The ephemeral vegetation of seasonal and semi-permanent ponds in tropical West Africa
Müller, Jonas V.; Deil, Ulrich
Seasonal ponds and semi-permanent freshwater lakes are a common landscape element in tropical West Africa. The vegetation, emerging in temporary waters and colonizing the amphibic shorelines of lakes with fluctuating water levels, was relatively poorly known for a long time, although these habitats offer an important resource for man and his livestock. Based upon the available phytosociological data from Senegal to Burkina Faso, a synopsis of the ephemeral wetland vegetation is presented. The associations and syntaxa of higher rank are published according to the ICPN. A schematic profile illustrates the spatial zonation (hydroseries) of these communities in the North Sudanian and Sahelian zones.To the north, water level fluctuations become stronger and the dry period is more extended. The dominant life forms are free-floating or rooting submerged pleustophytes, hydrogeophytes and short-living amphibic and terrestrial plants. Superimposition of several communities, dominated by different life-forms, is a common phenomenon.The hierarchical classification of 392 relevés resulted in 26 associations and 9 rankless communities, grouped into five different classes (Rhamphicarpo fistulosae-Hygrophiletea senegalensis, Ceratophylletea, Lemnetea minoris, Drosero-Xyridetea, Ludwigio octovalvis-Echinochloetea colonum) and in two orders of unclear class-position (Nymphaetalia loti, Papyretalia).Water depth, duration of flooding, and the trophic level are the most important differentiating ecological factors. Floating leaf communities (Nymphaeion micranthae, Nymphaeion guineensis), submerged macrophytic vegetation (Ceratophyllion demersi) and free-floating communities (Lemnetea) are concentrated in deeper ponds. The new class Rhamphicarpo-Hygrophiletea senegalensis combines the vegetation of shallow and temporary ponds and of amphibic environments with short-term inundation. The mesotrophic communities of the Echinochloion callopi occur in rapidly draining depressions over lateritic crusts. Nutrient-rich shorelines with muddy substrate in the North Sudanian and Sahelian zones are colonized by the Sagittario guayanensis-Nymphaeion maculatae. The creeping swards of the BrachiarioCynodontion develop on eutrophic, shortly inundated wet places around Sahelian seasonal lakes, submitted to heavy grazing. The sub-alliances of the Eragrostion squamatae reflect the extent to which the saline groundwater affects the ephemeral vegetation in interdune depressions. The Eriocaulo pumili-Ophioglossion gomezianum (Drosero-Xyridetea) is characterized by members of the families Eriocaulaceae, Xyridaceae, Droseraceae, Lentibulariaceae and Ophioglossaceae. These families are adapted to oligotrophic conditions in seasonally flooded rock pools. Many species which have their primary habitats in ephemeral ponds settle rice fields as their secondary habitats. Rice weed communities in West Africa belong to the order Melochietalia corchorifoliae. From the humid Guinean to the semi-arid Sahelian zone, two trends can be observed: an impoverishment in floristic diversity of the communities, and a reduction of the ecological amplitude towards the borders of the areal. Finally, the vegetation of seasonal and semi-permanent lakes of West Africa is compared with other regions. There is a clear floristic contrast to Mediterranean seasonal ponds. Similarities in life strategies, habitat conditions and floristics (on the family and genus-level) exist with other tropical regions of the world. Plant communities with closely related taxa grow in the Neotropis. Vicarious taxa occur for example in the families Scrophulariaceae (Gratiolioideae), Acanthaceae (Hygrophila), Apiaceae (Hydrocotyle), Elatinaceae (Bergia, Elatine), Lythraceae (Ammannia), Menyanthaceae (Nymphoides) and Alismataceae (Limnophyton, Sagittaria).