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\begin{document}

\title{Fluvial biogeomorphological feedbacks from plant traits to the
landscape: lessons from French rivers in line with A.M. Gurnell's
influential contribution}



\author[1]{Dov Corenblit}%
\author[2]{Johannes Steiger}%
\affil[1]{Laboratoire ecologie fonctionnelle et environnement}%
\affil[2]{Universite Clermont Auvergne}%


\vspace{-1em}



  
  \date{May 21, 2024}


\begingroup
\let\center\flushleft
\let\endcenter\endflushleft
\maketitle
\endgroup





\selectlanguage{english}
\begin{abstract}
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 last century, Angela M. Gurnell has made a major contribution to
fluvial geomorphology by considering, in addition to water flow and
sediment transport, explicitly riparian, and later also aquatic
vegetation and thus significantly promoted the fluvial
biogeomorphological approach from its beginnings until today. The
objective of the present paper is to present a set of studies and
results obtained over the last twenty years by the authors and many
collaborators, including Angela M. Gurnell, on a panel of French rivers:
Tech, Garonne, Is\selectlanguage{ngerman}ère and Allier Rivers. In particular, feedback
mechanisms between fluvial morphodynamics and riparian vegetation
dynamics were investigated directly in the field and also using high
resolution remote sensing at the scale of individual plants,
populations, communities and landscapes, as well as during
semi-controlled \emph{ex situ} experiments at the scale of individual
plants. Collectively, the authors' research conducted over the past 20
years contributed to elucidate some key aspects of the feedback dynamics
between the lowest and highest levels of the riparian ecosystem
organisation. This article presents and discusses those key aspects. The
gradually obtained results contributed to better understand and quantify
feedbacks 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 twenty years now, has clearly helped to contribute to the
enlargement of the discipline of geomorphology to ecology as well as
evolutionary ecology, and to the development of a more integrative
vision to study earth surface processes.%
\end{abstract}\selectlanguage{ngerman}%



\sloppy


\textbf{Fluvial biogeomorphological feedbacks from plant traits to the
landscape: lessons from French rivers in line with A.M. Gurnell's
influential contribution}

\textbf{Running title: Biogeomorphological feedbacks from plant traits
to the landscape}

Dov Corenblit\textsuperscript{a,b*}, Johannes Steiger\textsuperscript{b}

\textsuperscript{a}Universit\selectlanguage{ngerman}é de Toulouse, CNRS, Laboratoire Écologie
Fonctionnelle et Environnement, 31062 Toulouse, France.

\textsuperscript{b}Université Clermont Auvergne, CNRS, GEOLAB, 63000
Clermont\selectlanguage{english}-Ferrand, France.

\textbf{*Corresponding author:} Dov Corenblit:
jean-francois.corenblit@univ-tlse3.fr

\textbf{Acknowledgments}

The authors first warmly thank Angela M. Gurnell for her invitation to
contribute to this special issue and second are very grateful for the
long and very rewarding collaboration which started 25 years ago at the
University of Birmingham and largely contributed to determine our
professional trajectories. We also thank all co-authors from our papers
we have cited in this contribution for the successful achievement of the
presented biogeomorphological investigations. The authors acknowledge
the French National Research Agency (ANR) that supports the
Project-ANR-21-CE32-000 ``NUMRIP''.

\textbf{Abstract}

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 last century, Angela M. Gurnell has made a major contribution to
fluvial geomorphology by considering, in addition to water flow and
sediment transport, explicitly riparian, and later also aquatic
vegetation and thus significantly promoted the fluvial
biogeomorphological approach from its beginnings until today. The
objective of the present paper is to present a set of studies and
results obtained over the last twenty years by the authors and many
collaborators, including Angela M. Gurnell, on a panel of French rivers:
Tech, Garonne, Isere and Allier Rivers. In particular, feedback
mechanisms between fluvial morphodynamics and riparian vegetation
dynamics were investigated directly in the field and also using high
resolution remote sensing at the scale of individual plants,
populations, communities and landscapes, as well as during
semi-controlled \emph{ex situ} experiments at the scale of individual
plants. Collectively, the authors' research conducted over the past 20
years contributed to elucidate some key aspects of the feedback dynamics
between the lowest and highest levels of the riparian ecosystem
organisation. This article presents and discusses those key aspects. The
gradually obtained results contributed to better understand and quantify
feedbacks 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 twenty years now, has clearly helped to contribute to the
enlargement of the discipline of geomorphology to ecology as well as
evolutionary ecology, and to the development of a more integrative
vision to study earth surface processes.

\textbf{Keywords:} Fluvial biogeomorphology, biogeomorphological
feedback, riparian vegetation, \emph{Populus nigra} , black poplar,
ecosystem engineer, plant traits.

\textbf{Introduction}

In order to fully understand river behaviour and trajectories of change,
Angela M. Gurnell early pointed out the need to study interactions
between the biological and physical compartments of ecosystems, i.e.,
the responses of organisms to the physical environment and their effects
on that physical environment (Gurnell and Gregory, 1984, 1995; Gurnell,
1995). Research in fluvial biogeomorphology largely aims to promote our
understanding of the interactions between riparian vegetation and
fluvial morphodynamics within riparian ecosystems. Angela M. Gurnell has
made a major contribution by considering explicitly vegetation, first
riparian (living vegetation, propagules and large woods) and then
aquatic, in addition to water flow and sediment transport into fluvial
geomorphology and thus to promote the fluvial biogeomorphological
approach from the end of the last century until today (e.g., far from
exhaustive: Gurnell, 1997; Gurnell et al., 2000, 2001, 2004, 2005, 2006,
2012, 2013, 2016a, 2016b, 2018, 2019; Gurnell and Petts, 2002, 2006;
Gurnell, 2014; Gurnell and Grabowski, 2016; Gurnell and Bertoldi, 2020;
Gurnell and Bertoldi, 2022). A bibliographic research on the Web of
Science (all databases) using the combined terms ''riparian vegetation''
+ ''geomorphology'' + ''river*'' showed that out of a total number of
711 articles archived on 11\textsuperscript{th} November 2023, more than
12\% were authored or co-authored by A.M. Gurnell, which represents
86~articles. The second-best score is 33~articles (Fig. 1).

