Vegetation on islands in the perennial swamps of the Okavango Delta exhibits a marked zonation pattern. Island fringes are generally characterized by broad-leaf evergreen riparian communities. 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 or are completely devoid of vegetation with sodium carbonate (trona) ? encrusted soil surrounding a central pan of extremely high conductivity. Plant distribution; Vegetation types.