A.K. Natta, B. Sinsin and L.J.G. van der Maesen 1
Information was gathered on the flora and fauna, as well as the ecological and economic importance of forests along waterways in Benin, located at the discontinuity of the West African rain forest block, the so-called Dahomey Gap. The current trend is a general decrease in the structural complexity and extent, or the complete disappearance of these hygrophile and edaphic freshwater forests. The main causes are shifting cultivation, grazing, selective cutting of valuable or rare tree species, road and dam construction, and over-exploitation of non-timber forest products to satisfy the basic needs of rural people. Such degradation, in these fragile and endangered ecosystems, threatens rare plants and animals. Although small in area and linear in shape, riparian forests are rich, harbouring at least one-third of the estimated flora of Benin (3000 species) and containing numerous valuable, rare or even endemic plant species. Riparian forests in Benin are comparable to many tropical dense forests, particularly to the West African ones, in terms of family richness, species richness (129 to 358/ha), Shannon diversity index (H'= 2.4 to 5.4 bits), evenness index of Pielou (0.8), life forms, phytogeographical types, diameter frequency distributions (J-reverse type), and basal area. The presence of plant communities adapted to moist soil, permanence of water or a shallow water table, structural heterogeneity and species complexity providing ample food or substrate, make riparian forests a suitable refuge for many organisms. To control the current degradation trend of the remaining riparian forests is a challenge for conservationists that should be faced together with local people.
Centuries of intense human activity accompanied by a drying climate resulted in the loss of most of Benin's closed forests long ago. Small patches of moist forest remain today only on moist soils, or as numerous sacred forests and riparian forests. FAO (1980) estimated that only 470 km² (just 0.4% of Benin's area) remained under dense forest. Benin's flora is estimated to have close to 3,000 species (PFB 1997), which is relatively poor in comparison with more humid neighbouring countries. However, the important hydrologic network, which includes the Atlantic Ocean, the Ouémé, Zou, Couffo, Mono, Sota, Alibori, Mékrou and Niger rivers, and numerous streams, allows the presence of many riparian forests all over the country.
The importance of riparian forests in the functioning of river ecosystems and biodiversity protection is recognised all over the world by numerous authors. In Benin, the rapid changes in land use have led to their destruction and fragmentation. Their abusive overuse is widespread and growing, hence they are classified as endangered ecosystems (Natta et al. 2002). Riparian forests are often small in area, but are of extreme ecological and economic value for local people. The interest of riparian forests lies in their resources that are utilised by neighbouring inhabitants to satisfy their basic needs and as source of income. As a result biological resources, especially plant species, are diminishing before they can be inventoried and assessed. Until recently, they remained largely ignored and unmanaged as key ecosystems in biodiversity protection. This is partially due to insufficient scientific knowledge about species particular to riparian forests, which hinders appropriate planning and conservation initiatives. This paper presents an overview of the biological diversity of riparian forests in Benin, and discusses several issues associated with the threats of these forests as well as their status and protection.
In the Benin context, where savanna is the most dominant ecosystem, `riparian forests' are represented by any woodland (e.g. semi-deciduous, dry, and open forest, and woodland savanna) occurring at riverbanks or along streams. Although often fragmented and degraded, there are still good examples of undisturbed riparian forests. High human pressure has reduced the width of riparian forests, even up to a single tree, so they are often extremely narrow and quite linear (strip-like) in configuration. However, floristic data and the distinction between core and edge species may be important to identify riparian forest limits, when the structure is uniform beyond its edges. The main characteristics of typical riparian forest can be summarised as:
Concerning the life forms of riparian forests, phanerophytes, lianas and therophytes are the most species-rich. Riparian forest physiognomy is highly variable (i.e. on average 14 to 18 m tall, with occasional heights of 20 to 25 m), and the understorey is generally dense. Most riparian forest sites in Benin are influenced by adjacent more open ecosystems. This is shown by a high richness of Poaceae, low percentage of mega-phanerophytes and woody lianas as compared to dense tropical forests. Therefore they can be termed relatively low edaphic and hygrophile forests with irregular canopy. On the other hand, the floristic and phytogeographic affinities towards the guineo-congolian basin are indicative for a sufficient supply of water, which allows the establishment and long-term existence of moisture requiring species.