The biogeomorphological approach applied to rivers is necessarily
ambitious because requires skills in geomorphology and plant ecology and
because the modalities of interactions between hydrogeomorphological
components and riparian vegetation are multiple and nested at various
spatiotemporal scales. Thanks to Angela M. Gurnell's influential
research and contribution to fundamental and applied river sciences,
fluvial biogeomorphology is now well situated at the interface between,
geomorphology, ecology and river management (e.g., Gurnell et al.
2016a,b).\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image1/image1}

\end{center}
\end{figure}

\textbf{Figure 1.} Number of peer reviewed journal articles per year
using the combined terms ''riparian vegetation'' + ''geomorphology'' +
''river*'', as archived on Clarivate Web of Knowledge (all databases)
(https://www-webofscience-com.inee.bib.cnrs.fr/). A.M.~Gurnell's first
article using these terms was published in 1997 (see vertical black
arrow) and her name comes out first with more than 12\% of all articles
authored or co-authored. Accessed on 11\textsuperscript{th} November
2023.

The objective of this article is to present a set of studies and results
from the past 20~years obtained by the authors and many collaborators,
including Angela M. Gurnell, on a panel of French rivers: Tech, Garonne,
Is\selectlanguage{ngerman}ère and Allier Rivers. In particular, \emph{feedbackmechanisms}
between fluvial morphodynamics and riparian vegetation dynamics were
investigated directly in the field and used high resolution remote
sensing at the scale of individual plants, populations, communities and
landscapes as well as during semi-controlled \emph{ex situ} experiments
at the scale of individual plants. Collectively, the authors' research
contributed to elucidate some key aspects of feedback dynamics between
the lowest and highest levels of the riparian ecosystem organisation.
This article presents and discusses those key aspects.

\textbf{Feedbacks between vegetation and fluvial geomorphology at nested
spatiotemporal scales}

Riparian ecosystems are among the most dynamic and variable environments
on Earth's surface in terms of structure and function, making them a
target of many ecological restoration projects (Amoros and Petts, 1993;
Naiman and Décamps, 1997; Muñoz-Mas et al., 2017; Gonzalez et al., 2018;
González, et al. 2022; O'Briain et al., 2023). They represent land-water
interface zones where matter and energy are concentrated, transit, are
stored, and transformed. The flows of water, sediment, organic matter --
from dissolved organic matter to large woods --, and seeds that pass
through, both cyclically (hydrological regime) and stochastically
(floods), contribute to maintain a high degree of heterogeneity in the
mosaic of habitats (Townsend, 1989; Gurnell et al., 2005). This
spatiotemporal heterogeneity generates strong ecological gradients and
sometimes exceptional levels of biodiversity, including a variety of
survival and reproduction strategies in a shifting mosaic (Ward, 1989;
Schnitzler et al., 1992; Ward et al., 2002; Stanford et al., 2005;
Bornette et al., 2008).

Hydroecological theoretical models are based on the postulate that the
structure and functioning of aquatic and riparian communities can be
explained by considering the links between life history traits of
species and the geomorphological structures and processes that govern
flows of matter and energy (Vannote et al., 1980; Newbold et al., 1982;
Pickett and White, 1985; Amoros et al., 1987; Junk et al., 1989;
Townsend, 1989; Ward, 1989; Tockner et al., 2000). Models such as the
River continuum concept, Flood pulse and Flow pulse, Nutrient
spiralling, Patch dynamics, were nevertheless developed according to a
type of Physical habitat template approach (\emph{sensu} Southwood,
1977), that is, focusing strictly on the \emph{a priori} unidirectional
effect of the physicochemical heterogeneity on biodiversity and
ecosystem functioning. Previous research on riparian ecosystems has not
sufficiently considered how and to what extent the three spatial
dimensions of riverine ecosystems are controlled by riparian vegetation.

Certain plant species and assemblages have a profound and lasting impact
on the structure and function of riparian ecosystems by modulating
matter and energy fluxes and modifying substrate cohesion (Gurnell et
al., 2012). Riparian plant species (e.g., the black poplar \emph{Populus
nigra} L. in the Northern hemisphere) that significantly affect
hydrogeomorphological processes and fluvial landforms have been
identified as ecosystem engineers (\emph{sensu} Jones et al., 1994),
keystone species (\emph{sensu} Paine, 1966), and, more recently, as
potential niche constructors \emph{sensu} Odling-Smee et al. (2003)
(Edwards et al., 1999; Gurnell and Petts, 2006; Corenblit et al., 2009b;
Francis et al., 2009).

