Anne-Marie Planty-Tabacchi’s research while affiliated with French National Centre for Scientific Research and other places

Riparian ecosystems are highly dependent on feedbacks between vegetation dynamics and hydrogeomorphic components. Physical constraints on vegetation support a mosaic of heterogeneous habitats that support high biological diversity and provide many ecosystem services. Most European rivers have undergone anthropogenic alterations such as channelization and modification of the hydrosedimentary regime. In March 2019, the United Nations designated the period 2021-2030 as the "Decade for Ecosystem Restoration," and riparian ecosystems are among the priority targets for restoration actions. Actors responsible for riparian ecosystem restoration need decision support tools to guide their actions. There are gaps in knowledge about river functioning, particularly in understanding the feedbacks between vegetation dynamics and river morphodynamics. This understanding is essential to make predictions about the adjustment trajectories of riparian ecosystems. Current numerical models of river landscape dynamics incompletely reflect feedbacks between river morphodynamics and vegetation successions. In particular, the role of vegetation should be integrated through its impact on sediment dynamics, as well as on river forms and their evolutionary trajectories. We present here the conceptual framework and methodology of the NUMRIP project (2022-2025) funded by the French National Research Agency (ANR). This project is based on the conceptual model of "fluvial biogeomorphological succession" developed in the GEOLAB laboratory (UMR6042 UCA-CNRS) and on recent advances in the "functional trait" approach in plant ecology and remote sensing techniques of plant-geomorphology interactions. The core of the project is the development of a numerical model (based on cellular automata) of the dynamics of the river landscape by integrating physical, biological and human components. The project focuses on riparian vegetation, from the scale of individuals to plant communities. The model integrates vegetation as a dynamic component of the system, which responds to and affects hydrogeomorphic processes and fluvial forms. My post-doctorate, as part of the NUMRIP work package "Plant traits", aims at identifying and characterizing riparian plant guilds in relation to their responses and effects on hydrogeomorphological constraints, in particular on substrate stability and sediment trapping. The aim will be to associate functional plant traits (morphological, biomechanical, phenological and physiological traits) with hydrogeomorphological processes and fluvial forms using functional trait approach tools and remote sensing techniques. The lower Allier River (France) is used as a case study. It is one of the last free meandering river segments in Europe, and thus represents an opportunity to study the feedbacks between plant dynamics and river morphodynamics in a dynamic system in a temperate climate. The model can be used as a fundamental research tool but also as a decision support tool for managers. It will be able to predict potential evolutionary trajectories of river corridors in relation to, for example, a change in hydrological regime, an invasion of an exotic species or will be used for restoration work.

Plant communities and dynamics can be characterized according to species composition or plant traits. Here, we used species composition and plant traits to compare their effectiveness in discriminating the biogeomorphological (involving reciprocal feedbacks between physical and biological processes) and ecological (mainly biologically driven) phases of the fluvial biogeomorphological succession (FBS) model. The comparison was done between two French rivers, the largely unchannelized lower Allier and the channelized middle Garonne. One reach representative of each river section was selected for the study. Within each river reach, we chose two contrasted study sites in terms of channel and floodplain dynamics: a reference site (least altered channel and floodplain dynamics) and an altered site (laterally stabilized by riprap and constrained). In the four study sites, we sampled vegetation in 402 plots of 4 m2. The 512 species identified in the plots were characterized in terms of plant traits (20) from a literature review. When comparing reaches in unconstrained ordinations and permutational multivariate analyses of variance, both species composition and plant traits led to a similar identification of the biogeomorphological and the ecological successional trajectories. Nevertheless, the trait approach was less influenced by local and regional bioclimatic, hydrogeomorphological, and anthropogenic settings and thus produced a more comprehensive and general classification of the biogeomorphological and ecological phases of the FBS model. A lower than expected contrast between the four sites was found, because neither species composition nor plant traits could entirely characterize distinct successional trajectories occurring in our reference or altered sites. Furthermore, our results contributed to a better understanding of the multiple successional trajectories that can occur in midlatitude river corridors. It also showed that relating plant traits to their effects on fluvial landform dynamics remains a core challenge in explaining succession including feedback mechanisms between hydrology, morphodynamics, and vegetation dynamics.

