PLANT-DISTRIBUTION IN ISLANDS OF THE OKAVANGO DELTA, BOTSWANA - DETERMINANTS AND FEEDBACK INTERACTIONS

Vegetation on islands in the perennial swamps of the Okavango Delta exhibits a marked zonation pattern. island fringes are generally characterized by a broad-leaf evergreen riparian community of Syzigium cordatum, Ficus verruculosa, F. natalensis, F. sycamorus, Phoenix reclinata, Garcinia livingstonei and Diospyros mespiliformis. This gives way towards island interiors to a community dominated by Acacia nigrescens, Croton megalobotrys and Hyphaene ventricosa. The central regions are characterized either by short, sparse grassland dominated by Sporobolus spicatus or are completely devoid of vegetation with sodium carbonate (trona)-encrusted soil surrounding a central pan of extremely high conductivity. An indirect gradient analysis indicated the important determinants of vegetation distribution as being not only elevation, depth to groundwater and distance from the water source, but also as groundwater chemistry (conductivity and pH) and soil chemistry (sodium concentration). These studies were supported by a direct gradient analysis approach with species tolerances to depth to groundwater, groundwater conductivity and soil-surface sodium concentrations having been established. Feedback interactions of vegetation and groundwater elevation and chemistry, as well as soil chemistry, are evident and appear to be responsible for the vegetation dynamics in the study area.