How, and to what extent, the structure and function of riparian
ecosystems are controlled by abiotic-biotic feedbacks across various
spatiotemporal scales has become an important issue in fluvial
biogeomorphology. This question must be addressed from the fine scale of
the individual plant, the microhabitat and single hydrological event to
the coarse scale of plant community mosaic, the fluvial corridor and the
hydrosedimentary regime. In particular, the mechanisms by which engineer
plants respond to, and shape, the mosaic of riparian habitats are not
fully understood. Some evolutionary adaptations of plants in response to
the hydrogeomorphological disturbance regime such as floating seeds,
synchronization of reproduction with the flood regime (Junk et al.,
1986; Guilloy-Froget et al., 2002), vegetative reproduction in addition
to sexual reproduction strategies (Barsoum, 2002; Francis and Gurnell,
2006; Moggridge and Gurnell, 2009), and biomechanical stem flexibility
in order to avoid uprooting during flood events (Karrenberg et al.,
2002; Lytle and Poff, 2004; Bornette et al., 2008), help riparian plants
to survive despite high physical constraints. These adaptations,
combined with phenotypic plasticity, allow engineer plants to
mechanically resist to drag force in the flow and to influence their
riverine environment. By trapping sediments, organic matter, and seeds,
engineer plants can create, build, and maintain riparian habitats that
are beneficial to other plant species, including other engineer plants.

In addition to the role of vegetation assemblages (i.e., community
scale) in shaping the hydrogeomorphic features of riverine ecosystems,
specific life history, morphological and biomechanical traits related to
distinct engineer species also play an important role. In the absence of
large, destructive floods, the short-range actions of plants -- within
their own stands and immediately downstream -- typically result under
adequate conditions in the accumulation of fine sediment, nutrients,
organic matter, which improve habitat quality. In addition, with the
deposition of seeds (e.g., Goodson et al., 2001, 2002; Gurnell et al.,
2004, 2006; Corenblit et al., 2009a, 2016a), the improved habitat
quality, in turn, promotes the establishment and growth of new plant
species, including other engineer species. This process can increase the
likelihood of pioneer woody engineer species such as \emph{P.
nigra}reaching sexual maturity (Corenblit et al., 2014a, 2018).

The synchronized process of fluvial landform construction and plant
establishment/growth/succession can lead to the emergence of
characteristic biogeomorphological landforms such as fluvial islands
that are the result of engineering effects of riparian plants (Gurnell
et al., 2001). These positive abiotic-biotic feedback dynamics which
potentially affect the fluvial style (e.g., island braided
rivers\emph{sensu} Gurnell et al. 2019) are consistent with the action
of ecosystem engineers (\emph{sensu} Jones et al., 1994; Gurnell and
Petts, 2006) and, potentially, niche constructors (\emph{sensu}
Odling-Smee et al., 2003; Corenblit et al., 2015). In turn, the effects
of riparian engineer plants on the hydrogeomorphological components of
riverine ecosystems involve a set of responses in these plants, from the
individual to the community mosaic level and vice versa. This
abiotic-biotic feedback is organized in three stages (Fig. 2):

(1) Initial response of plants to hydrogeomorphological constraints: at
the beginning of growth and succession, plants must respond to a variety
of hydrogeomorphological constraints, including mechanical forces (e.g.,
stream flow, erosion and sediment burial) and physiological stress
factors (e.g., prolonged immersion and drought). These constraints act
as a powerful filter for the colonisation, establishment and growth of
engineer plants in exposed areas on alluvial bars. However, they can
also stimulate morphological and biomechanical responses that increase
plant resistance to these constraints.

(2) Effects of engineer plants on hydrogeomorphological components: the
mechanical resistance allows plants to act passively on their
hydrosedimentary and geomorphological environment. Once established,
engineer plants can modify the hydrogeomorphological components of their
environment in a variety of ways. For example, their roots can increase
substrate cohesion, which can help to stabilize bars and banks and
prevent erosion. Their trunks, branches and leaves can promote trapping
of sediment and organic matter, which can help to build up islands,
floodplains and create new habitats.

(3) Plant response to changes in the hydrogeomorphological environment:
as engineer plants modify their environment, they can create positive
feedback loops that further enhance their impact and growth (Corenblit
et al., 2009a, 2014, 2018) as well as negative feedback loops translated
into the transition toward stabilized and mature stages of the
vegetation succession on raised alluvial surfaces (Bendix and Hupp,
2000; Corenblit et al., 2007; Corenblit et al., 2009a). For example, the
increased stability of river bars and banks and their vertical accretion
provided by engineer plants can reduce erosion and sediment burial,
which can create more favourable conditions for the establishment of
other competitive plants. This can lead to the development of complex
and diverse riverine ecosystems with increased beta diversity at the
landscape scale (Gurnell et al., 2005).

\textbf{Hosted file}

\verb`image2.emf`  available at \url{https://authorea.com/users/701832/articles/688065-fluvial-biogeomorphological-feedbacks-from-plant-traits-to-the-landscape-lessons-from-french-rivers-in-line-with-a-m-gurnell-s-influential-contribution}

\textbf{Figure 2:} Multi-scale theoretical model of the feedback between
riparian vegetation dynamics and river morphodynamics.

\textbf{Individual plant and population level feedbacks: response and
effect traits of \emph{Populus nigra}}

In settings exposed to high tractive forces and sediment erosion or
burial during floods, high temperatures and hydric stress during summer
low flows, such as on alluvial bars, the completion of the life cycle of
certain woody pioneer riparian species such as \emph{P. nigra} requires
a progressive modification of hydrogeomorphological conditions and
habitat, from germination to sexual maturity. The latter is reached when
the habitat and its topographic surface attain higher, more stable
levels, characterized by the deposition and accumulation of finer
sediments, nutrients, and organic matter, and less exposed to the risk
of destruction by floods.

Individual plants may form together at the colonization stage more or
less compact and monospecific woody populations which are very relevant
for the study of biogeomorphological feedback from the individual to
population levels. In riparian contexts in the European Northern
hemisphere, \emph{P.~nigra} has become a model species for field studies
but also \emph{ex situ} experiments (Figs. 3 and 4).

