Fine-grained sediment deposition alters the response of plant CSR strategies on the gravel bars of a highly regulated river

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Prepared by Philippe Janssen, Hervé Piégay & André Evette

Along the Rhone, gravel bars with a high proportion of subsoil fine sediments at low elevation benefit stress-tolerant species such as Lythrum salicaria (Photo credit: Philippe Janssen).

Flow regulation by dams profoundly changes the functioning of river ecosystems. By reducing the intensity and frequency of the flood regime, flow regulation alters the level of disturbance and can promote the deposition of fine-grained alluvial sediments. These changes in environmental conditions can be particularly strong along riparian zones, i.e., transitional areas at the interface between freshwater and terrestrial ecosystems.

Using gravel bars along the Rhone River (France) as a case study, we investigated how the ecological strategies of riparian plants were shaped by environmental gradients. To do so, we studied the relationship between competitive, stress-tolerant, and ruderal vegetation strategies, as defined in Grime’s CSR theory, at the community scale and changes in elevation and proportion of fine sediments at the gravel bar scale, and in temperature at the corridor scale.

Our results showed that the response of plant communities to local elevation gradients was not the same if the gravel bar was weakly or strongly covered with fine sediments. Specifically, we measured a shift in species strategies from communities dominated by ruderal species to communities dominated by stress-tolerant species along the elevation gradient on bars with a low proportion of fine sediments and vice versa on bars with a high proportion of fine sediments. In addition, we found that gravel bars subjected to extensive deposition of fine sediments harboured plant communities with less diverse ecological strategies, resulting in biotic homogenization along the local flood gradient.

At the corridor scale, increased temperatures along the 250 km of the north-south river corridor studied primarily benefited ruderal plant communities. However, as along the local elevation gradient, the deposition of fine-grained sediments on the gravel bars mitigated the effect of the climatic gradient by promoting a less stressful environment (i.e., by increasing water retention capacity and nutrient availability) and favouring communities with less diverse ecological strategies.

Along the Rhone, gravel bars with a low proportion of subsoil fine sediments at low elevation benefit ruderal species such as Persicaria mitis (Photo credit: Philippe Janssen).

Overall, these results suggest that the deposition of fine-grained sediments reduces the intensity of stress not only along the flood gradient but also along the climate gradient, with potentially adverse consequences for riparian biodiversity and its conservation. Indeed, by increasing the similarity of plant communities within and across gravel bars, overbank fine sedimentation may further reduce the species pool and therefore the adaptability of riparian communities, i.e., by inducing a selection on plant strategies toward a limited range of values.

To avoid a general homogenization of riparian communities in these degraded ecosystems, it is necessary to restore their hydro-geomorphological dynamics. This could be achieved by increasing the intensity of disturbances, i.e., by reactivating bedload supply and transport, and by increasing the intensity of stress levels, i.e., by increasing flooding and promoting unstable base-flow. In regulated river systems, these actions would especially benefit the development of ruderal pioneer communities in the littoral zone, which are the most threatened by the overgrowth of competitive and stress-tolerant perennial species.

This is a plain language summary for the paper of Janssen et al. published in Applied Vegetation Science (https://doi.org/10.1111/avsc.12494).