Recolonisation of river banks by vegetation

Favourable conditions for rapid recolonisation

The valley's soils, deposited at the bottom of the lakes over the years, are very rich in nutrients (nitrogen, phosphorus) from the erosion of the surrounding agricultural soils. Well supplied with water and exposed to light, the soils of this valley have been naturally sown by an abundant production of various seeds. These seeds come from the local surrounding vegetation (vegetation surrounding the former lake, slope forest) but also from the surrounding landscapes, transported by water (run-off, tributaries, upstream of the river) or by the wind.

All these initial conditions were particularly favourable to the rapid return of new vegetation and the emergence of the variety of flora observed. Monitoring carried out in the former Vezins reservoir indicates two major phases in this recolonisation by vegetation.

1. First, from 2015 to 2018, the pioneering phase.  The bare sediment surfaces, exposed progressively during the emptying or suddenly following the sediment storage works carried outside of the Sélune River bed, were systematically and rapidly colonised by pioneer plants, i.e. annual herbaceous plants (e.g. knotweed, Bidens, Vergerette) and woody plants (willows, birch, alders).
2. Then, from 2018 to 2022, the annual herbaceous plants, which have a short life cycle, gradually gave way to perennial herbaceous plants (Baldingère, Joncs, etc.) and developing trees. This type of perennial vegetation has a deeper, denser root system that remains in place throughout the seasons.

During these two phases, a number of rare species, which are protected or of heritage interest in Normandy and characteristic of wetlands, developed in several sectors. These include 3 protected species (Limoselle aquatique, Léersie faux-riz, Scirpe à épis ovales) and 6 species of heritage interest (with "vulnerable" to "near-threatened" status) such as Elatine à six étamines and Oseille maritime.

Recolonisation influenced by human actions

The banks and slopes of the Sélune valley are very different from one area to another, both longitudinally, from upstream to downstream, and laterally, from the limit of the former lake to the present channel. In particular, differences in slope, substrate, bank shape and type have a strong influence on vegetation. These differences may be natural or linked to human action, in particular the work carried out to remove dams.

Thus, the emptying of the lakes, the reshaping of the banks, the storage of sediments or even one-off initiatives such as the crushing of vegetation to establish seeded cover, the establishment of grazing or mowing on the banks have largely conditioned the nature of the plants that re-colonise and the patterns of vegetation distribution. For example, alders, typical riverside trees, were able to establish themselves on the banks in the early stages of the emptying of the Vezins dam, which took place over several years. These trees are now found on the hillsides, sometimes a long way from the watercourse. Maintained thanks to springs and seepage from perched water tables, their long-term survival is not guaranteed, particularly in the context of climate change. Their presence right from the start of recolonisation has been a remarkable asset in maintaining coarse sediments, especially in areas with very steep slopes and low fertility. This singular fact has also favoured the development of a characteristic associated flora (Spaced sedge, Wood Angelica, etc.).

The new Sélune valley seems to have the potential to become a major reservoir of plant and animal biodiversity in the catchment area. However, each mechanical intervention on the evolving vegetation puts the plant dynamic back to where it started. This delays the natural establishment of trees on the banks of the river, and can benefit exotic species that can then become invasive.

What does this mean for the landscape of the Sélune valley?

The natural recolonisation by vegetation that we are currently observing on the banks is rich, heterogeneous and functional.

• It will thus help to improve water quality by retaining water in the soil (wetlands) and purifying it (denitrification in particular).
• During floods, it helps to limit downstream flooding and bank erosion thanks to the root networks of the plants and trees.
• In addition, riparian woodland absorbs some of the sun's energy and provides shade, limiting the warming of the river water.
  This is therefore a key factor in promoting aquatic and terrestrial biodiversity in the Sélune.

This recolonisation, which is currently heterogeneous, is not yet complete. On a valley-wide scale, although riparian woodland is generally expected to increase, the exact evolutionary trajectories remain largely unpredictable, given the dynamic nature of river valleys.