\textbf{3.1. Responses of aerial and underground parts of riparian
plants to hydrogeomorphological \emph{in situ} constraints}

When individuals of \emph{P. nigra} establish on alluvial bars in
dynamic rivers, they can strongly affect fluvial geomorphology by
trapping fine sediments on alluvial bars during floods, and thus build
new biogeomorphological landforms (Gurnell, 2014; Hortobágyi et al.,
2018a). The engineer effect of \emph{P.~nigra} on alluvial bars depends
on its architecture, biomass, and exposure to mechanical stress. The
species has a strong phenotypic plasticity that allows to adjust its
physiological, morphological and biomechanical traits according to local
hydrogeomorphological conditions. The quantification of the
morphological and biomechanical variation of response traits
within\emph{P. nigra} populations which depend on exposure to mechanical
constraints related to flow is essential to better understand why, to
what extent, and where the black poplar is able to modulate
geomorphology and thus gain in its survival, growth, and reproduction.

In an empirical study conducted on the Allier River, Hortobágyi et al.
(2017) quantified the variation in response traits of
\emph{P.~nigra}within a population that was two years old, based on
three levels of exposure to mechanical constraints. In highly exposed
areas, at the head of an alluvial bar, saplings developed response
traits that allow for greater resistance, including a reduced size
(avoidance strategy; see Bornette and Puijalon, 2011; Puijalon et al.,
2011), a flexible and inclined stem, and also a more robust root system
(tolerance strategy; see Bornette and Puijalon, 2011; Puijalon et al.,
2011). In a highly exposed context, avoidance morphological and
biomechanical responses led to a decrease in drag during floods. These
responses that increase the resistance of young individuals nevertheless
limit the engineer effect, i.e., their ability to trap fine sediment and
nutrients in relation with increased surface biomass and roughness. This
limitation of the engineer effect of black poplar in the most exposed
areas, at the head of the bar, is therefore translated by a limitation
of sediment trapping and niche construction at the very early stage of
establishment.

This observation suggests the existence of a functional compromise in
the expression of traits between resistance to floods (i.e., improvement
of survival) and niche construction (i.e., improvement of habitat
conditions under biotic control and therefore improvement of resource
acquisition and growth). In less exposed areas of alluvial bars -- in
the middle part and downstream of the bars as well as within near
secondary channels -- young poplars developed a wider, longer, less
flexible, and less inclined stem, as well as larger leaves. This
expresses a prioritization of resource acquisition and competition
abilities and at the same time the possibility for niche construction
during ordinary floods. The total length (underground part + aerial
part) of the plants is maximal at the tail of the bar with a notable
effect on the trapping of fine sediments.

Complementary observations of individual roots system on the same
alluvial bars showed that plant burial by fine sediment systematically
stimulates the production of adventitious roots and aerial biomass,
which in turn increases sediment trapping (Ding, 2014). This was
interpreted as a positive biogeomorphological feedback of sedimentary
accretion and plant growth. The construction of fluvial landforms by
black poplar cohorts seems to correspond to a positive niche
construction strategy leading to the progressive reduction of mechanical
constraints and to a modification of the habitat that positively
influences the survival, growth, and ultimately the sexual reproduction
of poplars (Corenblit et al., 2016b, 2018, 2020a,b). Biogeomorphological
processes occurring at a small scale caused by different morphological
and biomechanical plant traits therefore may have an influence on the
processes of fluvial landform construction at a larger spatial scale
(Merritt, 2013; Corenblit et al., 2015; O'Hare et al., 2016; Diehl et
al., 2017).

\textbf{3.2. Responses of the aerial and underground parts to
hydrogeomorphological constraints in experimental \emph{ex
situ}conditions}

As observed in the field, specific morphological and biomechanical
response traits of avoidance and tolerance (resistance traits)
allow\emph{P.~nigra} to effectively colonize alluvial bars and thus to
modulate hydrogeomorphological processes (Fig. 3). Quantitative
experimental studies of the modulation of riparian plant morphology and
biomechanics induced by hydrogeomorphological constraints permit to
better understand under semi-controlled conditions how plants can
colonize a physically highly exposed and naturally disturbed fluvial
landform. These modulations of individual plants potentially translate
the development of a resistance or avoidance strategy, or both, in these
plants, which ultimately can induce changes in fluvial morphodynamics at
the landscape scale.

Experimental research should particularly provide a quantitative answer
to two key questions such as: How does drag force caused by stream flow
affect the growth of \emph{P.~nigra} ? How do successive sediment
burials and the accumulation of sediment influence the growth
of\emph{P.~nigra} ? The response of \emph{P.~nigra} to these two
categories of mechanical constraint determines both the efficiency of
the growth of individuals to sexual maturity and the evolution of
fluvial landforms. Garófano-Gómez et al. (2018) conducted an \emph{ex
situ} experiment in semi-controlled conditions at the GEOLAB laboratory
in Clermont-Ferrand, France, to dissociate the effects of
hydrogeomorphological factors acting on anchorage and plant growth. The
experiments targeted the morphological and biomechanical responses of
young \emph{P.~nigra}individuals. One hundred and twenty-eight
\emph{P.~nigra} cuttings of Jean Pourtet variety were planted in one
hundred and twenty-eight flexible bags of 0.6 m\textsuperscript{3}
filled with Allier River sand with an automatic irrigation system
adapted to a filtering substrate. This system included a microporous
ring for automatic irrigation for each bag. The young \emph{P.~nigra}
individuals were subjected to two types of constraints during a full
growing season (from March to November), i.e., three treatments in
total: (1) no constraint (this population without any treatment served
as a control); (2) sediment burial with a calibrated grain texture; (3)
mechanical constraint reproducing a calibrated hydraulic constraint
repeated at a certain frequency and intensity; (4) the two constraints
combined on a population of cuttings.