Background and aims The study aimed to assess the effects of the potential replacement of dominant native plants by co-occurring exotic invasives on organic matter degradation in a riparian area. The questions were: i) Is there a relationship between the litter breakdown rate and species origin? ii) Is the chemical composition more relevant than species origin? iii) Does species origin influence the associated saprophagous invertebrate community? Methods Within the riparian zone of the Garonne River (France), we conducted a litterbag experiment using pairs of native and exotic species selected in the stages of the successional gradient. Results Native and exotic species did not differ in their primary chemical composition and breakdown rate. Litter breakdown rates were primarily driven by the C:N ratio with no influence of species origin. The abundance and diversity of saprophagous invertebrates of exotic and native species did not differ, but the composition did. Only one plant litter was completely degraded much earlier than expected (next litterfall) while remnant litter were still present at this time for the other species. Conclusions The replacement of a dominant native by an exotic invasive plant species does not generally result in noticeable changes in the litter breakdown rate, which mainly depends on litter composition. Litter from quickly degrading exotic species may cause a resource discontinuity in invaded areas stabilized by river regulation.

QuestionsThe high competitiveness of exotic invasive species has often been demonstrated, but usually with respect to native species known to have low competitive ability. Considering five exotic and five native riparian species with close characteristics regarding competitive ability, habitat and growth form, we addressed the following questions: (i) do the selected invasive plants produce more biomass than the selected native dominants under competitive pressure; and (ii) are the selected invasive species better competitors than the selected native dominants? LocationCommon garden experiment at the Henri Gaussen Botanical Garden, Toulouse, France. Methods We selected five native dominant species and five exotic invasive species co-occurring along a riparian successional gradient of the middle Garonne River (SW France). Young plants of each species were planted in pots in ten intra- and 17 inter-specific combinations in conditions of high water and nutrient availability. To simulate the effects of hydrological disturbance during earlier growth stages, a partial cutting of plants was applied 6 weeks after planting. We measured above-ground and below-ground biomass of individuals of each species after 6 mo of growth. ResultsThere were large disparities among species performances, regardless of whether the species were exotic or native. The exotic species produced more above-ground and below-ground biomass than the natives species for 73% of the selected species pairs. The exotic species had higher competitive ability than the native species, mainly related to the high competitive effect of I. glandulifera. The two species with the highest biomass production and competitive ability were invasive exotics, whereas the two species with the lowest were dominant natives. Conclusions Our results predict that competition among young individuals could play a major role for the invasion success of the studied exotic species in European riparian areas.

Background/Question/Methods Temperate riparian corridors include ones of the most species-rich plant communities. Aside species turnover due to natural dynamics, the biodiversity of these ecosystems is increasingly altered due to direct human pressure and recent global environmental changes. However, mid-term surveys carried out at the ecosystem scale in order to demonstrate such overall changes in community structure remain scarce. In 1989, 1999 and 2009, we exhaustively investigated the flora along the riparian and aquatic corridor of the 350 km long Adour River, SW France. We analyzed species turnover rates, morphotypes, ecological strategic groups and species geographical status (natives versus introduced) to resume major changes in upstream-to-downstream gradients expressed by the 2,000 species identified. Results/Conclusions Surprisingly, total species richness remained comparable though time, suggesting a compositional stability of the riparian corridor flora. However, high species turnover rates depicted strong changes due to riparian species replacement by external, ruderal plants. The longitudinal trends of most biological indicators were consistent among dates, indicating the robustness of the ecological response of each functional compartment within the regional community. However, we observed overall significant quantitative shifts over time among spatial trends, and subtle local changes, both in relation to major mid-term environmental changes. Whilst riparian and aquatic stress-tolerant and competitive species tended to decrease in proportion, the contribution of ruderal and non-native species –including invasive ones– significantly increased. This can be explained by the alterations of riparian corridor physical permeability and of natural disturbance regime and by modifications in land use. These changes are also in agreement with independent predictions we made at the regional scale from climate change scenarios. This was more specifically the case for the increase in non-native species occurrence. Addressing the concept of ecological resilience, we discuss the possible consequences of the settlement of such a novel biodiversity on temperate river system functioning. Possibly, newcomers could sustain riparian ecological functions and structure as far as a minimum natural disturbance is preserved. However, low beta diversity due to community homogeneization would have consequences on successional dynamics.