Abstract
Riparian zones are formed by interactions between fluvio-geomorphological processes, such as sediment deposition, and biota, such as vegetation. Establishment of invasive alien plant (IAP) species along rivers may influence vegetation dynamics, evidenced as higher seasonal or inter-annual fluctuations in native plant diversity when IAP cover is high. This could impact the overall functioning of riparian ecosystems. Conversely, fine sediment deposited in riparian zones after floods may replenish propagule banks, thus supporting recruitment of native species. The interactive effects of invasion and fine sediment deposition have hitherto, however, been ignored. Vegetation surveys across rivers varying in flow regime were carried out over 2 years to assess changes in community composition and diversity. Artificial turf mats were used to quantify over-winter sediment deposition. The viable propagule bank in soil and freshly deposited sediment was then quantified by germination trials. Structural Equation Models were used to assess causal pathways between environmental variables, IAPs and native vegetation. Greater variation in flow increased the cover of IAPs along riverbanks. An increased in high flow events and sediment deposition were positively associated with the diversity of propagules deposited. However, greater diversity of propagules did not result in a more diverse plant community at invaded sites, as greater cover of IAPs in summer reduced native plant diversity. Seasonal turnover in the above-ground vegetation was also accentuated at previously invaded sites, suggesting that a legacy of increased competition in previous years, not recent sediment deposition, drives above-ground vegetation structure at invaded sites. The interaction between fluvial disturbance via sediment deposition and invasion pressure is of growing importance in the management of riparian habitats. Our results suggest that invasion can uncouple the processes that contribute to resilience in dynamic habitats making already invaded habitats vulnerable to further invasions.
Highlights
Riparian zones are complex and dynamic habitats noted for their high biodiversity (Naiman and Decamps 1997), yet are threatened by land use and channel management practices, altered hydrology, climate change, and biological invasions (Flanagan et al 2015)
Greater variation in flow increased invasive alien plant (IAP) cover in year 1 (Beta = 0.40, SE = 0.15, P = 0.02), which led to higher IAP cover in summer year 2 (Beta = 0.44, SE = 0.12, P B 0.01)
This subsequently led to a decline in the diversity of native vegetation (Beta = - 0.26, SE = 0.13, P = 0.05), and flow regime indirectly reduced diversity of native vegetation in summer through its direct effects on IAP cover
Summary
Riparian zones are complex and dynamic habitats noted for their high biodiversity (Naiman and Decamps 1997), yet are threatened by land use and channel management practices, altered hydrology (e.g., abstraction and flow regulation), climate change, and biological invasions (Flanagan et al 2015). Their biodiversity is intimately linked to high spatial and temporal heterogeneity driven by fluvio-geomorphological processes and the potential for waterborne dispersal (hydrochory). It is critical to understand how they contribute to the resilience of riparian vegetation in the face of invasion and increased sediment loading
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