The leaves of Gnetum africanum and G. buchholzianum are highly valued as nutritious green vegetables across Central Africa and are the subject of considerable cross-border trade. This trade has increased dramatically in recent years and the resource base has been seriously threatened by unsustainable harvesting methods and the gradual disappearance of the forests in which they thrive. In order to ensure and enhance the sustainability of these two Gnetum species, research is being carried out on their selection, genetic improvement, vegetative propagation and ex-situ management.
To date, the two species have been successfully propagated and subsequently planted out mostly in on-station trial blocks. They respond well to cultivation and many of the plants have produced large amounts of biomass and have produced flowers and fruit. Germplasm of over eighty-five provenances has been established for future selection and genetic improvement for the mass production of broad-based planting materials. On-farm trials are now being established and appropriate harvesting methods are being introduced to allow for quick sprouting of vines. These efforts are aimed to ensure, not only the future availability of Gnetum, but that the cultivated sources are also genetically superior.
Key words: Gnetum africanum, Gnetum buchholzianum, domestication, field trials, sustainability
Gnetum is the lone genus in the family Gnetaceae. There are about thirty species in the genus, which occurs throughout the tropics in Asia, South America (Mialoundama and Paulet, 1986) and in Central Africa (Watt and Breyer-Brandwijk, 1962). The majority of the species of Gnetum are lianas. The plants are dioecious, with the male plants producing catkins of stamens and the females catkins of ovules barely protected by an envelope (Letouzey, 1986).
There are two species of Gnetum in Africa, G. africanum and G. buchholzianum and they are distributed in the humid tropical forests from Nigeria through Cameroon, Central African Republic, Gabon, DR of Congo to Angola (Mialoundama, 1993). Both species are understorey lianas, although in some cases some individuals have been found to scramble into the crowns of emergent trees (author, pers. obs.). These two species are very similar and can only be distinguished by the shape of the leaves and characters of the male reproductive parts (Lowe, 1984).
Both Gnetum species have significant value to many forest-based communities and have a number of vernacular and trade names. In the Central African Republic, Gabon, Congo, DR of Congo and Angola, the two species are locally called koko. (Bahuchet, 1990). In Anglophone Cameroon, they are known as eru, while in Francophone Cameroon the name okok is applied. In Nigeria, the two Gnetum species are called ukasi by the Igbo tribe while the Efik/Ibibio tribes call them afang.
In Nigeria, the leaf of G. africanum is used in the treatment of an enlarged spleen, sore throats and as a cathartic (Burkill, 1994). In Ubangi (DR Congo), it is used to treat nausea and is considered to be an antidote to some forms of poison (Burkill, 1994). In Congo-Brazzaville, the leaves of both species are used as a dressing for warts and boils and a tisane of the cut-up stem is taken to reduce the pain of childbirth (Bouguet, 1969). Gnetum africanum is also reported to be used for medicinal purposes in Mozambique (Watt and Breyer-Brandwijk, 1962).
However, it is for its edible value that Gnetum is most prized. The leaves are either eaten raw or are finely shredded and added to soups and stews (Burkill, 1994). The leaf of G. buchholzianum is also edible, although it is reported to be less common in commerce (Busson, 1965). The leaves of both species have very high nutritional value and constitute an important source of protein, essential amino acids and mineral elements (Busson, 1965; Fokou and Domngang, 1989; Mialoundama, 1993; Ouabonzi et al., 1983).
The leaves of both Gnetum africanum and G. buchholzianum are a very important article of trade in the Central African region, particularly in Cameroon where the leaves are harvested on a daily basis and sold in local and regional markets. As the leaves of both species are evergreen they are available throughout the year. The volume of export trade in these leafy vegetables has significantly increased in recent years.
There are two main ports of exit in Cameroon. Idenau, a coastal fishing village in SW Province, exports large quantities of Gnetum to Nigeria; and Campo, near Kribi in the South Province, exports to Gabon and Congo-Brazzaville. Once exported, the Gnetum leaves are then traded in large border town markets. These markets are well-organised and are frequented by a wide range of nationalities trading in the product.
