Autochthonous versus allochthonous organic matter in recent soil c accumulation along a floodplain biogeomorphic gradient: an exploratory study

The mechanisms controlling soil succession in floodplains remain much less studied than in uplands due to the complexity that flooddriven erosion and sedimentation bring into soil development processes. The amount of organic matter and C generally grows with soil ageing and is controlled by multiple and interacting allogenic and autogenic factors, but to what extent the production of organic matter by in situ vegetation contributes to soil formation in floodplains remains unknown. The objective of this work was to explore the importance of autochthonous organic matter versus allochthonous organic matter in organic C accumulation of floodplain forest soils along a vegetation succession and hydrogeomorphic connectivity gradient. Physicochemical analyses of sediment collected after one single flood event in a large Mediterranean floodplain (Middle Ebro, a 9th order regulated river reach in NE Spain) were used to estimate the proportion of organic C found in the topsoil (first 10 cm) samples of young (50 yr) floodplain forests that had an allochthonous (i.e., % of organic C deposited by floods) or autochthonous (i.e., % of organic C produced in situ by vegetation) source. Results of this exploratory study showed that the accumulation of autochthonous organic C in the floodplain topsoil only occurred in floodplain forests older than 50 year-old, but even then, it was more than six-fold less abundant than that with an allochthonous origin. Moreover, a linear mixed effect model showed that, although autochthonous organic C accumulation was mainly explained by the forest structure, a small proportion of it was also controlled by an allogenic factor, the groundwater table depth. Then, groundwater table depth variations could be partly controlling autochthonous organic matter production and decomposition in this Mediterranean floodplain. Although flow regulation and embankment has dramatically limited the hydrogeomorphic dynamics of the river, allogenic overbank sedimentation during flood events still controls floodplain soil succession and organic C accumulation in the floodplain

Autochthonous Versus Allochthonous Organic Matter in Recent Soil C Accumulation Along a Floodplain Biogeomorphic Gradient: An Exploratory Study

The mechanisms controlling soil succession in floodplains remain much less studied than in uplands due to the complexity that flooddriven erosion and sedimentation bring into soil development processes. The amount of organic matter and C generally grows with soil ageing and is controlled by multiple and interacting allogenic and autogenic factors, but to what extent the production of organic matter by in situ vegetation contributes to soil formation in floodplains remains unknown. The objective of this work was to explore the importance of autochthonous organic matter versus allochthonous organic matter in organic C accumulation of floodplain forest soils along a vegetation succession and hydrogeomorphic connectivity gradient. Physicochemical analyses of sediment collected after one single flood event in a large Mediterranean floodplain (Middle Ebro, a 9th order regulated river reach in NE Spain) were used to estimate the proportion of organic C found in the topsoil (first 10 cm) samples of young (<25 yr), mature (25-50 yr) and old (>50 yr) floodplain forests that had an allochthonous (i.e., % of organic C deposited by floods) or autochthonous (i.e., % of organic C produced in situ by vegetation) source. Results of this exploratory study showed that the accumulation of autochthonous organic C in the floodplain topsoil only occurred in floodplain forests older than 50 year-old, but even then, it was more than six-fold less abundant than that with an allochthonous origin. Moreover, a linear mixed effect model showed that, although autochthonous organic C accumulation was mainly explained by the forest structure, a small proportion of it was also controlled by an allogenic factor, the groundwater table depth. Then, groundwater table depth variations could be partly controlling autochthonous organic matter production and decomposition in this Mediterranean floodplain. Although flow regulation and embankment has dramatically limited the hydrogeomorphic dynamics of the river, allogenic overbank sedimentation during flood events still controls floodplain soil succession and organic C accumulation in the floodplain

Species composition and plant traits: Characterization of the biogeomorphological succession within contrasting river corridors

[EN] 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 per mutational 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.French National Centre of Scientific Research; French Ministry of Ecology, Environment, Sustainable Development and PlanningTabacchi, E.; González, E.; Corenblit, D.; Garófano-Gómez, V.; Planty-Tabacchi, A.; Steiger, J. (2019). Species composition and plant traits: Characterization of the biogeomorphological succession within contrasting river corridors. River Research and Applications. 35(8):1228-1240. https://doi.org/10.1002/rra.35111228124035