The size-class distribution of trees, drawn from 7765 stems, is a decreasing reverse-J curve, typical of natural forest regenerating from seed. This distribution type is also indicative of a mature stand, with many small individuals and few large ones. Similar results were found in riparian forests of Belize, and following Meave and Kellman (1994), we might characterise riparian forest in Benin as a low-biomass with many small-stemmed communities, and that is not surprising for forest formations in the Dahomey-Gap. The overall mean basal area of riparian forests in Benin, was 41.5 _ 17.9 m2 ha-1 and the mean density 586 _ 192 stems ha-1. The Shannon index of the riparian forest sites varied from 2.4 to 5.4 bits. Similar results were found in many African as well as South American and Asian tropical dense forest sites.
From the viewpoint of phytosociology, numerical classification and ordination of a data set of 818 plant species and 180 relevés of riparian forests yielded 12 plant communities, among which many have not yet been scientifically described (Natta, in prep.). The study also revealed that five important environmental gradients best differentiated these plant communities: size of waterways, relief, topography, latitude and longitude.
From a survey of 19 ha of riparian forests altogether 1002 species (about 1/3 of the estimated Benin flora) from 120 families and 513 genera have been recorded. We recorded 224 tree species with dbh _ 10 cm. The most species rich families were Papilionaceae, Poaceae, Rubiaceae, Euphorbiaceae, Cyperaceae, Asteraceae, Acanthaceae, and Caesalpiniaceae. Most woody species belong to families with one or a few species in the country. The most common species of riparian forests in Benin are Pterocarpus santalinoides (Papilionaceae), Cola laurifolia (Sterculiaceae) and Syzygium guineense (Myrtaceae). The other most common species along waterways originate from upland forests. Many forest, volunteer or pioneer species in the guinean region and sudano-guinean zone of southern and central Benin are found in streamside vegetations in the savanna region of northern Benin. Some species that have completely disappeared from plateaux in densely populated areas are found in riparian forests far from settlements or at higher latitudes. Therefore riparian forests present niches conserving species common in the dense forest in less humid landscapes. Meave and Kellman (1994) already found that most riparian forest species in a Neotropical savanna of Belize are not specialised to the riparian environment, and there is a significant supplement of species having other habitat affinities (i.e. rain and non-rain forest taxa). For Bersier and Meyer (1994), riparian forests are mosaics composed of patches of different vegetation types, which cover the entire range of forest succession.
Some endemic or valuable species are found almost exclusively in riparian forests. Among the extinction-menaced plants in Benin, Khaya senegalensis, Voacanga africana et Rauvolvia vomitoria (IUCN 2001) are abundant in riparian forests. The attractive ornamental Thunbergia atacorensis (Acanthaceae) is a rare endemic species in Benin, mainly found in riparian forests at hill's feet. The valuable Dissotis anthennina (Melastomataceae) is important in medicine, while Pentadesma butyracea (Sapotaceae) and Xylopia aethiopica (Annonaceae) are major Non Timber Forest Products in West Africa.
Summing up, riparian forests in Benin are characterised, on the one hand by a flood-dependent flora widely distributed all over the country, on the other hand by many forest and savanna species. As they combine plants from various forest and savanna types riparian forests are more diverse than vegetation formations belonging to only one ecosystem, and will provide more different categories of food for a variety of animals than other forest types.
It is generally agreed that species extinction is largely related to the reduction and fragmentation of their habitats. Therefore the protection of species can best be done through protecting habitats. Waterways and their forested banks are rich in birdlife, and attract animals of many kinds. The importance of riparian forests as habitat for a variety of animals is reported by numerous authors and confirmed by our own field observations. The reproduction, growth and nutrition of many animals depend on the more or less dense and hospitable vegetations of riparian forests.
In the north of Benin and the south of Burkina Faso, flood plains, shallow lakes and riparian forests are the principal habitats of the 278 bird species found in the Pendjari and Arli National Parks (Roggeri 1995). Gallery forests and forest edges are reportedly the main habitat of the Python sebae, bushbuck, red-flanked duiker, yellow-backed duiker, kobs, Nile crocodile, hippopotamus, African elephant, as well as vervet, patas, and mona monkeys (Sinsin et al. 1997). Likewise Cercopithecus erythrogaster erythrogaster, an endemic primate still found in Benin spends part of its time in gallery forests and forest patches in the Ouémé valley region. Certain protected or extinction-menaced animals in Benin, like elephants, pythons, hippotamus, Tragelapus spp., Cercopithecus spp., (IUCN 2001) share part of their life time in riparian forests.
