A Comment on the Impact of Development Schemes on the Floodplain Forests of the Tana River of Kenya

Abstract

This paper presents the preliminary results of an ecological study in the floodplain forests of the lower Tana River of Kenya. The different forest types within the floodplain are described in detail and attempts are made to understand the recent environmental history ofthe forests. It is suggested that the forests are dependent on periods of high sustained floods for regeneration of major canopy species. It is postulated that a wetter climate in East Africa at the end of the last century (Lamb, 1966) promoted widespread forest regeneration in the floodplain but that the wetter climate experienced since the 1960s has not promoted similar regeneration because of human influences in the form of local population increase and construction of development schemes. Dams built in the Upper Tana are already reducing flood levels and a series of proposed dams will probably accentuate this process. A large-scale irrigation scheme in the Lower Tana is accelerating forest destruction through wood collection for fuel and house-building poles. NUMEROUS STUDIES HAVE been carried out on floodplain forests in temperate latitudes, especially in North America (Sigafoos, 1964; Wilson, 1970; Carter Johnson etai, 1976; Teversham and Slaymaker, 1976; Nanson and Beach, 1977; Teskey and Hinkley, 1979) but few detailed studies have been carried out in tropical floodplain forests. In temperate areas, it has been possible to build up detailed histories of floodplain vegetation through tree-ring analysis and to relate regeneration of floodplain species to sedimentation patterns and flood regimes. In many tropical areas, poor hydrological data and problematical use of tree-rings (Eckstein, 1980; Broecker et al., 1980; Mariaux, 1980; Ogden, 1980; Tomlinson and Longman, 1980) make studies of floodplain ecology very difficult. This paper presents some preliminary findings on the composition, structure and dynamics of the Lower Tana Floodplain forests in Kenya and attempts to relate them to environmental variables such as flood regimes, deposition and climate. Data collection has been carried out over a period of two years but a detailed analysis of the data has not yet been completed. The Tana River of Kenya rises on Mount Kenya and the Aberdares and flows into the Indian Ocean north of Malindi (Fig. 1). The lower reach of the river is generally considered to lie below the Kora Rapids. From here to the sea, it meanders through a semi-arid alluvial plain and supports a narrow band of evergreen forest. In its upper reaches, a series of dams is being built and in the lower reaches, large-scale irrigation schemes are presently under construction. Both directly and indirectly, these developments are likely to have adverse effects on the floodplain forests. The obvious economic problems arising from forest destruction must claim the attention of those developing the Tana River Basin. These problems include loss of firewood and house-building poles for local people inhabiting the river banks and potential increase in river bank instability, affecting irrigation works and the agricultural practices of the local people. On a less obviously pragmatic level, the floodplain forests are very important to the wildlife of the area, notably to some endemic primate species, the Tana Mangabey (Cercocebus galeritus galeritus) and the Tana River Red Colobus (Colobus badius rufomitratus), which are classified as 'critically endangered' and 'rare' respectively in the Red Data Books of the International Union for the Conservation of Nature (Simon, 1966). The data collected in the present study are used to make a preliminary assessment of the impact that the development schemes in the Tana may have on the ecology of these floodplain forests.