ECOTONE VERSUS ECOCLINE - VEGETATION ZONATION AND DYNAMICS AROUND A SMALL RESERVOIR IN TANZANIA

Data from transects on the shores and drawdown area of a small reservoir in Tanzania with a strongly fluctuating water level were used to illuminate a spatial and short term temporal variation in the vegetation of a border zone. Vegetation data were related to the position of the sample plots in relation to a water level gauge and to time (seasonal changes and changes between two consecutive years). Few perennial species survived the rainy season in the zone exposed to the fluctuating water level. Most plants were annuals that colonized yearly. The vegetation under the full supply level (=FSL) was sparse and related to Eriochloetum nubicae and Ecliption albae. An observed great variation between sites and years of this kind of vegetation seems to be due to the unstable character of the sites with frequent recolonization of bare land. The need to distinguish formally between ecotones (in a strict sense) and ecoclines is stressed. The drawdown area has (a) a peak in beta diversity just below FSL, (b) no beta diversity in a zone below FSL that was not flooded the preceding rainy season and (c) a certain amount of beta diversity further down. (a) and (b) are considered typical of an ecotone. (c) is interpreted as a 'short term ecocline' of colonizing annuals between two rainy seasons. The system below FSL as a whole is an ecotone. The shore above FSL is an ecocline with high alpha and beta diversity. A downward movement of plant species during the dry season is demonstrated using canonical correspondence analysis. Thus, this ecotone is driven by the environment and no internal (autonomous) control is likely to occur.