The growth of the aerial and root parts was precisely quantified by
measuring numerous morphological and biomechanical traits of the
underground and aerial parts: morphological and architectural soft
traits (e.g., maximum height, cumulative length of stems, growth rate,
stem diameter, root branching rate, root depth, ratio of elongation and
biomass between the aerial and underground parts, number of structural
roots, ratio of biomass between fine roots and structural roots, root
growth orientation, specific leaf area); anatomical and biomechanical
traits (flexural stiffness, stem breakage resistance, and cell size).
Hard traits were also measured at the end of the experiment, including
resistance to pulling out with a winch test. The measurement of
morphological and architectural parameters of the aerial part was
carried out continuously on the experimental site during growth. The
measurements of the root part were performed in the laboratory at the
end of the growing season. 2D and 3D modelling techniques, including
multi-image photogrammetry, were used to automatically reconstruct and
measure plant architecture.

Results indicated a significant effect of the treatments on the size,
position, and type of roots. Sediment burial (SB) induced the production
of numerous fine adventitious roots, and the application of stem bending
(DF) induced the production of more solid basal structural roots.
Morphological responses were more pronounced in the sediment burial
treatment than in the stem bending treatment. The control (C) showed
reduced aerial biomass compared to the treatments, but a root-to-shoot
biomass ratio in favour of the roots. The combination of bending and
sediment burial (DFSB) produced a positive surface growth response. DF
showed a significantly higher biomass of basal roots than SB and DFSB,
while SB had a higher biomass of lateral roots.

These findings are consistent and complementary with the \emph{in
situ}observations (Ding, 2014; Hortobágyi et al., 2017; Corenblit et
al., 2020b). They provide evidence of the high morphological and
biomechanical plasticity of the black poplar, which ensures its
establishment in various conditions of exposure to shear stress and
sediment burial and thus its capacity in high density to collectively
affect sediment transport.

\textbf{Hosted file}

\verb`image3.emf`  available at \url{https://authorea.com/users/701832/articles/688065-fluvial-biogeomorphological-feedbacks-from-plant-traits-to-the-landscape-lessons-from-french-rivers-in-line-with-a-m-gurnell-s-influential-contribution}

\textbf{Figure 3:} Observed feedback at the individual level. (a)
PopTrait \emph{ex situ} experiment in Clermont-Ferrand for
understanding\emph{P. nigra} responses to biomechanical constraints; (b)
sediment tail downstream of a multi-stemmed black poplar on an alluvial
bar of the Tech River. The morphology of the sediment tail is correlated
with morphological traits of the black poplar individual. Photographs:
Virginia Garófano-Gómez (a) and Aurélien Chabanon (b).

\textbf{3.3. Effect of \emph{Populus nigra} at the scale of the alluvial
bar}

Corenblit et al. (2016b) established on alluvial bars of the Garonne
River: (i) the modality of variation of annual sedimentation and
topographic aggradation rates within \emph{P.~nigra} cohorts of
different ages, and (ii) the physiological response (growth rate,
production of adventitious roots, suckers, and surface stem density)
associated with hydrogeomorphological modifications under\emph{P.~nigra}
control. Within the strongly channelized river section of the Garonne
River downstream of Toulouse studied (Steiger et al., 2000, 2001),
populations of black poplars establish on alluvial bars and
predominantly modulate the dynamics of fine sediments and fluvial
landforms. Following the cessation of massive gravel extraction in the
main channel during the 1980s, successive dense cohorts
of\emph{P.~nigra} have successfully colonized the entire alluvial bars
since mid of the 1990s. These very dense cohorts, which settle from
upstream to downstream on point bars in successive meander loops, induce
very important sedimentary accretion and landform stabilization.

Over a period of thirty years, the geomorphological characteristics of
wooded alluvial bars have evolved in close relationship with the
dynamics of cohort establishment and plant growth. The geomorphological
characteristics of these bars (topography, thickness, and texture of
sediments) are statistically linked to the spatial distribution and
intensity of establishment by the successive cohorts of \emph{P.~nigra}
. In return, geomorphological changes strongly influence the physiognomy
of cohorts (density, diameter, size, and architecture of individuals) as
well as the floristic structure of the herbaceous layer, with the
transition from open units with high plant biodiversity of opportunistic
annual and biannual species to closed units under mature black poplar
cover dominated in the understory strata by \emph{Rubus} , \emph{Urtica}
,\emph{Convovulus} , and \emph{Galium} spp. This reciprocal causal
relationship between geomorphological dynamics and plant dynamics is
suggested by a strong correlation between key geomorphological and
ecological variables measured \emph{in situ} and from aerial photographs
taken by the IGN between 2000 and 2010 (Corenblit et al., 2016b). In
addition, this study found that recruitment and establishment dynamics
of \emph{P.~nigra} are controlled by the engineer effects of this
species through a facilitation effect linked to the obstacle and
protection effect offered to new seedlings and saplings by upstream
cohorts.

The construction of the biogeomorphological units ``wooded point bars''
is the result of a positive biogeomorphological feedback of
stabilization and closure of the environment linked to the massive
trapping of sand by dense \emph{P.~nigra} cohorts and the vertical
growth and to a lesser extend to lateral expansion of vegetation on the
landforms under construction. \emph{In situ} root excavations
demonstrated the systematic formation of numerous adventitious roots
that explore the new layers of sediment (Corenblit et al., 2018) -- with
a tropism linked to the higher organic level as observed on the Allier
River by Ding (2014). At the same time the dense network of adventitious
roots combined to the structural roots increase the resistance of
individuals to uprooting. In the current context, the recurrent massive
trapping of fine sediments seems to improve the anchoring, growth and
resistance performance of \emph{P.~nigra} populations to floods with a
return period of twenty years.