In the past, Syncerus cafer nanus (African buffalo) was usually seen in woodlands and humid sites in the south and central Benin, and used to follow gallery forests up to the north. According to Raynaud and Georgy (1969), Potamochoerus porcus was found in dense forest and guinean savanna, and gallery forests as far as in the extreme north of Benin. For Depierre and Vivien (1992), various squirrels, civets, as well as numerous bird species are mainly seen either in dense forest or riparian forests. Generally in regions where dense forest does not exist, riparian forests play the role of residual forest and provide, when they are undisturbed, the best conditions for wild animals, which live in forest zones.
In Benin, misuse of riparian systems is chronic and accelerating. No recent study estimated the rate of degradation, but in 52 out of 232 relevé sites studied in 2001 and 2002 (i.e. 22.4%) agricultural practices were evident. Decades of human influence are usually seen as the main causes that have reduced riparian forests in both size and structural complexity. The interest of riparian forests lies in their resources: fertile and moist soils, water, wood and non-timber forest products vital for rural populations. These resources allow shifting cultivation, irrigation and cattle ranching, selective tree cutting and hunting. Also, disruption of watershed vegetations by damming of watercourses and/or diversion of stream flow, excessive lowering of aquifer levels through pumping, channelisation and levee construction on watercourses often cause significant damage to riparian systems.
As a result indigenous multilayered plant communities have been completely removed in many parts and are replaced by open fields, shrubs or grass savanna of much less ecological value. Therefore ecological consequences are serious both at local and regional levels with regard to the control of water sources, watersheds, water runoff and quality, mineral nutrient flows, stream bank erosion, floods, pollution, etc. Then riparian areas no longer act as routes for movement of terrestrial plants and animals across the landscape, and cease to harbour plant species adapted to a more moist climatic regime (Forman and Godron 1986).
The government of the Republic of Benin decreed a new forest law (no. 93-009) in July 1993. This law recognises the uniqueness of riparian forests as refuge ecosystem for plant and wildlife of many kinds. For example, clearance of wood and shrubs is not allowed within 25 m at both sides of any waterway (article 28). Moreover in the management plans of most forest reserves in Benin, gallery forests are to be left uncut, and rare species (e.g. Khaya spp. (Meliaceae), Milicia exelsa (Moraceae)) outside the gallery forests will not be cut either (Natta et al. 2002). Despite the strictness of conservation policies against clearance of riparian forests, in reality uncontrolled and unplanned, mostly illegal, utilisation goes on, especially in non-protected areas.
To conserve riparian forests as habitat to many plants and animals, we should effectively preserve not only the river front, interior and riparian forest edge but also parts of the adjacent upland forest or savanna. Moreover, in many riparian forest sites, a dense and continuous canopy extends to more than 100 m away from the water course. This should induce legislators to augment the distance to be protected at river sites. In the meantime, foresters, rural extension workers, and NGOs dealing with natural resource protection must promote local people's rights and duties specified by the forest law. Effective conservation can best be achieved if the local population is either collaborating or in control. Initiatives have to be taken to rise awareness concerning their interest or benefit in protecting riparian forests (e.g. plant and animal protection, landscape beauty, watershed protection, water sources protection, etc.). The application of the forest law and good collaboration within local natural resources management will surely prove positive for forested areas including riparian forests.
In line with the Convention on Biological Diversity (UNCED 1992), which emphasised the need for every country to take responsibility for conserving its own biodiversity, a prudent approach to biodiversity conservation necessitates the preservation of all potential rich or endangered ecosystems. Riparian forests in Benin are important conservation sites that need more care than currently available (Natta et al. 2002). In the majority of savanna ecosystems, efforts toward the conservation of riparian forests will increasingly become a priority in land use planning, this will allow a wide range of plants and animals to benefit from the protection of forests along rivers and streams (Natta 2000). Despite the new forest law of 1993, its implementation related to the protection of riparian vegetation, leaves to be desired. Different stakeholders dealing with biodiversity conservation in partnership with local residents should take initiatives for the preservation of the remaining rich and diverse riparian forests especially in hitherto non-protected areas.
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1 Agronomic Engineer, MSc. FSA/UAC 01BP526, TCotonou Benin; [email protected]