Role of vegetation

Figure 1: The Tagliamento river (Italy); riparian plants grow on the sediment bars in the middle of the river and along the river margins.

Vegetation is a key element in rivers. Riparian vegetation (such as herbaceous plants, shrubs and trees) can be found on sediment deposits, on banks and along river margins (see the examples in Figure 1 and 2), while aquatic vegetation grows in or near water.

Figure 2: The Cecina river (Italy); riparian plants (herbaceous plants, shrubs and trees) colonize the sediment deposit, the floodplain and the banks while woody debris (dead trees) are deposited in the main channel.

In river systems, vegetation interacts with hydromorphology in different ways:

• river hydromorphology plays a significant role in all plant life stages, from seed dispersal to colonization, recruitment, growth, succession and mortality. Successful riparian plants often adopt a combination of adaptive strategies during different life stages in order to ensure their survival. Examples are high dispersal rates, adaptations to resist stress, and vegetative reproduction;
• vegetation actively interacts with river hydromorphology. Above-ground plant biomass can affect the flow, by blocking it or changing flow resistance and sediment transport, for instance inducing sediment deposition. Below-ground plant biomass modifies the hydraulic and mechanical properties of the substrate, including the sediments retained by the above-ground biomass, and consequently the moisture regime and erodibility of the soil on the river bed and on banks.

Plants therefore can actively interact with river morphology by changing river bed topography, river planform shape and bed sediment composition, and by promoting the development of distinctive vegetated landforms.

Although there is still much to be learnt about the interactions between plants and hydromorphology, various models are available in the scientific and technical literature for the interpretation of these interactions. Examples are models for evaluating the effect of vegetation on flow resistance, river bank dynamics, river bed development and river planform evolution (see also Vegetation Models).

More details on the recent advances on modelling riparian vegetation - hydromorphology interactions can be found in:

• Chapter 2 of the REFORM Deliverable 2.2 Part 1;
• Solari L, Van Oorschot M, Belletti B, Hendriks D, Rinaldi M, and Vargas-Luna A. 2015. Advances on modelling riparian vegetation – hydromorphology interactions. River Research and applications. DOI:10.1002/rra.2910;
• Gurnell AM., O’Hare MT., Corenblit D., García de Jalón D., González del Tánago M., Grabowski M., and Szewczyk RC. 2015. A conceptual model of vegetation-hydrogeomorphology interactions within river corridors. River Research and applications. …….
• Gurnell AM. 2014. Plants as river ecosystem engineers. Earth Surface Processes and Landforms 39(1): 4–25. DOI: 10.1002/esp.3397;