SA Water is planning to upgrade Mt Bold dam, with one of the objectives to increase flood mitigation. During high rainfall events, resultant reservoir water levels will exceed the current full supply level and temporarily inundate surrounding native vegetation. The impact of short-term inundation on native terrestrial vegetation is not well understood and SA Water bears risk to native vegetation impacts if offsets are not made for potential losses under the Native Vegetation Act.
The aim of this study was to investigate the response of terrestrial species to inundation similar to what the fringing vegetation would experience as a result of the dam upgrade for a one in 100-year flood in a mesocosm experiment. Species were selected based on abundance in the area surrounding the reservoir; with the two dominant overstorey species (Eucalyptus obliqua and Eucalyptus leucoxylon) and five perennial understorey species (Allocasuarina muelleriana, Dianella revoluta, Dodonea viscosa, Hakea rostrata and Xanthorrhoea semiplana) selected.
One year old plants (except Xanthorrhoea semiplana, where five year old plants were used) were completely inundated for two, four and six days. Following inundation, plants were removed from the tanks and survivorship, leaf number and height recorded at two, four, eight and 12 weeks after inundation.
Eucalyptus obliqua was the least tolerant to submergence with 100% mortality of plants from all treatments except the control. Dianella revoluta and Xanthorrhoea semiplana were the most tolerant with no mortality across treatments. Results suggested that the vegetation composition around the edge of Mt Bold Reservoir may change in the short-term because of the upgrade to the dam as all species, except Dianella revoluta and Xanthorrhoea semiplana, were impacted by inundation. Whether species, or only cohorts are temporally or permanently impacted requires more research, in particular field studies that can complement this experiment.