agricultural production guidelines
m
Veld in KwaZulu-Natal
Co-ordinated Extension KwaZulu-Natal Veld 7.3 1999
GROWTH PATTERNS OF WOODY PLANTS
C R Hurt
ARC - Range and Forage Institute, Cedara
Seeds and
Germination
Vegetative Stage
Reproductive Stage
INTRODUCTION
Woody plants form a natural component of most of the vegetation types in KwaZulu-Natal. Severe climate or soil conditions, as well as various veld management factors, may, however, prevent these plants from predominating in certain areas. The lower rainfall areas such as the major river valleys, and the Lowveld in particular, are characterized by savanna and woodland vegetation of varying densities and structures.
A major problem in most savanna areas is the loss of valuable grazing lands when grass production declines as a result of bush encroachment. Acacia species are important in the initial stages of the bush thickening process, and broadleaf plants predominate later in the successional sequence. Once the latter phase has been reached, grass production is drastically reduced (refer to Production Guideline 7.4 in this series). In order to manage the woody vegetation to maintain an open nature, it is important to have an understanding of the growth processes involved in the life cycles of woody plants.
Browsing animals have a high potential in the savanna areas of KwaZulu-Natal, and can contribute significantly towards red-meat production or trophy hunting. A sound understanding of the growth patterns of this source of forage is important for a high level of management that will be conducive to maximum animal production. The objective of this Production Guideline is therefore to examine various stages in the life cycles of woody plants, and to relate these to the effects of management on the vegetation. As woody plants in savanna situations are of prime importance to agriculturalists, the ensuing discussion will concentrate on this vegetation type.
Seed banks
Seeds represent a dormant developmental stage in the life cycles of plants, whereby unfavourable periods for growth are survived. The population of seeds of any species makes up the seed bank of that species. From time to time individuals are recruited to the growing population from the seed bank when germination and growing conditions become favourable.Seed dormancy
Although a few seeds may germinate soon after being released from the parent plant, many enter a dormant phase and contribute to the seed bank. Depending on the species involved, the dormancy may be broken by various factors (e.g. breaking of the seed coat, day length, light intensity, presence of hormones).Most of the pioneer woody plants, and particularly the acacias, tend to have seeds with extremely hard coats that allow them to remain viable for long periods. In most cases the seed coat needs some form of treatment before moisture can enter the seed and initiate germination. This usually involves the breaking of the seed coat, and may occur in a number of ways (e.g. fire and abrasion by sand).
Germination
Germination occurs once the seed embryo starts growing and the new root forces its way out of the seed coat. At this stage, the young plant is totally reliant on the nutrients stored in the endosperm of the seed. The cotyledons which contain the nutrient reserves emerge next, and contain chlorophyll for photosynthesis. The period immediately following germination until the first leaves are formed is the critical stage when the growing plant is most vulnerable to impacts such as defoliation and drought.Establishment
Once sufficient leaves have grown, the plant is able to start manufacturing its own nutrients and become less dependent on the reserves in the cotyledons for further growth. Prior to this happening, the seedling is readily killed by defoliation. Before the young seedling is established, it must compete with shallow-rooted grasses, forbs and other woody plants for water in the upper soil layers. Once the seedling's root system has extended beyond the level of the grass roots, however, competition is reduced and the young plant becomes less vulnerable to water stress.
Following establishment, the young plant enters into a period of vegetative growth. This period is required to increase the total photosynthetic surface and root system prior to going into the reproductive (flowering and fruiting) stage. This developmental stage is extremely variable in woody plants, and its duration, and the form of the individuals in this phase, is largely dependent on the management applied as well as prevailing environmental conditions. For example, in savanna where fires are a fairly regular feature, certain woody plants may have a high chronological age, yet be no more than a few centimetres in height. Individuals of the same species which are of a similar age but have escaped the effects of fire, could well be several metres in height.
Initial growth
The woody seedling initially develops a strong root system. This is particularly evident in the pioneer woody plants which usually grow in disturbed or hostile growing conditions. The establishment of a strong root system is designed to place the seedling at a competitive advantage over neighbouring plants, and will permit it to continue growing during normally adverse growing conditions. A well-developed root system will also permit the plant to survive, and recover after, impacts such as fire and browsing.Woody plants and water
Water in the lower soil horizons is available for use by woody plants due to their large, well-developed root systems which extend down to these levels. This places the established plants at an advantage over grasses and forbs (which have relatively shallower root systems) during periods when soil moisture is restricting to growth. For example, Acacia karroo (sweet thorn) can produce a tap root of over 2.2 m only 17 months after germination, while the aerial portions of the plant are only 150 mm high (Trollope 1981).Storage of nutrients
Most savanna species are deciduous (i.e. they drop their leaves and enter a dormant phase during winter). During the active growing period, the plant moves nutrients from the leaves to storage organs (roots and stems) prior to the leaves dropping. At the start of the following growing season, growth is initiated by increasing temperatures and daylight, and the plant makes use of either available groundwater, or the first spring rains, or both to start the new flush of growth. At this stage, the stored nutrients are moved to the growing points where they are needed to produce the new leaves and stems. The plant therefore enters a vulnerable stage when nutrient reserves are low, and before sufficient leaves have been produced to take over the function of supplying the energy requirements for continued growth, and replenishing the storage organs. Unlike grasses, most trees and shrubs are able to store relatively more nutrients, so that they have the capacity to recover from a series of defoliation impacts. This factor can be important in the management of woody plants, particularly for the provision of a sustainable source of forage for browsers.Defoliation of woody plants
It is during the vegetative stage of the plant's life history that defoliation becomes important (i.e. in terms of providing sustainable browse, or the reaction of the species to some bush control measure). Defoliation refers to the removal of the aerial parts of the plant and may occur in any of the following ways.