No significant vegetation destruction was observed on the Garonne
following vicennial floods since the 1990s. These biogeomorphological
units are functional from an ecological point of view because at the
population scale \emph{P.~nigra} is able (i) to strengthen its own
inherent ability to resist hydrogeomorphological disturbances and thus
is able to grow to sexual maturity in favourable conditions, and (ii) to
strengthen its resilience capacity resulting from successful sexual
reproduction through the facilitation effect offered by the upstream
more mature cohorts.

However, the very strong spatial expansion by \emph{P.~nigra} , in a
context of channelization that no longer allows the channel to
sufficiently erode its banks, has led, after thirty years, to a very
imminent situation of blocking of the biogeomorphological succession in
the post-pioneer and mature stages with reducing possibility of
rejuvenation and regeneration of open alluvial surfaces (Corenblit et
al., 2016b). It was suggested that only a very exceptional flood with a
return period of about 100 years may be able to reset the succession
today.

The observations of \emph{P.~nigra} population and landform dynamics on
the Garonne River are concordant with the concept of biogeomorphological
life cycle (BLC) proposed by Corenblit et al. (2014). The BLC model
applies at the population level and aligns with the concept of positive
niche construction \emph{sensu} Odling-Smee et al. (2003). Natural
selection acting on riparian trees over their life cycle would, over the
long term, lead to an increased probability of survival and sexual
reproduction in individuals exhibiting suitable trait variations
(Corenblit et al., 2009b, 2020b). The fluvial environment, characterized
by strong physical constraints, exerts a predominant selective pressure
on riparian trees. However, the influence of riparian trees on the
hydrogeomorphological components of their environment can modulate this
pressure throughout the life cycle. Indeed, the life history of poplars
in a riparian context consists of a succession of characteristic
developmental phases where physical components of the
hydrogeomorphological environment and biological characteristics of
riparian trees interact (Corenblit et al., 2014a). The establishment of
a population of \emph{P.~nigra} on exposed sediment surfaces is not an
exclusively unidirectional process in which seedlings solely undergo
physical selection pressures. The feedback dynamics
between\emph{P.~nigra} trait values and population physiognomy and
geomorphology during its life cycle lead to the emergence of
biogeomorphic entities in the river corridor, such as vegetated fluvial
islands, bars and wooded floodplains. The BLC describes the fundamental
phases of propagule dispersal, recruitment, and establishment up to the
sexual maturity \emph{of P.~nigra} , outlining the hierarchy of
interactions and feedbacks between biotic and abiotic components during
these phases. This model elucidates how the modification of the
hydrogeomorphological environment by the black poplar enhances its
fitness (determined by its survival, growth, and sexual reproduction),
resulting in a positive niche construction.

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\verb`image4.emf`  available at \url{https://authorea.com/users/701832/articles/688065-fluvial-biogeomorphological-feedbacks-from-plant-traits-to-the-landscape-lessons-from-french-rivers-in-line-with-a-m-gurnell-s-influential-contribution}

\textbf{Figure 4:} Observed feedback at the population level. (a)
Established dense pioneer black poplar cohort with a very high growth
rate on a gravel bar of the Garonne River; (b) same cohort in winter,
with well-distinguishable massive sediment trapping in the vegetation
unit. Photographs: Dov Corenblit.

\textbf{Community-level feedbacks: responses and effects of biodiversity
on the functioning of riverine ecosystems}

At the community level, riparian vegetation can decisively modulate
fluvial dynamics, and in return, adjusts its structure (floristic
composition, physiognomy and mean trait value) to new environmental
configurations evolving under biotic control according to characteristic
resilient succession patterns. Research carried out on the Tech and
Allier Rivers permitted to identify, at the community level, a number of
key elements of the abiotic-biotic feedback which control the structure
and functioning of riverine ecosystems in different and more or less
artificialized hydrogeomorphological contexts. As suggested by Tilman
(1996), Loreau et al. (2001; 2021) and Hooper et al. (2005), our
investigations showed that the diversity of response and effect traits
of engineer organisms influences the functioning and stability or
resilience of the ecosystem (Fig. 5).

\textbf{4.1. Function of herbaceous plants: facilitation and resilience}

Fieldwork carried out on the Tech River, South France, that is, in a
Mediterranean-mountainous torrential context, has highlighted the
functional role of herbaceous formations on alluvial bars (Corenblit et
al., 2009a). According to the hydrosedimentary regime, herbaceous units
may be able to induce a facilitation effect for the recruitment and
survival of herbaceous and woody seedlings and promote the resilience of
plant biodiversity on exposed alluvial bars through habitat
stabilization and seed trapping. In the case of the Tech River,
vegetation starting at the herbaceous stage on highly exposed gravel
surface, had a very significant impact on surface sediment dynamics and
the seed bank, i.e., the set of seeds contained in the substrate, which
may contribute to the ecological and biogeomorphological resilience of
the river system.

