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 reservo ir in Tanzania with a strongly fluctuating water level were used to il luminate 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 spe cies survived the rainy season in the zone exposed to the fluctuating water level. Most plants were annuals that colonized yearly. The veget ation under the full supply level (=FSL) was sparse and related to Eri ochloetum nubicae and Ecliption albae. An observed great variation bet ween 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 s ense) and ecoclines is stressed. The drawdown area has (a) a peak in b eta diversity just below FSL, (b) no beta diversity in a zone below FS L that was not flooded the preceding rainy season and (c) a certain am ount of beta diversity further down. (a) and (b) are considered typica l of an ecotone. (c) is interpreted as a 'short term ecocline' of colo nizing annuals between two rainy seasons. The system below FSL as a wh ole is an ecotone. The shore above FSL is an ecocline with high alpha and beta diversity. A downward movement of plant species during the dr y season is demonstrated using canonical correspondence analysis. Thus , this ecotone is driven by the environment and no internal (autonomou s) control is likely to occur.