Utilisation de la photogrammétrie pour l’étude de la végétation riveraine

International audienceThis study presents the results of methodological tests in aerial photogrammetry with the objective of a diachronic survey of riparian vegetation in three dimensions, i.e. its spatial distribution and height. The study is undertaken at two complementary scales. The first one corresponds to a 10 km reach of the Allier river floodplain. At this scale, aerial photographs were taken from a small airplane. The second level of analysis corresponds to a wooded point bar located within the 10 km reach. At this scale, the photographs were taken from an unmanned aerial vehicle (UAV). Canopy height models (CHM) are produced at two scales of analysis. The accuracy of the CHMs is then analysed by comparison with vegetation height measurements collected during a filed campaign. The results show significant agreement for the models derived from two approaches, with an error of one meter at the floodplain scale and few centimetres at the bar scale. This variation in error is mainly due to different photographic resolutions. The two approaches appeared to be complementary. The first one is adequate for mapping vegetation structure at the floodplain scale, the second for mapping vegetation in smaller areas but at a higher resolution.Cette étude présente les résultats de tests méthodologiques réalisés en photogrammétrie aérienne avec pour objectif le suivi diachronique de la végétation riveraine dans ses trois dimensions, c’est-à-dire sa distribution spatiale et sa hauteur. L’étude est menée à deux échelles d’analyse. La première échelle est celle d’un tronçon longitudinal de 10 km de la plaine alluviale de la rivière Allier. À cette échelle de travail, un avion a été utilisé pour les prises de vue aérienne. La deuxième échelle d’analyse est celle d’un banc boisé, localisé dans la zone de 10 km. À cette échelle, la couverture photographique a été faite à l’aide d’un drone. Pour les deux objets d’étude des modèles de hauteur du couvert végétal (MHC) sont produits. La précision des MHC est ensuite vérifiée à partir de la mesure de la hauteur d’arbres sur le terrain. Les résultats révèlent une très bonne précision des modèles pour les deux approches, de l’ordre du mètre à l’échelle du tronçon de 10 km et de quelques centimètres à l’échelle du banc. Cette inégalité s’explique surtout par des résolutions photographiques différentes pour les deux missions. Il ressort de ce travail que les deux approches sont complémentaires. La première permet de cartographier la structure de la végétation à l’échelle de la plaine alluviale, la deuxième permet une cartographie de la végétation à plus haute résolution mais seulement à une échelle plus réduite

Une analyse écologique orientée vers des processus d’un système riverain dynamique: la rivière Allier aval (France)

International audienceRiparian ecosystems are highly dynamic ecosystems subjected to hydrogeomorphological processes. Their geomorphological, sedimentological and hydrological heterogeneity makes them one of the richest ecosystems in terms of species diversity. In addition, riparian zones also provide numerous ecosystem functions and services to society. In the European context, the lower river Allier (France) is one of the last remaining rivers with laterally dynamic sections. Its historical evaluation has shown repeated river bed displacements during the last century. Spatio-temporal processes on a highly mobile river section of the lower river Allier have been studied analysing the mosaic of vegetation types and successional phases as well as physical habitat parameters. Understanding riparian ecosystem functioning and evolution in natural or nearly natural systems is essential for river restoration practices in highly degraded rivers. This understanding may lead to the establishment of better sustainable river rehabilitation targets that consider societal needs and natural processes.Les écosystèmes riverains sont des écosystèmes dynamiques soumis aux processus hydrogéomorphologiques avec une forte hétérogénéité géomorphologique, sédimentologique et hydrologique et de nombreux services écosystémiques rendus à la société. Ils font partie des écosystèmes les plus riches en termes de diversité des espèces. Dans le contexte européen, la rivière Allier aval est une des dernières rivières avec des sections à forte mobilité latérale. L'analyse historique a démontré cette forte mobilité latérale des chenaux durant le siècle dernier. Des processus spatio-temporels d'une section de rivière très mobile ont été étudiés à travers l'analyse de la mosaïque des types de végétation, les phases de succession, ainsi que les paramètres physiques des habitats. Comprendre le fonctionnement et l'évolution des écosystèmes riverains dans des systèmes naturels ou proche de l'état naturel est essentiel pour la restauration des cours d'eau très dégradées, afin de permettre la mise en place de stratégies de réhabilitions durables qui considèrent les besoins de la société et les processus naturels

Succession processes of a dynamic riparian ecosystem: the lower Allier River (France)

Riparian ecosystems are highly dynamic and are subject to hydrogeomorphological processes. In Europe, the lower Allier River (France) is one of the last remaining meandering rivers with laterally dynamic sections. For this study, we developed an original method to analyse the vegetation assemblages and succession processes. The analysis includes both spatial and temporal evolution in order to evaluate the shifting paradigm of habitat mosaic at the study site over a 60-year period. On the long-term, floods with low recurrence intervals tend to be important for small scale habitat changes, whereas major flood events lead to major habitat changes. Flood events did not change the overall habitat composition of the active tract but their spatial distribution. The lack of major hydrogeomorphological disturbance observed during the last two decades, however, led to a higher stability of floodplain ecotopes

Bringing the margin to the focus: 10 challenges for riparian vegetation science and management

