- FLOODPLAIN REHABILITATION IN NORTH CAMEROON: IMPACT ON VEGETATION DYNAMICS - 33 Applied Vegetation Science 3: 33-42, 2000 © IAVS; Opulus Press Uppsala. Printed in Sweden Floodplain rehabilitation in North Cameroon: Impact on vegetation dynamics Scholte, Paul 1,2,3* , Kirda, Philippe 2 , Adam, Saleh 2 & Kadiri, Bobo 2 1 Centre of Environmental Science, Leiden University, P.O 2300 RA Leiden, The Netherlands; * Corresponding author: Nieuwe Teertuinen 12C, 1013 LV Amsterdam, The Netherlands; 2 Waza-Logone Project, P.O. Box 284 Maroua, Cameroon; 3 Ecole de Faune, P.O. Box 271 Garoua, Cameroon Abstract. Since the construction in 1979 of a dam in the Logone floodplain in the Sahelo-Sudanian zone of Cameroon, annual inundations have decreased, reducing perennial veg- etation as important grazing source for nomadic herds and wildlife during the dry season. Presently, possibilities exist to release excess water for floodplain rehabilitation. In 1994 a pilot release was executed, reflooding 200 km 2 , to verify predicted advantages. Vegetation has been studied from 1984 onwards along a transect covering flooded, recently reflooded and desiccated parts of the floodplain. Since 1993, the floristic composition has also been monitored in a grid in the centre of the impact zone. Cover of perennial grasses, most notably Echinochloa pyramidalis and Oryza longistaminata increased from 41 to 61% in the reflooded zone. Vetiveria nigritana, a tussock grass that used to be dominant, disappeared slowly after the dam construction and has not shown a comeback. The cover of annual species, most notably Sorghum arundinaceum, a dominant annual grass only since the mid-1980s, decreased in the reflooded zone from 58% to 34%. If the present conver- sion rate of annual into perennial grassland is extrapolated, recovery towards a 100% perennial state may be reached after the 2003 flooding season. Apart from favourable climatic conditions, recovery might be dependent on the restoration of soil fertility, limiting an approach focusing on flooding depth only. Keywords: Dam; Flooded grassland; Re-colonization; Resil- ience; Rhizomatous grass; Tussock grass; Wetland. Nomenclature: Geerling (1982) for woody species; van der Zon (1992) for grasses. Introduction Seasonally flooded grasslands are amongst the most productive ecosystems known in sub-Saharan Africa (Denny 1985). They are essential habitats for water birds and can harbour large concentrations of antelopes. Many people have adapted to life on the floodplains and are exploiting them with increasing intensity, compet- ing with wildlife. Extreme cases of competition occur when the water supply is cut off for irrigation or hydro- electric purposes, causing abrupt environmental changes due to desiccation and lack of silt input (Breen et al. 1988; Middleton 1999). After widespread food short- ages in the early 1970s, such water control projects were undertaken in almost all major river systems in sub- Saharan Africa (Drijver & Marchand 1985). Following a change in general attitudes in which the value of downstream, small-scale human activities as well as of wildlife is recognized, more diversified approaches of river basin development are currently being developed (Adams 1996). In several river systems possibilities exist for the controlled release of water to deliberately flood downstream areas, potentially rehabilitating the degraded environment and restoring its functions. Such controlled flood releases have been conducted on the Phogolo river in South Africa (Bruwer et al. 1996), in the Senegal valley (Hollis 1996) and in the Hadejia- Nguru wetlands in Nigeria (Polet & Thompson 1996). They are reported as being ‘successful’, but remain ecological ‘black boxes’ with available information re- stricted to water flows (in) and local production in- creases (out). In 1979, the Waza-Logone area followed the fate of many African floodplains, with the construction of a dam (forming Lake Maga) and an embankment along the Logone river, as part of a parastatal rice scheme (Fig. 1). In combination with lower than normal rainfall for the last two decades (Beauvilain 1995) this dam has led to a reduction in the depth and extent of flooding over an area of 2000 km 2 . Previously productive perennial grasslands were replaced by annual-grass dominated stands, limiting regrowth in the dry season and reducing the carrying capacity for wildlife and livestock (Scholte et al. 1996a, b). A pilot reflooding scheme was started in 1994 by the opening of a watercourse, formerly closed by the embankment along the Logone River, reflooding an area of ca. 200km 2 . Based on initial positive impres- sions, options were formulated for a further 1000 km 2 of floodplain rehabilitation. No ecological information following reflooding has been reported from elsewhere in (West) Africa (John et al. 1993). Expectations of the Waza-Logone rehabilitation were, however, optimistic based on the impressive dy- namics of the floodplain (Breen et al. 1988). In the reflooding