The biotic and abiotic legacy of a large infrequent flood disturbance in the Sabie River, South Africa

Abstract

Large infrequent disturbances (LIDs) leave heterogeneous, patchy legacies because the event may have different severities in different locations, or may influence ecosystem components differently. Biotic and abiotic legacies form the template upon which subsequent ecological processes take place and thus, LIDs can have a long-lasting influence on ecosystems. This study determined the geomorphological (channel type), landscape state (mosaic of physical and riparian vegetation patches) and riparian vegetation legacies of an extreme flood in the Sabie River. The linear proportions of geomorphological channel types were altered by the flood because of complex patterns of sediment erosion and deposition related to bed slope and channel width. Biotic landscape states (tree, shrub, reed, herbaceous vegetation) were stripped to expose the underlying physical template. The persistence and arrangement of landscape states varied amongst channel types because of the interaction between flood flows and the geomorphological attributes of each channel type. Overall, some riparian species decreased in density because of removal by the flood, and some increased in density because of post-flood recruitment or re-establishment. The flood also changed the location of many species in relation to their characteristic pre-flood elevation within the channel. In accordance with the expectations of LIDs, the effects of the flood were not uniform. Instead, the flood left a legacy of juxtaposed biotic and abiotic components that varied spatially amongst channel types and at different elevations within the channel.