[EN] Riparian zones are the paragon of transitional ecosystems, providing critical habitat and ecosystem services that are especially threatened by global change. Following consultation with experts, 10 key challenges were identified to be addressed for riparian vegetation science and management improvement: (1) Create a distinct scientific community by establishing stronger bridges between disciplines; (2) Make riparian vegetation more visible and appreciated in society and policies; (3) Improve knowledge regarding biodiversity¿ ecosystem functioning links; (4) Manage spatial scale and context-based issues; (5) Improve knowledge on social dimensions of riparian vegetation; (6) Anticipate responses to emergent issues and future trajectories; (7) Enhance tools to quantify and prioritize ecosystem services; (8) Improve numerical modeling and simulation tools; (9) Calibrate methods and increase data availability for better indicators and monitoring practices and transferability; and (10) Undertake scientific validation of best management practices. These challenges are discussed and critiqued here, to guide future research into riparian vegetation.COST Action CONVERGES, Grant/Award Number: CA16208; Horizon 2020 Framework Programme of the European Union; Portuguese Foundation for Science and Technology, Grant/Award Number: 2020/03356/CEECIND;PTDC/ASP-SIL/28593/2017;UIDB/00239/2020; CSIC: PTIECOBIODIVRodríguez-González, PM.; Abraham, E.; Aguiar, F.; Andreoli, A.; Bale Entiene, L.; Berisha, N.; Bernez, I.... (2022). Bringing the margin to the focus: 10 challenges for riparian vegetation science and management. Wiley Interdisciplinary Reviews Water. 9(5):1-14. https://doi.org/10.1002/wat2.16041149

Réponse du peuplier noir (Populus nigra L.) aux contraintes hydrogéomorphologiques : une expérimentation ex situ semi contrôlée

International audienceBased on the hypothesis of an eco-evolutionary feedback between woody riparian species and fluvial geomorphology, a semi-controlled ex situ experiment has been planned to quantify key response functional traits (morphological and biomechanical) of Populus nigra L. cuttings to simulated hydrogeomorphological constraints, as well as to dissociate the specific responses to them. The constraints tested are sediment burial and drag force exerted by floods. The characteristics of the experiment are presented as well as the experimental design. The hypothesis of a positive niche construction by P. nigra is tested to a certain degree. The results from this research will improve our understanding of riparian ecosystem functioning and specifically of the role of this key woody pioneer species within active floodplain rivers.Basé sur l’hypothèse d’existence d'une rétroaction éco-évolutive entre les espèces végétales riveraines ligneuses et la géomorphologie fluviale, une expérimentation ex situ en conditions semi-contrôlées a été planifiée afin de quantifier les traits de réponses fonctionnels (morphologique et biomécanique) de boutures de Populus nigra L. soumises à des contraintes hydrogéomorphologiques simulées : l’enfouissement sédimentaire, la force de traînée, et la combinaison des deux contraintes. Les questions scientifiques et le protocole expérimental sont présentés ici. L’hypothèse d’une construction de niche positive de P. nigra est testée à un certain degré. Les résultats de cette recherche contribueront à améliorer notre compréhension du fonctionnement des écosystèmes riverains et plus particulièrement du rôle de cette espèce ligneuse pionnière clé au sein de la bande active des cours d’eau

Bringing the margin to the focus: 10 challenges for riparian vegetation science and management

Riparian zones are the paragon of transitional ecosystems, providing critical habitat and ecosystem services that are especially threatened by global change. Following consultation with experts, 10 key challenges were identified to be addressed for riparian vegetation science and management improvement: (1) Create a distinct scientific community by establishing stronger bridges between disciplines; (2) Make riparian vegetation more visible and appreciated in society and policies; (3) Improve knowledge regarding biodiversity—ecosystem functioning links; (4) Manage spatial scale and context-based issues; (5) Improve knowledge on social dimensions of riparian vegetation; (6) Anticipate responses to emergent issues and future trajectories; (7) Enhance tools to quantify and prioritize ecosystem services; (8) Improve numerical modeling and simulation tools; (9) Calibrate methods and increase data availability for better indicators and monitoring practices and transferability; and (10) Undertake scientific validation of best management practices. These challenges are discussed and critiqued here, to guide future research into riparian vegetation

Fluvial biogeomorphological feedbacks from plant traits to the landscape: Lessons from selected French rivers in line with A.M. Gurnell's influential contribution

International audienceAbstract Research in fluvial biogeomorphology largely aims to promote our understanding of the interactions between riparian vegetation and fluvial morphodynamics within riverine ecosystems. Starting at the end of the last century, Angela M. Gurnell has made a major contribution to fluvial geomorphology by considering water flow and sediment transport in combinaton with riparian and later also aquatic vegetation and thus significantly promoting the fluvial biogeomorphological approach from its early days until today. The objective of the present paper is to present a set of studies and results obtained over the last 20 years mainly by the authors and many collaborators, including Angela M. Gurnell, on a panel of French rivers: the Tech, Garonne, Isère and Allier. In particular, feedback mechanisms between fluvial morphodynamics and riparian vegetation dynamics were investigated directly in the field and by using high‐resolution remote sensor systems at the scale of individual plants, populations, communities and landscapes as well as during semi‐controlled ex‐situ experiments at the scale of individual plants. Collectively, the authors' research conducted over the past 20 years has helped to elucidate some key aspects of the feedback dynamics between the lowest and highest levels of the riparian ecosystem organization. This article reviews and discusses those key aspects. The gradually obtained results have contributed to a better understanding and quantification of feedback between river morphodynamics and vegetation at nested spatiotemporal scales, from plant species traits to the riverine landscape. Furthermore, the biogeomorphological approach advocated for more than 20 years now has clearly facilitated the extension of the discipline of geomorphology to ecology in general and evolutionary ecology in particular and thus contributed to a more integrative vision of earth surface processes