass layer is in a poor condition due to overgrazing, or because it has become moribund as a result of infrequent burning, relatively fewer roots will be present and will therefore offer less competition to the woody seedling. Seedlings will stand a greater chance of surviving any stress period and will be able to establish their root systems in the subsoil layers. Once this takes place, the woody plant is unlikely to die as a result of competition from other plants.
Why does the bush thicken up?
Once woody plants become established in areas that were previously grassland or open savanna, the grasses are placed at a disadvantage due to competition from the woody plants (i.e. for moisture, nutrients and light). This is due to the fact that the roots of the woody plants are present in both the upper and lower soil layers. This increased competition results in a further decline in grass production, and promotes the creation of better conditions for the establishment of seedlings of woody plants. Eventually a situation is reached where the woody species start to compete with one another for moisture, nutrients and light, and their growth becomes retarded. This is the case where a short, dense shrub layer with a closed canopy virtually prevents the growth of any grass plants. Such situations are now common over a large part of the KwaZulu-Natal savanna areas, and can be ascribed primarily to poor veld management practices and old land situations.Can trees control trees?
The density of woody individuals which develops depends on the potential of a piece of land to provide the resources needed for growth (moisture and nutrients). This potential is called the carrying capacity. Once the density of woody species reaches the carrying capacity of the land, individuals will also start competing with one another for moisture, nutrients and light. At this stage, growth is reduced considerably and the trees do not exceed a certain height, depending on the species and the number of individuals of each species present. Virtually all the energy which is produced by metabolic processes in the plant is used for maintenance, but minor fluctuations will occur in sympathy with the erratic nature of the rainfall. It is only on the death of an individual that its resources, which were previously unavailable to other plants, become available. The individuals in its immediate vicinity will then be able to continue growing until the carrying capacity is once again reached.To illustrate this concept, consider the following example. The carrying capacity of one hectare of land dictates that it is able to support, for example, 2 000 sweet thorn trees 1 m high, or 1 000 sweet thorn trees of 1.5 m, or 500 trees of 2.5 m, or 200 trees 3.5 m high. Various intergrades are possible, but in general, there is a distinct relationship between the size of the individuals and the number of individuals present. Obviously, as the trees increase in density, the spaces between them decrease. Conversely, as the trees grow larger, their root systems and canopies extend over a larger area and the spaces between individuals become greater. They therefore make a greater demand on the resources of the area, and thus the larger the individuals are, the fewer individuals will be supported by the available resources.
Thus, once a situation is reached where the carrying capacity of the land has been attained, existing trees will outcompete or suppress the growth of younger woody plants that germinate between them. On the other hand, when a tree dies of natural causes, other woody plants will be able to take its place. Although little research has been conducted on this theory in KwaZulu-Natal, it has potential as a long-term means of keeping areas open and productive. The concept here is rather to maintain a low density of large, widely-spaced trees, than to encourage a high density of smaller individuals by repeatedly removing all, or most of, the smaller trees. Widely-spaced trees will favour grass growth and access by animals.
In conclusion, it can be seen that it is critical that the vitality of the grass sward be maintained in order to keep a competitive balance between the woody and herbaceous components of savanna vegetation.