Dense herbaceous structures colonizing the most exposed areas of
alluvial bars protected fine substrate very effectively from erosion
during floods. Once established, living herbaceous structures, even
senescent or dead, contributed recurrently, every year, to the
deposition of new layers of sandy-silty sediments (on average 3 cm
yr\textsuperscript{-1}). These deposits occurred mainly at the edge of
the main channel and secondary channels during quasi-annual morphogenic
torrential floods where the erosion process is normally dominant on
coarse-grained surfaces. The structure of the seed bank had been
described in detail under a binocular microscope in the laboratory LEFE,
Toulouse, from numerous sediment samples taken in the deposition zones.
The deposition of very numerous seeds (sometimes up to 1,000 seeds and
50 species per 10 cm\textsuperscript{3} sample) accompanied this process
of stabilization and retention of fine sediments at the top surface of
the gravel bars near the water resource. Erosion (on average 20 cm
yr\textsuperscript{-1}) measured in the adjacent areas with low plant
cover suggested that the herbaceous cover locally represented a critical
parameter which controlled the threshold of mobilization of coarse
sediment, surface deposition, and the stabilization of fine sediment.

The action of herbaceous structures on sediment dynamics had so far been
neglected in high energy fluvial contexts. However, it turned out to
play an important ecological role in promoting the resilience of plant
biodiversity on alluvial bars that are regularly swept by floods and
strongly constrained in summer by possible severe temperatures and
hydric stress. This process is similar to the development of a
regeneration niche under strong biotic control (Lavorel and Chesson,
1995; Grubb, 1997; Silvertown, 2004). It seems appropriate to establish
here the relationship with the facilitation process in constrained
environments as described by Bertness and Callaway (1994) and Bruno et
al. (2003).

The facilitation process self-sustains itself as long as a large
torrential flood does not destabilize the entire vegetation-sediment
complex. In some locations on alluvial bars, a few native herbaceous
species (e.g., \emph{Polygonum lapathifolium} , \emph{Echinocloa
crus-galli} ) and exotic species (e.g., \emph{Cyperus eragrostis}
,\emph{Bidens frondosa} , and \emph{Paspalum paspalodes} ) with high
abundance alone fulfilled the function of stabilizing and retaining fine
sediments and seeds. However, in most cases, this function was ensured
at the community scale by a multitude of opportunistic herbaceous
species with low abundance that form a heterogeneous, dense, and complex
mat structure due to its architectural and biomechanical
characteristics. This latter observation suggests that the combination
of different morphological groups and biomechanical abilities of
herbaceous plants, or even life histories, within the same plant unit
positively influenced the resistance and resilience of the entire
vegetation structure to hydraulic constraints and the effectiveness of
sediment and seed retention.

The areas subjected to the effects of herbaceous formations also
represented a favourable support for the initiation of the niche
construction process by pioneer woody vegetation as well as the
associated herbaceous plant succession. These biogeomorphological units
with an herbaceous cover and an associated layer of fine sediments
offered more favourable habitat conditions for the summer survival of
seedlings of \emph{P.~nigra} , \emph{Salix} spp., and \emph{Alnus
glutinosa} . Herbaceous units on the edge of the channel showed a
tendency to evolve in less than three years -- at least in the absence
of a too large flood -- towards dense pioneer shrub formations dominated
on the Tech by the three pioneer riparian species mentioned above. These
mixed herbaceous/shrub pioneer units (between 2 and 3 m in average
height three years after their recruitment) have a very high surface
roughness and remained perfectly in place during the flash floods via a
very high biomechanical resistance of the woody structures. In addition,
they showed a strong resilience of the herbaceous floristic structure
constituting the lower layer, i.e., the local seed bank and new seed
inputs during floods permitting recovery.

\textbf{4.2. Function of woody plants: biogeomorphological succession
and niche construction}

After their establishment on alluvial bars, following the critical
recruitment phase sometimes facilitated by herbaceous formations,
pioneer woody riparian plants act as ecosystem engineers by trapping
large amounts of fine sediments. Studies conducted on the Tech, Garonne
and Allier Rivers have shown that pioneer woody riparian vegetation is a
predominant factor in the stabilization and construction of habitat and
plant succession in dynamic riparian environments.

For example, on the Tech River, Corenblit et al. (2009a) observed a
strong correlation between net annual sedimentation rates and the
intercepted plant biovolume by floods. In the Allier River fluvial
corridor, biotic-abiotic feedbacks also occurred between pioneer woody
vegetation that is strongly influenced by hydrogeomorphological
constraints (sediment transport and deposition, shear stress,
hydrological variability) and fluvial geomorphology which is itself
modulated by established vegetation. Hortobágyi et al. (2018a) studied
16 alluvial bars on the Allier River in the Natural National Reserve of
the Val d'Allier (RNNVA) to assess the ability of three pioneer riparian
engineer species (\emph{P.~nigra} , \emph{S.~purpurea} and\emph{S.~alba}
) to establish and act as ecosystem engineers by trapping sediments and
building/stabilizing fluvial forms on alluvial bars. The objective was
to identify empirically the preferential establishment window
(recruitment and establishment zones) of these three engineer species,
as well as their biogeomorphological feedback window (zones where these
species act significantly on geomorphology). The results showed that the
establishment and feedback windows of the three species varied
significantly along the longitudinal profile of alluvial bars, namely
that relating to the upstream-downstream exposure to
hydrogeomorphological flows, as well as along the transverse gradient,
i.e., the gradient of hydrological connectivity from the main water
channel to the floodplain. At the bar scale, the evolution of fluvial
landforms was controlled differentially according to the morphological
and biomechanical attributes of each of the three species. These latter
exhibited an inherent (genotypical) variation modulated according to age
and degree of exposure on the bar (phenotypic plasticity).

