It never fails to come as a surprise to newcomers to the region that many of the woodland trees start their growth cycle in August and September, long before the onset of the rains in November. The flush of new foliage, in spectacular shades of red, is a wonderful sight, and the evening fragrance of the Brachystegia flowers three weeks later lends an air of magic after the heat of the day.
Miombo woodlands are generally considered to be deciduous, but they are neither strictly evergreen nor deciduous. They are best regarded as semi-evergreen. Muputu (Brachystegia spiciformis) is evergreen in good years and on the more favourable sites, and deciduous in dry conditions.
The name “miombo” is the plural for “muombo”, the Bemba name for Brachystegia longifolia, a tree which dominates extensive areas of the Zambesian plateau. Miombo is regarded as woodland, in spite of its closed canopy (with crowns touching), because of its light foliage which allows sufficient light to reach the ground to support a continuous ground cover of grasses and other herbs. Since this herbaceous ground cover dries out and burns most dry seasons, miombo woodland is regarded as a “fire climax”, a vegetation type which is maintained by regular fires.
Some woodlands on steep or shallow soils are naturally protected against burning, but retain their woodland structure because of the nature of the soil. Other areas cannot burn because of heavy grazing pressure by cattle. Such areas tend to become heavily invaded by shrubs.
Miombo woodland is defined as any woodland which is dominated by species of three related genera in the family Fabaceae (the pea family): Brachystegia, Julbernardia and Isoberlinia. Unlike most other leguminous plants, these do not develop nitrogen-fixing nodules on their roots.
Two features which these trees have in common are the characteristic mushroom-shaped crown, and that they disperse their seeds by the explosive dehiscence of their pods. The violent twisting of the two valves of the pod flings the seeds to a distance of up to 25 metres.
Miombo woodland is also rich in herbs and subshrubs. Regular burning is necessary for their maintenance. Unburnt dead grass suppresses new growth. Mowing has the same effect as burning, indicating that it is the heat of sunlight on the ground which stimulates new growth. Grazing, however, can be detrimental to the more sensitive herbs, such as orchids and milkweeds. Miombo woodland provides poor grazing except during the rainy months, when the grasses are young. Since this is also the growing season of most other herbs, they are most vulnerable to damage by trampling at that time.
Miombo Forest Products
Miombo woodland produces a great range of valuable forest products. The chief source of indigenous hardwood timber is mukwa (Pterocarpus angolensis), which is logged by pitsawyers in almost every district in the country. Government restrictions on exports have resulted in a serious undervaluation of mukwa which has led to wasteful usage. Another very valuable product of miombo are edible caterpillars, or ifinkobala, of emperor moths of the family Saturniidae, which are harvested in great quantities in certain areas, and sold dried in the urban markets. The foodplant of the most important commercial species is mutondo (Julbernardia paniculata). This species, which may well be the most common tree of Zambia, is also the most important source of nectar for honey. Unlike the other miombo dominants it flowers after the rains, thus providing a second honey flow. Because there is little else in flower at that time the honey is less contaminated than Brachystegia honey. Traditional bark-hive beekeeping has been practiced by the Lunda people of Mwinilunga and Kabompo districts for centuries.
Hills and Escarpments
Steep slopes and rocky outcrops are generally protected from fires by the sparseness of the grass cover. The good drainage on slopes ensures that the soil remains friable and free from compaction. These areas are consequently rich in many of the more fire-sensitive plant species. Smooth-barked trees and thicket clumps are characteristic features of the vegetation. Hills of limestone and other basic rock types may develop deciduous thickets, with pockets of rich herbaceous vegetation.
Characteristic of these hill slopes are the smooth-barked species of Brachystegia (B. bussei, B. glaucescens and B. microphylla), and the white-barked Sterculia quinqueloba. Miombo species can only be supported where there is sufficient soil to sustain them. Elsewhere deciduous species predominate.
Dry Ever-Green Forests
The best quality miombo woodland may grade into dense evergreen forest, especially at dambo margins, or where there are laterite pavements. Like other forest types they are protected from fire by having no flammable ground cover of grasses and herbs. Two of the most characteristic species are mufinsa (Syzygium guineense subsp. afromontanum), which is frequently dominant, and mofu (Entandrophragma delevoyi), which occurs as an occasional emergent, and is one of the tallest of Zambia’s indigenous trees. Some of the best of these forests, which are seriously threatened by clearing for cultivation, are to be found in the southern parts of Copperbelt Province.
Owing to the flatness of the terrain drainage on the plateaux is provided by flat-bottomed valleys called dambos. The water level in dambos rises and falls with the seasons. This annual rise and fall in the water level has a number of consequences. Since trees cannot withstand flooding for any length of time the absence of trees marks the highest level to which the water rises.
