Trees – Namibia’s big fiveJune 21, 2012
Up close and personal with Namibia’s coastJune 21, 2012
Fire is an integral environmental variable that drives savannah dynamics throughout the world. However, says Antje Burke of the Ministry of Environment and Tourism, although fires are a natural condition, most of today’s savannah fires are ignited by humans. They consequently occur in certain areas more frequently than in the past, and often at times of the year when natural fires would not occur.
Whoever visits the vast woodlands in north-eastern Namibia, for example in the Kavango and Caprivi regions, will no doubt notice obvious and perhaps not so obvious signs of the burning that has maintained and transformed these woodlands. They consist of tropical savannahs that are characterised by varying proportions of trees and grasses, and it has been suggested that these landscapes have been transformed to a considerable extent.
A multitude of cascading effects has altered obvious features such as vegetation structure, composition and re-growth. These effects have also resulted in more subtle changes affecting soil formation, nutrient cycling and less obviously, small plants and animals. However, the clock cannot be turned back and a pragmatic approach linking fire management and control with traditional farming practices is necessary. This, in the long term, may sustain savannah ecosystems, while at the same time providing people with the vital goods and services they need.
Savannah dynamics are extremely complex. This is why scientists have not yet been able to understand all facets of savannah ecology and provide solutions to some of the problems. The key assumption is that there is intense competition between grasses and trees. The balance between the tree and grass components of savannahs and the factors that determine the dominance of one or the other component, have been debated for many decades.
One classical hypothesis postulates that competition between roots is responsible for maintaining this balance. According to this theory grasses obtain moisture largely from the upper layers of the soil, while trees draw their moisture from deeper sources. Depending on the nature and timing of rains, either one or the other component would gain an advantage. This simple concept has now been revised. Other factors such as the season in which plants use water, competition between trees and particularly disturbances such as fire, grazing, shifting cultivation and timber extraction, as well as climate change, are believed to influence this tree-grass relationship. We do know today that savannahs would probably not exist without the impact of fire. The question is, whether or not altered fire regimes have negative effects in the long term, and if so, how this could be reversed.
So what has changed in Namibian woodlands? Human-induced fires have been documented on the Southern African continent for at least 1.5 million years. Due to low population pressure in the savannah areas, impacts were very localised. Hunter-gatherers of later periods, such as the San in Namibia and Botswana, used fire extensively to hunt, improve grass growth and maintain diverse habitats that would enable them to obtain a wide array of veld products.
The major difference between naturally ignited, for example through thunderstorms, and human-induced fires relates to the season of burning and intervals between fires. Thunderstorms in the early rainy season have been named the major cause of natural fires, thus restricting the burning season to this period. Humans light fires mainly during the dry season when plant material and atmospheric conditions are dry. In all probability fire frequency has also been altered. Recent analyses of satellite images revealed that the majority of woodlands in Namibia’s north east are currently burned on an annual basis. Whether or not the same short intervals would be maintained by natural fires is questionable.
The effects of these changes in the fire regime on Namibian woodlands have not yet been fully investigated and understood. However, some signs are evident. A shift in the composition of the vegetation to the more fire-tolerant species and a change of the structural diversity of the vegetation to largely two size classes, instead of a range of shrub and tree heights, are perhaps some indications of the effects of a changed fire regime. It has also been suggested that the re-growth of some important timber trees, such as African teak (Baikiaea plurijuga), has been reduced. On the resource side, grazing for livestock and wildlife is lost, as well as thatching grass, veld products and woody resources. People have realised that this is affecting their livelihoods directly. In some instances fires kill people and livestock.
Although all the effects are not yet known, programmes to control fires are practised in Namibia. Raising awareness amongst the rural population and involving the people in fire control, for example by creating firebreaks, have been some of the tools used to tackle the problem with uncontrolled and too-frequent fires. The awareness component of these programmes has been particularly effective and the programme has been extended from initial pilot areas in the East Caprivi to other fire-prone regions.
Livestock grazing can also be used as a tool to control fires. The interplay between grazing and fire has been regarded as being of critical importance in determining vegetation structure in some woodlands of East Caprivi.
• Firstly, all things being equal (i.e. the same vegetation type, substrate, rainfall and no other disturbances), it has been suggested that heavy livestock grazing and total suppression of fires result in bush encroachment in this type of woodland.
• Secondly, minor or no grazing results in dense and tall swards of grass. Coupled with more frequent and intense fires, the woodlands would be transformed into open woodland with few tall trees and a dense cover of shrubs and grasses. In this scenario, the abundant grass facilitates intense and frequent fires, maintaining re-sprouting trees and shrubs in the flame zone and thus hindering their ability to escape the damaging effect of the flames. Re-growth of trees is severely hampered.
• Thirdly, moderate to heavy grazing pressure in combination with fires, on the other hand, would result in open to closed woodland, showing a wider spread of tree and shrub heights. Here low availability of fuelling grasses results in less intense, and probably less frequent fires, thus enabling some trees to escape the flame zone. However, one must not forget that the negative effects of overgrazing could well outweigh the benefit of fire suppression and that this strategy has to be evaluated on a case-by-case basis with clear management objectives in mind.
Recognising the problem is the first step. We hope that finding solutions will not require many more generations, by which time changes in the woodlands may well be irreversible.
This article appeared in the 2004/5 edition of Conservation and the Environment in Namibia.