We examined the effects of grazing exclusion over a period of 14 years on the species richness and community composition of three plant communities with different dominant species and water regimes in the Gwydir Wetlands in eastern Australia. Responses to grazing exclusion varied among the three plant communities, and were most likely to be evident during dry periods rather than during periods of inundation. In frequently flooded plant communities, there was an increase in phytomass following exclusion of domestic livestock, but changes in plant community composition and species richness due to livestock exclusion varied depending on the morphological attributes of the dominant plant species. In a plant community where tall sedge species were dominant, livestock exclusion further increased their dominance and overall species richness declined. In contrast, where a prostrate grass species such as 'Paspalum distichum' was dominant, species richness increased following livestock exclusion, due to an increase in the abundance of taller dicotyledon species. However, livestock exclusion in a community where flooding was less frequent and native grass species have been largely replaced with the introduced species 'Phyla canescens', resulted in no significant changes to phytomass, species richness or community composition among the grazing exclusion treatments over time. Our results indicate responses to exclusion of domestic livestock are dependent upon the dominant species within the plant community and will likely vary over time with the extent of wetland inundation. Grazing exclusion alone, without increased flooding, is unlikely to restore floristically degraded floodplain plant communities.

Extensive floodplain wetland systems are a characteristic feature of the major rivers in semi-arid regions of Australia's Murray-Darling Basin. Plant communities in these wetlands are dynamic in nature, having evolved under a highly variable flow regime. Their ecology is characterized by pulses of productivity driven by inundation patterns following river flooding. Typical of floodplain ecosystems in many semi-arid regions, they are sites of high biodiversity. However, water resource developments have had a dramatic impact on the flow regime of almost all rivers in the Murray-Darling Basin, holding back floodwaters, and reducing the frequency and duration of floodplain inundation. Environmental flows have been introduced as a means of restoring river-floodplain connectivity with the aim of supporting wetland ecological processes. However, interactions with other land use factors may potentially diminish the benefits of this action. 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 plant communities in the Gwydir Wetlands, an inland terminal wetland system on the Gwydir River in the northern Murray-Darling Basin, Australia.