Dambos are features of intense biological activity. It has been observed that elephant prefer to dig for water at the dambo margins rather than taking surface water from the channels. The reason for this is that the water entering the dambo contains minerals, which are adsorbed by clay or taken up by living organisms in their passage through the dambo soil.
Dambos, which are permanently wet but have sufficient slope to avoid being flooded, develop into acid peat bogs. These provide habitats for raffia palms (Raphia farinifera), orchids and many other interesting plants. Denitrifying bacteria deplete the soils of nitrogen compounds, thus providing habitats for insectivorous plants, such as Drosera and Utricularia.
Water draining these peat bogs is often black owing to high concentrations of tannins which are leached from the vegetation. Tannins are defensive chemicals which inhibit the digestion of protein by animals. Tannin-producing plants are prevalent in areas of nutrient deficiency, where competition for available nutrients is particularly intense. Black-water rivers are well known to be deficient in animal life.
The grasses and sedges of poorly-drained acid dambos are extremely unpalatable to herbivores. However when the soils of these areas are disturbed by heavy trampling, which breaks down the soil structure, palatable grasses may invade. These areas attract grazing animals, thus extending the replacement of sedges by palatable grasses.
Much of the upper and middle course of the Kafue River, upriver of Kafue National Park, is flanked by wide dambos, consisting of sedge grassland, whereas in the Park the vegetation along the river is mainly short grasses. The latter is kept in a palatable state by grazing and by drainage provided by paths made by hippo to and from the river. Where these animals have been eliminated by hunting the drainage is poor, the soils turn acid and the vegetation unpalatable.
Dambos in the higher rainfall area frequently have patches of swamp forest (mishitu in Bemba). These, as the name implies, have wet floors. Some are very rich in species while others consist of just one, musombo (Syzygium cordatum). Some of the largest and richest dambos occur in the Mpongwe area of Copperbelt Province. Since the soils cannot be used for cultivation, people do not make much use of these forests and rarely enter them. The dominant musombo does, however, produce fine honey.
The many extensive floodplains of Zambia have formed where large rivers cross flat plateaux. The Zambezi River and several of its tributaries form a very extensive system of plains in Western Province and the western part of North-western Province. The Kafue River has large floodplains on two of its tributaries, the Lufupa (Busanga Plain) and the Lukanga, in addition to the Kafue Flats. The Chambeshi River enters the Bwela Flats near its source, and spills in the vast Bangweulu Swamp, which is drained by the Luapula River.
These plains all have rich and distinctive floras. Large areas of floodplain may be inundated for the whole period of the dry season during wet spells, but may not flood at all during dry years. Such areas have two complementary floras, one aquatic and the other adapted to dryland conditions. The aquatic flora consists of a variety of species, such as wild rice (Oryza longistaminata), which have stems that elongate to keep pace with the rising flood, so that the leafy tips are always above water. The water may rise 4 metres or more. As the flood recedes the vegetation lies down to form a dense mat.
Characteristic of the elevated levees and the higher levels of the plains are groves of fan-palms, Borassus aethiopum and Hyphaene petersiana, the former with a characteristic swelling on the trunk. Termite mounds on sandy floodplains frequently have the wild date-palm Phoenix reclinata.
Most woodlands and many dambos have large termite mounds, which are frequently covered with dense woody vegetation. As a general rule the plants growing on these mounds are forest species of trees and climbers, but the variety of termite mound vegetation is enormous in these raised areas of richer soils. What is nearly always apparent is that it is very different from the surrounding vegetation. Yet there are exceptions even to this rule.
There are large tracts of miombo woodland on the plateaux which lack termite mounds. The most extensive areas are the Kalahari Sands where the soils simply have not enough clay to support stable mounds. In many rocky areas, and on steep slopes, where the soil depth is limited, termite mounds are generally absent. Mounds are most consistently found at the dambo margins where the optimal conditions of drainage and an abundance of suitable clay exist.
Termite mounds undoubtedly have long lifespans, measured in centuries. Old trees associated with mounds suggest this, and archaeological evidence confirms it.
Some species are highly adapted to termite mounds, such as the large succulent tree Euphorbia ingens, which superficially resembles some of the New World cacti, but is in fact unrelated. Other trees, notably the Proteas, are never to be found either on or even near termite mounds.
Termite mounds accumulate mineral salts. They are frequently rich in lime even when the surrounding soils are deficient. This accounts for the preponderance of lime tolerating plants on mounds. In some areas mounds are rich in salt, and these attract animals which eat the soil. Even well-fed cattle on commercial ranches cannot resist salty termite mounds, eating away the soil to form a grotto and eventually demolishing the whole mound.