agricultural production guidelines
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Veld in KwaZulu-Natal
Co-ordinated Extension KwaZulu-Natal Veld 7.6 1999
FIRE AND BUSH
C R Hurt
ARC - Range and Forage Institute, Cedara
Plant Adaptations
Burning as a Management Tool
INTRODUCTION
Fire is a natural component of savanna ecosystems. Many of the common species in these systems have evolved with fire, and have developed morphological mechanisms to escape the effects of fire. Because fire is a natural part of the system, it must be considered in the management of the system. Fire affects woody plants in a variety of ways, depending on the time and conditions of the burn, as well as the frequency that a certain area is burnt. In gaining an understanding of the vegetation (and therefore being able to manage it properly), the effect of fire on both the bush and the grass needs to be addressed. Furthermore, questions such as the reasons for burning, when and how often to burn, and, finally, what to do after the burn, need to be answered. The objective of this Production Guideline is to clarify these issues, and it should therefore be read in conjunction with Production Guideline 7.5 in this series.
Grasses
Adaptations of the grass plant to burning have been comprehensively covered in Production Guideline 7.5 in this series. A normal fire will not affect the grass plant provided that it is burnt while the plant is dormant. The plant becomes dormant during periods of stress (usually drought or cold) and, during this phase, growth ceases. A fire at this time will not harm the plant as most of the nutrients will have been translocated to storage organs at the base of the stems. Once normal growth has been resumed, however, the stored nutrients are transported to the new leaves for growth, and thus the storage depots become depleted. Should the plant be burnt at this stage, the developing leaves are destroyed and the plant is placed under stress when growth continues, as fewer reserve nutrients are available for recovery. This consequently leads to a decline in vigour and may even result in the death of the plant.Grass plants that are not defoliated regularly tend to accumulate old dead leaf material which eventually smothers the plant, and either reduces its vigour, or kills it completely. From a grazing perspective this accumulation of old material is undesirable because animals find it unpalatable and it restricts access to the new, palatable growth within the tuft. Removal of the dead growth stimulates the plant to produce new, palatable lateral tillers which are highly acceptable to animals.
Woody plants
In certain areas, fire will actually kill trees if it is sufficiently intense, and if the trees are burnt after spring growth has commenced. This generally occurs in areas where spring rains are late and ground water is used by woody plants for initial growth. In these cases, the grass layer will still be dry enough to sustain an intense fire because spring growth would not have been initiated due to the lack of water in the topsoil. However, in most of the KwaZulu-Natal savannas, spring growth of the woody plants coincides with the comparatively early rains. It now becomes difficult to implement an intense burn at the onset of the first growth of the woody plants, as the spring rains would also have stimulated the grass plants to start growing. The grasses should never be burnt once they have started to grow.Mechanisms evolved by woody plants to survive fire are diverse, and the more important concepts are discussed below.
Dormant buds
The aerial parts of woody plants (branches, leaves, flowers, fruit) all result from growth initiated from meristematic tissue at the growth tips of the plant. These growth tips or apical buds usually produce chemical hormones which suppress growth in buds which are situated lower down on the branch. This causes the lower buds to remain in a dormant condition, a situation referred to as apical dominance.As the plant increases in size so the apical buds are found at an increasing height above ground level. If such a plant is exposed to a fire, topkill of the canopy will often result (unless the individual plant is large, or the fire relatively cool), and apical dominance is removed. This situation stimulates growth in the dormant buds lower down on the stem, provided they have not been killed by the fire. Coppicing from basal dormant buds is common in savanna trees and shrubs. These buds are situated at the stem base (at ground level) where temperatures during the fire are relatively lower than in the canopy, and, consequently, chances of survival are higher.