To meet the high demand, the search for Gnetum has extended to more remote parts of Cameroon so that it is now difficult to find either species in the forests of the Littoral and South West Provinces, where they were previously abundant. It is common to see vehicle loads of Gnetum heading to the border market of Idenau on Wednesdays and Thursdays every week from the forests of the Centre, East and South Provinces of the country. It is estimated that 600 tonnes a year leave from this port alone with a local market value of
1 800 000 000 CFA (Bokwe and Ngatoum, 1994). This large volume of trade offers valuable employment to many young people in Cameroon and surrounding countries. As much as
450 000 CFA / month is reported to be made from the sale of Gnetum by one of the full-time traders in the product in the Idenau market in 1997 (pers. comm.).
Gnetum africanum and Gnetum buchholzianum thrive in a wide range of habitats, including farm fallows or abandoned farmland, secondary forests, and closed forest. The vines of both Gnetum species climb supporting big and small trees, dead trees, saplings, shrubs, other climbing vines such as rattan palms, and a host of other plant materials in the complex tropical humid forest, where they grow luxuriantly and produce great quantities of leaf biomass.
Under wild conditions, both species grow and form underground tubers or roots that store plant food reserves. These can remain alive for many years when the vegetation and the Gnetum vines above ground are cleared and the soil surface is laid bare. It has been reported that some local tribes in East Cameroon and the Congo eat these tubers as wild yams, particularly during lean seasons (Bahuchet, 1990). In the course of harvesting the vines, the buds on the tubers are damaged and it may take a long time for new buds to develop into a vine. In some cases, the forceful pulling of vines creates wounds on the tuber/root for fungal attack that can cause tuber/root rot disease. Hence the effects of harvesting does not ensure the growth and supply of future Gnetum leaves. It is, therefore, not a sustainable means of collecting the leaves.
On occasion, during the collection, the trees that the Gnetum vines are growing on are often felled, creating widespread damage. It is clear then, that the harvesting of Gnetum from the wild is not sustainable. In addition, much of the forest in which the Gnetum occurs is being degraded by illegal and uncontrollable exploitation of timber, farming, road construction and other forms of economic development.
To begin to alleviate some of the problems of the over-exploitation of Gnetum, a domestication programme has been developed. It is hoped that cultivated sources of supply will not only reduce pressure on the existing wild resource, but will also contribute to the incomes of local communities through the establishment of village-based co-operative cultivation systems.
During the first trials to assess the potential of Gnetum for cultivation, many multiplication techniques were studied. These included seed germination and rooting of leafy vine cuttings. In addition, selection of the best rooting medium and the identification of the cheapest and most efficient propagators that could be transferred to local farmers, were also determined.
The initial propagation trials ruled out the multiplication of Gnetum by seeds as they did not germinate under nursery conditions despite being commonly found germinating on the forest floor.
From our work, it is clear that the vegetative propagation technique of rooting leafy vine cuttings provides the optimum means of Gnetum multiplication. During the period 1994-95, 65 provenances were collected from the forest zone of South West and Littoral Provinces. These provenances were selected based on the fact that they are the most commonly harvested as palatability varies widely in Gnetum. Thirty-five cuttings of each provenance were put into simple and cheap non-mist and portable propagators made of wooden frames, with separate wooden frame covers framed with polythene sheets (see Figure 1).
Whilst in the propagators, the cuttings were watered twice a day for a maximum period of six weeks when enough roots were formed to allow transplanting to pots to take place. After an initial period of hardening-off (4-6 weeks) these are then ready for planting.
Aside from the initial propagation work, further studies have been undertaken to evaluate the cultivation potential of Gnetum in the field. Five cuttings representing twenty-eight provenances were randomly assigned to one of twenty-eight study plots under five different hardwood species planted for timber production studies in the Southern Bakundu Forest Reserve. After planting, they have been managed by staking and weeding. Vine growth, leaf
Figure 1 Rooting cuttings of tropical trees (from Longman, K.A. 1993)
biomass production and survival have been monitored, and data so obtained has been recorded on data sheets designed for this purpose.
A harvesting regime has also been implemented to determine the effects of leaf removal and the rate of re-growth. Five regimes were implemented:
· The removal of alternate leaves only;
· The removal of two pairs of leaves were removed leaving behind a pair so that there remained a pair of leaves after every two nodes;
· The harvest of all the mature leaves;
· Cutting of the vine tops just above the height where the mature leaves end;
· Cutting of vines at ground level.
