Disconnected from the Rhine floods, the remarkable habitats on Rohrschollen Island have a tendency to dry out. The very existence of the forest populations typical of alluvial habitats is threatened. The project will help to demonstrate that, by restoring an alluvial dynamic based on the Rhine's natural system, the habitat will regain its functionality.

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Threat 1: Disappearance of the pioneer stage of alluvial forest succession

The canalisation and containment works on the Rhine have profoundly disrupted the river's hydrological functionality (pulse-driven water system) by considerably alleviating flooding (in terms of frequency, duration and dynamic) and the fluctuations in the water table. The floods, which once rejuvenated the habitat with their morphogenic action by creating banks of sand and gravel conducive to the installation of these habitats, have disappeared. These pioneer stages of the alluvial forest no longer encounter conditions conducive to their installation and development and are therefore doomed to disappear in the medium term.
Alluvial softwood forest has not appeared on Rohrschollen Island for 30 years, since the last natural floods. The absence of flooding has already brought about the progressive disappearance of the initial stages of vegetation succession.
If no action is taken, this tendency will increase and we will witness the progressive disappearance of the pioneer species (Black Alder, White Willow, Black Poplar), blocking the forest in its climacic hardwood stage.
Restoration of the alluvial and dynamic function of the hydrosystem (Bauerngrundwasser and its secondary arms) will help to expose the sandy-gravely to gravely areas and restore exchanges with the water table, providing conditions conducive to the installation of hygrophilous associations of pioneers attached to the Salicetum albae.

Threat 2: Degradation of habitats in the open and forest habitats typical of alluvial habitats
The practically total eradication of flooding on Rohrschollen Island has led to a general drying out of habitats, causing a change in their composition. For the open habitats, the tendency to dry out causes a loss of naturalness in the habitats, which are also threatened by forest colonisation in the alluvial forest. The presence of Giant Goldenrod (Solidago gigantea), an invasive plant, also threatens the open habitats by preventing Mesobrometum erecti-type indigenous herbaceous associations.
For the forest habitats, the lack of flooding leads to a homogenisation of the forest (loss of its mosaic character) and impairs its alluvial character. The alluvial forest habitat also suffers from the presence of an invasive plant, the Japanese Knotweed (Fallopia japonica), which has started to establish itself in the humid areas.
If no action is taken, this trend will increase and we will witness a total loss of the alluvial functionality of forest formations and a change to mesophilous formations.
The return of dynamic flooding to the island will help to restore the renewal dynamic of the habitats and get rid of species unsuited to the dynamic function of the Rhine alluvial system.

Threat 3: Alluviation of the arms supplying the Bauerngrundwasser and its secondary hydrographic network
The water diversions supplying the secondary arms of the Bauerngrundwasser fail to provide a regular flow rate: the arms supplying the Bauerngrundwasser are undergoing alluviation, which threatens their hydraulic and ecological functionality.
Moreover, the maintenance and regeneration of the secondary hydrographic network (depressions, ditches, ponds…) are no longer guaranteed by floods: there is a danger that we will witness a homogenisation of the surface horizons, with the progressive disappearance of areas in depressions in contact with the water table.
Analysis of the natural habitats on the island clearly shows the change from an active habitat to a passive habitat in which the total surface area of aquatic habitats is being considerably reduced (reduction of two thirds of the surface areas covered by water on the island between 1921 and 2004). The lack of water movement between the Rhine and the island's hydrographic network may bring about an increased eutrophication of the Bauerngrundwasser. Alluviation of the secondary hydrographic network will also contribute to the general regression seen in alluvial habitats, directly threatening habitat diversity.
Restoration of the connections between the Rhine and the secondary arms will help to guarantee a regular flow into the secondary arms of Bauerngrundwasser in such a way as to re-establish functional exchanges with the river. The return of dynamic flooding to the island will also help to maintain and regenerate the secondary network by reconnecting the depressions.

Threat 4: Degradation of the exchange conditions between the Bauerngrundwasser and the water table
The lack of flow dynamics is conducive to the clogging of the river bed by silt and fine sediments. Moreover, the various development works on the Rhine have considerably diminished the extent of variation in the water table, down by at least 50 cm. Water table-river exchanges are in danger of disappearing altogether owing to the progressive clogging of the river bed and the lack of ground water upwellings.
The Bauerngrundwasser is a relatively non-dynamic hydrosystem, reflected in its calm or flowing waters, poor in species, turbid and eutrophic, partially colonised by Nuttall's Waterweed, an invasive species. The degradation of exchanges with the water table will increase the eutrophic character of the water course and lead to a loss of diversity in the species and habitats present.
Re-establishing a regular flow into the Bauerngrundwasser and restoring dynamic flows during episodes of flooding on the Rhine will help to diversify the aquatic habitats and re-establish water table-river exchange conditions by unclogging the bed of the Bauerngrundwasser and restoring its phreatic supply to the depressions.


Threat 5: Disappearance of the Spined Loach (Cobitis taenia)

The tendency in the habitats to dry out and the lack of dynamic flows may, in the medium term, lead to the loss of functionality and the eventual disappearance of habitats favourable to the reproduction of the Spined Loach, listed in Annex II to the Habitats Directive. Indeed, the Spined Loach lives in slow-flowing or calm habitats with a sandy bed where it buries itself during the day and lays its eggs in the sand and on roots. Rohrschollen Island offers habitats conducive to the development and reproduction of the Spined Loach: five specimens of Spined Loach were observed in the Bauerngrundwasser in 2004, in the vicinity of the Old Rhine. Nevertheless, the species seems localised and relatively few in number. The lack of flooding in the main channel, the drying out of the humid areas and the silting up of the secondary arms and depressions present risks for the conservation of this species, the presence of which is unique in this sector of the Natura 2000 site.
The return of dynamic flows during episodes of flooding on the Rhine will help to conserve favourable, functional habitats for the Spined Loach. Moreover, a specific study of definitions of complementary works will help to provide optimal protection for the habitats specific to this species.

Photographs:
Sand lizard © Jean-Pierre VACHER (BUFO)
Black-necked Grebe © Sylvain HELLIO
Lesser Purple Emperor