Changes in the distribution of emergent aquatic plants in a brackish South African estuarine-lake system

Abstract

Vegetation mapping in the Wilderness Lakes indicated that, between 1975 and 1997, prominent increases occurred in the distribution of the mapping units Phragmites australis (53.9ha; +53%), grass or fields (23.1ha; +35%) and scrub or trees (12.2ha; +45%). Over the same period the area of human habitation more than doubled (10.8ha to 23.3ha). Substantial declines occurred in the distribution of Juncus kraussii (76.2ha; –243%), Schoenoplectus scirpoideus (10.1ha; –38%) and low scrub or fynbos (7.8ha; –66%). Six new plant species were recorded in stands large enough for identification from aerial photographs. The most prominent changes occurred at Langvlei and the Serpentine Channel. Effective management requires the setting of goals pertaining to the dynamics of wetland vegetation, identifying the causes of change, defining practical management options and monitoring the effectiveness of management actions. These issues are discussed in relation to information and decision-making requirements for ecosystem management. Probable causes of change in the distribution of wetland plants in the Wilderness Lakes include the natural tendency of plants to colonise new areas, as well as anthropogenic manipulation of physical, chemical and biological processes, including the cessation of disturbance by large herbivores, water-level stabilisation, changes in soil salinity and the accumulation of plant litter within wetland areas. Potential management control methods include the flooding of wetland areas, periodic burning of wetland plants, mechanical disturbance and physical disturbance by large herbivores.