An African Rain Forest in a South African Botanical Garden
Volume 2 Number 8 - July 1997
Johan Kluge & John Burrows.
Creating a rain forest in Africa may sound rather like developing a desert in Saudi Arabia! But South Africa is essentially a country of long and erratic dry periods and the establishment of a rain forest poses several difficulties that have been overcome by the National Botanical Institute's (NBI's) Lowveld National Botanical Garden in Nelspruit in the north-eastern province of Mpumalanga, South Africa.
In a change from the Institute's past policy of representing only South African plants in its eight botanical gardens, the concept of developing a representative African rain forest in the Lowveld National Botanical Garden was accepted as a logical step in expanding the NBI's involvement in promoting and conserving the African forest flora, much of which is fast disappearing in the face of uncontrolled deforestation through logging and population growth. In addition, South Africa is experiencing a marked increase in tourism and it was felt that the National Botanical Institute's botanical gardens should provide a glimpse of the magnificent tropical rain forests that are becoming increasingly difficult for the average tourist to experience.
In addition, there is an increasing need for better facilities for environmental educators in South Africa. By providing an ecologically sound and structurally accurate African rain forest, visiting teachers would be better able to convey to their pupils the complexity of rain forests and value that they have for people throughout the world.
The African Rain Forest Project has been carried out by the NBI in collaboration with the Lowveld branch of the Botanical Society of South Africa. The Society is a voluntary organisation which supports the development and maintenance of the NBI's network of botanic gardens. Work on the Project started in 1993. Nelspruit is situated in the hot but dry Lowveld of north-eastern South Africa at an altitude of 620 metres and with mean annual rainfall of 795 mm. Thus Nelspruit has acceptable temperatures but its rainfall is well short of the minimum needed. At the start it was appreciated that in order to create a functional forest system that was structurally correct, it was essential to provide a supplementary water supply over the canopy of the forest and not at ground level, as had been done in similar but unsuccessfull attempts elsewhere in South Africa.
The site selected for the African rain forest was a patch of existing riverine scrub forest on the banks of the Crocodile River which forms one boundary of the existing botanical garden. Water supply was therefore not a problem. The problem was how to get the water onto the canopy of the proposed rain forest. With the aid of a local firm of consulting engineers, four 17-metre high steel towers were designed and installed along the 4-metre high bank that runs along the edge of the 30-metre wide river levee on which much of the forest was to be established. On each tower was mounted a large 'rain gun' sprinkler which delivers 27.5 cu. metres of water per hour over an area of 35 metre radius. An electric submersible 11 kW pump, controlled from a central switchbox, was installed in the adjacent river. By adding the height of the tower to the height of the bank and the arc of the water spray, we were achieving a height of about 23 metres - hopefully sufficient to irrigate over any forest canopy that would grow under our climatic conditions.
It was realised from the outset of this project that the South African climate, with its low temperatures (light frosts of up to -2ºC have been recorded) and shorter day length during winter, would be a limiting factor in the growth rate of trees that were accustomed to high temperatures and a roughly constant day length throughout the year. We could not, therefore, expect that every species of plant we attempted to grow would grow, or that those that did grow successfully would always achieve the height normally attained in their natural habitat.
By October 1995, after some initial adjustments, the sprinklers began successfully delivering what amounted to a very realistic tropical downpour over the existing tree tops. Designed to provide an additional precipitation of 1,500 mm/annum at 5.33 mm/hour, we now had the basic requirement for our rain forest.
At the same time that the irrigation system was being installed, a raised wooden boardwalk was being constructed through the length of the rain forest area. Even before the irrigation scheme was commissioned, much of the area set aside for the rain forest was naturally marshy from ground water seepage. With the addition of 1,500 mm of 'rain' each year, the area would become impassable for visitors unless a raised walkway was provided. The raised boardwalk also serves to keep the public away from fragile groundcover plants.
And so to the plants. In an area of slightly more than 2 hectares (c. 5 acres), it would be impossible to have all the African rain forest species that we wanted - we would have to select species that were typical of the tropical African forests as well as plants of economic importance or educational value. We also wanted to replicate a natural pattern of forest succession by establishing forest pioneer trees in the many gaps that had been unavoidably created during construction.
Through contacts with various state organisations elsewhere in Africa, such as the Entebbe Botanic Garden in Uganda and the Departments of Forestry in Kenya and Malawi, we began to assemble a wide range of tropical African plants. Members of both the Lowveld National Botanical Garden and the Lowveld Branch of the Botanical Society undertook trips into central Africa to collect seed. We now have a fine selection of plants from Kenya, Uganda, Ivory Coast, Fernando Po, Cameroon, Gabon, Zambia, Malawi and Zimbabwe. All the seed collected has been germinated and grown in the nursery of the Lowveld National Botanical Garden. By the end of 1995 three pioneer forest species had been planted out: Polyscias fulva (Araliaceae), Croton megalocarpus (Euphorbiaceae) and Maesopsis eminii (Rhamnaceae). Several canopy species had also been planted out in the shade of the existing trees along the river, including Khaya anthotheca (Meliaceae), Chrysophyllum gorungosanum (Sapotaceae), Maranthes goetzeniana (Rosaceae), Craibia brevicaudata (Fabaceae) and Ficus chirindensis (Moraceae).
Groundcovers and shrubs are currently represented by members of the Zingiberaceae (Afromomum, Costus), dracaenas (Dracaena laxissima, D. monostachya, D. steudneri) and several Impatiens spp. Some epiphytic orchids and ferns have also been established in the existing riverine trees.
Thus by the beginning of 1996, a fledgling African rain forest had been established in the Lowveld National Botanical Garden. Although a similar ecosystem-based forest has been recreated in the Harare Botanical Garden, Zimbabwe, representing the regional forest flora of Zimbabwe, this is the first time that a rain forest that represents the forests of tropical Africa has been established in a botanical garden. It is also the first time that an irrigation system has been devised and installed that provides a simulated rainfall throughout the entire forest structure, from the canopy to the forest floor. We hope that as the forest grows and develops into a complete forest ecosystem, it will become a valuable educational asset serving this region of South Africa, as well as a tourist attraction which will increase the flow of visitors through the Garden's gates and help maintain the Lowveld National Botanical Garden's reputation as one of the finest botanical gardens in Africa.