- Browsing. Animals that feed predominantly on the leaves and twigs of woody plants are called browsers (e.g. goats, kudu, black rhino, eland - refer to Production Guideline 8.3 in this series). The defoliation of the plant by browsers is mainly limited to the outer leaves and small twigs, although larger branches, or even whole trees, may be broken off by animals such as elephant, kudu or eland.
- Mechanical defoliation. This is a deliberate anthropogenic action with the aim of removing bush to combat bush encroachment, or to gather firewood in rural areas. Invariably, the plant is cut off at the base. Bush encroachment control can take the form of selective thinning where individual trees are removed, or radical thinning where machinery, such as bulldozers, is used to effect total removal of the woody plants in an area (refer to Production Guideline 7.7 in this series).
- Fire. In savanna vegetation, fire may have an effect on the canopy of the plant, depending on the time and conditions of the burn, but only rarely is the plant actually killed (refer to Production Guideline 7.6 in this series). In most cases, that part of the canopy that is exposed to the flames will be killed. Many pioneer woody plants have evolved mechanisms to escape the effects of fire. Examples include the development of a thick, corky, insulating bark in species such as Acacia sieberana (paperbark thorn) and Maytenus heterophylla (common spikethorn), or a suffrutex root stock in species such as Elephantorrhiza and Parinari.
When the outer shoots of the plant are removed in any of the above manners, natural processes in the plant stimulate buds lower down on the branches or the trunk to start growing in order to replace the lost stems and leaves. When defoliation occurs as a result of fire or mechanical removal, this process often takes place at the base of the trunk, a phenomenon known as coppicing. This usually results in the original single trunk being replaced by four or five new stems, and thus an overall increase in woody stem density. Such treatment may be advantageous in systems where browsers are important as it lowers the tree canopy and new, nutritious growth is made available to browsing animals.
The reproductive stage of a woody plant begins when the first flowers are produced by a vegetative plant. From a management perspective, this developmental stage is no different to the previous stage, except perhaps that plants in the reproductive stage are able to actively contribute to the seed bank.
Most species have distinct flowering periods during the growing season. The timing of flowering is a trait that plants have evolved, and may be dependent on one or more environmental factors. Day length is the most common flowering initiation mechanism, and when the day (or night) length exceeds (or is less than) a certain threshold length, flowers are produced. Other species are dependent on increasing temperatures to initiate flowering, while yet others require moisture for flowering (this is most common in arid and semiarid environments). Unseasonal flowering may also be initiated through some stress which might be imposed on the system (e.g. a period of drought, or a fire during the growing season).
Following successful fertilisation of the flowers, seed set and fruit development begins. Many species produce fruit as an attractant for animals in order to disperse their seeds. In savanna regions, this can be observed in species such as Acacia nilotica (scented thorn), Acacia tortilis (umbrella thorn), Sclerocarya birrea (marula), and many others. The passage of the dormant seed through an animal's gut may also kill seed predators and thus increase the percentage of viable seeds added to the seed bank. Other species make use of wind in order to disperse their seeds. In these cases, the fruit has one or more wings or a pappus of hairs in order to increase the surface area : mass ratio, and allow the seeds to be blown away. Seed predation forms an important regulating mechanism in plants, and can assume various forms. In savanna ecosystems, bruchid beetles are significant predators of acacia seeds, and are possibly the cause of the prolific seed production by these plants. Various other insects including weevils, longhorn beetles and wasps feed on different species.
The nature, time, and frequency of defoliation will usually have an effect on the ability of a woody plant to recover and continue growing. The factors affecting the growth of woody plants and their response to different methods of defoliation will be used as a basis for Production Guidelines 7.4, 7.6 and 7.7 in this series.
LITERATURE CITED
TROLLOPE, W.S.W. 1981. The growth of shrubs and trees and their reactions to treatment. In: Tainton, N.M. (ed.). Veld and pasture management in South Africa. Pietermaritzburg : Shuter & Shooter.