A temporal comparison of the influence of flows and grazing on vegetation communities in the Gwydir Wetlands, NSW, Australia

Abstract

Floodplain plant communities in Australia’s Murray Darling Basin are dynamic in nature, showing high levels of spatial heterogeneity and temporal variability. Typical of floodplain ecosystems in many semi-arid regions, their ecology is characterized by pulses of productivity, driven by inundation patterns following river flooding. However, river regulation has significantly altered the flow regime of almost all major rivers in the Murray-Darling Basin, holding back floodwaters and consequently reducing the frequency, duration and spatial extent of floodplain inundation. While environmental flows have been introduced as a means of ameliorating the impacts of river regulation and supporting wetland ecological processes, other land use factors may potentially diminish any benefits. Grazing of domestic livestock, particularly cattle, has taken place on many of these floodplains for over 160 years. In conjunction with flooding and drought, grazing may be one of the most important agents of disturbance that shape floodplain plant communities. This study examined three wetland plant communities in the Gwydir Wetlands in New South Wales, Australia, where long-term trials have investigated whether grazing by domestic and native herbivores alter the response of vegetation to natural flooding and environmental flows. The pattern of change in plant community composition over time indicated that inundation regime is the most important factor shaping the distribution and abundance of extant vegetation. In contrast, grazing by domestic livestock and by native herbivores had a relatively minor impact. At sites where environmental water allocations resulted in wetland inundation, changes in plant community composition occurred across all grazing treatments. These changes mirrored the responses seen following major floods, dominated by pronounced increases in the cover of amphibious species and a concomitant decline in cover of terrestrial species. Significant differences between plots open and closed to grazing mostly occurred during dry periods between flood events. While grazing can influence species composition in the in the short-term, inundation from both natural flooding and environmental flows plays a far more significant role over the long-term in shaping wetland plant assemblages.