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River systems are amongst the most dynamic and productive ecosystems in the world and provide habitats for numerous fluvial species. River flow and river shape determine the conditions that affect plant growth and survival. In turn, riparian plants can actively influence river flow and sedimentation and erosion processes, a process called ‘ecosystem-engineering’. These continuous interactions create dynamic landscape patterns with a heterogeneous habitat mosaic. In many rivers, natural dynamics have been altered by the construction of dams, fixation of river banks and by indirect pressures such as climate change and alien invasive species. We need more understanding of how natural interactions among plants, water flow and sediment produce river landscapes, and predictive models that simulate these processes to design management plans and to predict effects of ecological restoration. Therefore, this study aimed at better understanding of emergent patterns in vegetation and fluvial morphology arising from the interaction between hydro-morphological processes and eco-engineering species and how they affect habitat suitability of other species. To study how river landscapes develop as a product of plant-river interactions, a model was developed that simulates colonisation, growth and death for eco-engineering plant species with life-stage specific characteristics. River discharge and river morphology dictate where vegetation colonizes, and survival is determined by flooding, desiccation, uprooting, burial and scour. The vegetation interacts through hydraulic resistance with the hydro-morphodynamic processes modelled in Delft3D-FLOW. This combined model is the first that is able to simulate long-term interactions between detailed hydro-morphodynamics and detailed vegetation processes. The model produces natural patterns in fluvial morphodynamics and vegetation with realistic vegetation age distribution across the floodplain. Moreover, it simulates sustained long-term meandering dynamics due to conversions from channel to floodplain and vice versa, as opposed to models with static vegetation or without vegetation. Simulation results demonstrate that river landscapes are sensitive to vegetation properties such as settlement conditions, density, growth and survival conditions. When the river system is under pressure, the native riparian vegetation cover, river morphodynamics and the resulting habitat mosaic change. After introduction of invasive alien plant species in the model, native riparian vegetation becomes outcompeted when the invaders are abundant, but facilitated when the invaders are less abundant. Seasonal plant characteristics strongly affect water levels and sediment transport, depending on season. Dams and climate change affect vegetation and river dynamics by altering the natural flow regime. Scenarios with climatic drying and dams show a reduction in riparian vegetation cover and a shift of vegetation ranges across the floodplain. Higher vegetation covers arise when extreme events in high and low flows increase. Fluvial species are most substantially affected when the flow timing mismatched with critical life-history events. Combinations of multiple flow alteration pressures and natural flow regime restoration have complex nonlinear effects due to multiple adaptation timescales. The new understanding and the model reported in this thesis will allow us to study biogeomorphological interactions, and identify and test river management strategies and river restoration plans to help increase or maintain the ecological value of rivers and their floodplains. |