Corky bark
Certain savanna trees and shrubs have evolved a thick, corky bark which effectively insulates the stem from the high temperatures caused by the fire. A prime example of this phenomenon is provided by certain Australian eucalypts, which have a thick, corky bark only to flame height, and a smooth, thin bark above that point. In KwaZulu-Natal, many species exhibit this survival mechanism. Examples are paperbark thorn (Acacia sieberana), scented thorn (A. nilotica), common spikethorn (Maytenus heterophylla) and cabbage tree (Cussonia spicata), among others.Suffrutices
A suffrutex is a large, woody, underground rootstock which is essentially a storage organ. Plants with suffrutices are therefore well-adapted to survive fire, as the growing points of the plant are situated below the critical heat level of the fire, and reserve nutrients are well-supplied. KwaZulu-Natal examples of these plants include sumach bean (Elephantorrhiza elephantina), mobola plum (Parinari capensis) and ploughbreaker (Erythrina zeyheri).
As the woody vegetation in the savanna areas of KwaZulu-Natal has evolved with fire, it is important that fire is included in the management of this vegetation type. Important considerations when burning savanna are the reasons for burning, when to burn, how frequently to burn, and how to manage the post-burn vegetation.
This is the problem that faces most farmers and other land owners in any area where extensive farming systems make use of the veld. In bushveld areas (i.e. BRG’s 21, 22 and 23), permission must be obtained from the local extension office of the KwaZulu-Natal Department of Agriculture before burning. There are two justifiable reasons for burning in the savanna. Firstly, fire can be used as a management tool to remove old, dead, unpalatable grass material that has no use as forage. This residual material tends to smother the grass plant (it becomes moribund), and will eventually kill it. Secondly, fire may be used as a tool to manipulate the bush (i.e. kill trees or lower the canopy to provide browse).Why burn?
As has been discussed above, fire is not effective in killing most woody plants, but it rather causes topkill in small individuals. Certain broadleaf woody plants arekilled by fire, and therefore fire can be used as a management tool in situations where broadleaf plants predominate. Lowering the canopy of the woody component is achieved by fire and is an economical method of providing good-quality, accessible forage in browser enterprises. The main objective of burning where browsers are not widely used, therefore, is to maintain a healthy herbaceous layer by removing dead material.
A further consideration in sweetveld areas is that setting fire to potential reserve grazing is similar to burning money. Very often, the lost production in real terms following a burn will exceed the costs of implementing another form of bush control (refer to Production Guideline 7.7 in this series).
When to burn, and how often?
From the above discussion it should be obvious that the best time to burn is during the winter or early spring, while the grass plants are still in the dormant phase. In fact, it should be theoretically possible to burn during a mid-summer drought period and not affect the vigour of the grass plants, provided that the plants are dormant when burnt. Timing the burn can be difficult in these areas due to the unpredictable and erratic nature of the rainfall. In many cases, a farmer might burn a portion of the farm for bush control, only to be faced with a subsequent shortage of grazing as a result of late spring rains.The question of how often to burn is difficult to answer simply. The erratic nature of the rainfall has a direct influence on grass production. As a result one cannot prescribe a regular burning cycle (for instance every four or five years). It is a better strategy to adopt an ‘opportunistic’ policy and to burn whenever it is considered necessary (i.e. when there is an accumulation of old grass which might be a threat to the survival of the desirable plants, or during wet cycles). If stocking rates on the farm are sufficiently low to result in a fairly rapid accumulation of fuel, fires should be fairly regular. This will ensure that when a fire is implemented, it will not be too intense and thus will not be a threat to the larger trees. At the same time, these fires will effectively control broadleaf woody plants.
Post-burn management
Veld management after a fire should be aimed at maintaining a vigorous, competitive sward which will provide a sustainable source of forage, and actively prevent woody seedlings from establishing. The post-fire management of the herbaceous component has been covered in Production Guideline 7.5 in this series, and the principles also apply to savanna. Management of the post-fire woody vegetation depends mainly on the objectives of the enterprise. If the burn was implemented in order to provide forage for browsers, the burnt vegetation should be included in a suitable browsing system. The vegetation should be provided with a rest period following the burn in order for adequate browse to be produced. If insufficient browsable material is available, the animals will be forced to feed relatively more on the sprouting grass before the browsable plants have recovered from the burn.
CONCLUSION
From the above discussion it can be seen that the farmer must think very carefully before burning in the bushveld areas. In the farming situation, the main reason for burning would be to maintain a healthy, vigorous grass sward, which would provide maximum yields of forage, and provide strong competition for invading woody species.