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1. Restoration of degraded plant communities requires understanding of community assembly processes. Human land use can influence plant community assembly by altering environmental conditions and species' dispersal patterns. Flooding, including from environmental flows, may counteract land use effects on wetland vegetation. We examined the influence of land use history and flood frequency on the functional composition of wetland plant communities along a regulated river. 2. We applied fourth corner modelling to determine species' trait-based responses to flooding and land use by combining data on (i) the occupancy and abundance of species in propagule banks and standing vegetation, (ii) species traits, and (iii) environmental conditions of 22 standing vegetation and 108 soil propagule bank study sites. We used analysis of deviance to test how well each dataset characterised trait-environment interactions, and generalised linear models to identify traits related to species' responses. 3. The occupancy and abundance of native species in the propagule bank and standing vegetation increased with flood frequency and decreased with duration of agricultural land use. Species in standing vegetation with water-borne propagule dispersal (hydrochory) showed similar trends. In contrast, species with higher specific leaf area were associated with longer land use duration. 4. Identifying trait-based differences in the propagule bank and standing vegetation can help disentangle effects of dispersal and environmental filters. The occupancy and abundance of hydrochorous species in standing vegetation were negatively related to land use duration, but hydrochorous species were positively related to land use duration based on their abundance in the propagule bank. This suggests that land use does not limit plant dispersal, but acts as an in situ abiotic filter, limiting species present in standing vegetation. 5. Synthesis and applications. Land use duration and flood frequency have opposite effects on plant community traits in floodplain wetlands of the Macquarie Marshes, Australia. Legacies of agriculture can impede restoration of plant communities. Environmental flows that increase flooding may alleviate these impacts, especially in areas that have been used for agriculture for over 20 years, by providing dispersal and environmental filters that favour native wetland species. More flooding will likely be required to restore floodplains with longer histories of agricultural land use compared to floodplains less impacted by agriculture.

Dominance of invasive species is often assumed to be due to a superior ability to acquire resources. However, dominance in plant communities can arise through multiple interacting mechanisms, including disturbance. Inter-specific competition can be strongly affected by abiotic conditions, which can determine the outcome of competitive interactions. We evaluated competition and disturbance as mechanisms governing dominance of Phyla canescens (hereafter lippia), an invasive perennial forb from South America, in Paspalum distichum (perennial grass, hereafter water couch) meadows in floodplain wetlands of eastern Australia. Water couch meadows (in the study area) are listed under the Ramsar Convention due to their significance as habitat for migratory waterbirds. In the field, we monitored patterns of vegetation boundaries between the two species in response to flooding. Under controlled glasshouse conditions, we explored competitive interactions between the native water couch and lippia subject to different soil moisture/inundation regimes. We did this using a pairwise factorial glasshouse experiment that manipulated neighbor density (9 treatments) and soil moisture/inundation (4 treatments). In the field trial, inundation increased the cover of water couch. Under more controlled conditions, the invader had a competitive effect on the native species only under dry soil conditions, and was strongly inhibited by inundation. This suggests that dry conditions favor the growth of the invader and wetter (more historical) conditions favor the native grass. In this system, invader dominance is governed by altered disturbance regimes which give the invader a competitive advantage over the native species.