In October 1997, the number of leaves produced by the Gnetum planted in August 1996 were counted and subjected to an analysis of variance test after data transformation. Not surprisingly, there was considerable variation in vine length between the provenances, with some growing much more vigorously than others. The rate of growth of the rapidly-growing provenances was twice the rate of those that were slow-growing. The rate of growth has an enormous affect on the potential for increased yield and recovery from harvest. In general, survival rates were good (when measured 18 months after planting). The highest survival rates of 78.2% and 76.7% were recorded in the two most vigorous provenances, while the poorest survival rate was 57.1%.
Generally, the vines that were left with some leaves after harvest produced new leaves and some new vines were also produced at the nodes where leaves were harvested. As far as the pruning method is concerned, a maximum of three new vines sprouted from the first three nodes below the cut surface. There was no regrowth in the vines cut at ground level. It was also observed that when cuttings were taken from cloned vines in the nursery or propagation unit, many sprout vines and many more leaves were produced. This demonstrates that Gnetum biomass production can be sustainably harvested if the appropriate harvesting techniques are applied.
The results of this work have shown clearly that the two Gnetum species are easily domesticated. On-station and on-farm trials have also shown that Gnetum has considerable potential for inclusion into agroforestry and subsistence agriculture systems and, if suitably managed, can provide a sustainable supply of leaves for both household use and sale.
It is recommended that more research be undertaken on the cost-benefits of cultivation and whether the cultivated sources of Gnetum can indeed alleviate the reliance on the wild resource whilst contributing to the household economy.
Financial support for this work was provided by the European Union, the British Council and Darwin Initiative of the Government of the United Kingdom, to all of whom the author expresses his thanks.
Bahuchet, S. 1990. The Akwa pygmies: Hunting and Gathering in the Lobaye Forest. In Food and Nutrition in the African Rain Forest. Food Anthropology Unit 263, UNESCO.
Bokwe, A. & D. Ngatoum. 1994. Effectuée autour du Mont Cameroun (Province du Sud-Ouest) relatif au recensement de certaines especes des produits forestiere secondaires en voie de disparition. Rapport de la mission. MINEF, Yaounde.
Bouguet, A. 1969. Féticheurs et medicines traditionnelles du Congo (Brazzaville),Paris: ORSTOM.
Burkill, H.M. 1994. The Useful Plants of West Tropical Africa. Volume 2: Families E-I. Kew. Royal Botanic Gardens, Kew.
Busson, F. 1965. Plantes alimentaires de l' Ouest Africain: Etude botanique, biologique et chimique. Marseille: Ministere de la Cooperation.
Dutta, A.C. 1979. Botany for Degree Students 5th Ed. Oxford University Press.
Fokou, E. & F. Domngang. 1989. In vivo assessment of the nutritive value of proteins in situ in the leaves of Solanum nigrum L., Xanthosoma spp. and Gnetum africanum L. Indian Journal of Nutrition, and Dietetics 26 (12): 366-373.
Letouzey, R. 1986. Manual of Forest Botany: Tropical Africa, Vol. 1. General Botany. CTFT, Norgent Surmarne, France.
Lowe, J. 1984. Gnetum in West Africa. Nigerian Field 49 (1-4): 99-104
Mialoundama, F. et al. 1984. The periodic induction of dormancy during the rhythmic growth of Gnetum africanum. Physiologia-Plantarum 61(3): 309-313
Mialoundama, F. & P. Paulet. 1986. Regulation of vascular differentiation in leaf primordia during the rhythmic growth of Gnetum africanum. Canadian Journal of Botany. 64 (1): 208-213.
Mialoundama, F. 1993. Nutritional and socio-economic value of Gnetum leaves in Central African forest. In Hladik, C.M. et al., Tropical forests, people and food: Biocultural interactions and applications to development. Carnforth, UK: Parthenon Publishing Group
Okafor, J.C. 1980. Edible indigenous woody plants in the rural economy of the forest zone. Forest Ecology and Management. Vol. 3: 45-55.
Ouabonzi, A., M.L. Bouillant & J. Hapin. 1983. C-Glycosyflvones from Gnetum buchholzianum and Gnetum africanum. Phytochemistry. Vol. 22, No.11. pp 2632-2633. Pergamon Press Ltd.
Watt, J.M.A & M.G. Breyer-Brandwijk. 1962. The medicinal and poisonous plants of Southern and Eastern Africa. Edinburgh: E & S Livingstone.