In the current hydrogeomorphological context of the Allier
River,\emph{P.~nigra} is the most abundant pioneer woody species and is
the dominant engineer species. It affects the geomorphological dynamics
at the scale of the entire alluvial bars, except in their upstream part
where individuals remain small due to the direct exposure to very high
hydrodynamic forces. \emph{S.~purpurea} established and acted as an
ecosystem engineer in the most exposed locations of the alluvial bars,
i.e., upstream and near the main channel. \emph{S.~alba} established in
the downstream areas of the bars, near the secondary channels, and
affected the geomorphology within patches of mixed vegetation
with\emph{P.~nigra} and \emph{S.~purpurea} . The establishment and
feedback windows of the three engineer species also overlapped in
places, especially in the intermediate part of the bars. Thus, the total
establishment and feedback window of the three species corresponds to a
larger spatial extent than that defined individually by each species on
the alluvial bars. This study highlighted the role of the diversity of
functional response and effect traits of riparian engineer species and
the extent of niche construction by these three species along
hydrogeomorphological gradients.

On the Tech River, Corenblit et al. (2009a) observed peaks in specific
diversity and abundance in the seed bank within post-pioneer woody units
that are less directly exposed to hydrogeomorphological flows than
alluvial bars. This was explained by a large amount of floating seeds
that were deposited there during quasi-annual flood peaks following the
low water level, low flow velocities, and high plant roughness.
Floristic distance analyses based on species presence-absences at the
surface and in the seed bank indicated that, unlike the areas
immediately adjacent to the water channels on alluvial bars, a very
small percentage of this diversity contained in the seed bank was
expressed at the surface. The resistant seeds remained in part in
dormancy in the substrate. This result suggests that post-pioneer
biogeomorphological units forming a riparian forest partially
disconnected from hydrogeomorphological perturbations (post-pioneer
fluvial islands and floodplain) potentially fulfil a function as a
reservoir of plant diversity at the scale of the corridor. The storage,
or accumulation of seeds, could contribute to ensure the ecological
resilience of plants following low-frequency, high-amplitude flow
disturbances that cause massive destocking and maximum seed dispersal
within the fluvial corridor.

In a study of the seed bank on the Allier River, Corenblit et al.
(2016a) demonstrated that pioneer woody individuals of
\emph{P.~nigra}and \emph{Salix} spp. that establish on the alluvial
coarse-grained bars of this high-energy river promote the retention of
fine sediment during quasi-annual floods, within their stands and just
downstream in the form of sediment tails, i.e. obstacle marks. The
retention of fine sediment is accompanied by the deposition of a large
number of seeds transported by the flow, i.e., by hydrochory. Here, the
hypothesis was validated that, on exposed alluvial bars, pioneer
riparian trees, either isolated or growing in dense groups, form
obstacles to flow and predominantly control the deposition of seeds
within sediment tails composed of fine sands and silts depositing on a
gravel surface. The structure of the seed bank was described in detail
at the binocular microscope in the laboratory and from a germination
test performed in a greenhouse. It was compared between samples
collected in the sediment tails and control samples collected on the
bare surfaces of the alluvial bars. At the surface (at 2 cm depth), the
abundance of seeds and species richness were significantly higher in
areas subjected to the tree obstacle (number of seeds N: 693 ± 391;
number of species S: 17 ± 3) than on bare surfaces (N: 334 ± 371; S: 13
± 5). Surface and sub-surface samples (at 20 cm depth) were also
significantly different, with the sub-surface samples being almost
devoid of seeds (N: 514 ± 413, S: 15 ± 5 and N: 3 ± 6; S: 1 ± 2).

These results suggest a biogeomorphological feedback loop between
sediment dynamics controlled by riparian trees, the retention of
associated seeds, and the growth of herbaceous formations from these
seeds. This feedback loop is complex and involves, in a first step, the
process of hydrochory and sediment retention by trees; in a second step,
the growth of herbaceous vegetation on sediment tails and the local
dispersion of seeds by barochory; and in a final step, the contribution
of herbaceous formations to the stabilization and construction of the
sediment tail during quasi-annual floods. Corenblit et al. (2016b) have
demonstrated that this feedback loop promotes the resistance and
resilience of herbaceous plant diversity on the bars of the Allier
River.

Altogether, the results of these \emph{in situ} studies on the Tech,
Garonne and Allier Rivers, targeting vegetation on the surface and in
the seed bank, highlight a characteristic and recurring correlation
between geomorphological adjustments and growth, physiognomy and plant
composition on the surface and in the seed bank. The floristic structure
in the seed bank and on the surface, the plant physiognomy and the
distribution of strategies along the longitudinal and transversal
gradients of disturbance do not result from a unidirectional effect of
the hydrogeomorphological compartment, but rather from a feedback loop
with the hydrogeomorphological components under biotic control.

The process of sediment accretion within pioneer plant structures leads
to a transition from a phase dominated by hydrogeomorphological
processes, in which seeds and seedlings are unidirectionally subjected
to physical constraints (submersion, erosion, sedimentary burial, water
table fluctuations), to a phase where abiotic-biotic interactions are
multiple, intense, and bidirectional. These pioneer units dominated by
opportunistic strategies evolve into more robust post-pioneer tree units
in just a few years (between 4 and 5 years), reach an average height of
7 m and develop on topographically raised surfaces due to the rapid
accretion of sand and silt. This phase, which is generally transient, is
characterized by strong landscape heterogeneity and high plant diversity
at the scale of the bar. In just a few years and the absence of
destruction caused by an exceptional flood or lateral erosion due to
channel displacement, abiotic-biotic feedbacks drive biogeomorphological
units towards a more stable and ecologically mature phase, which is more
resistant to hydrogeomorphological disturbance events, physiological
stress and invasion by exotic species (Corenblit et al., 2014b). Within
these units, partially disconnected from frequent flood events, sediment
deposits become negligible for quasi-annual floods. At this stage,
competitive strategies take over in terms of floristic structure and
plant diversity decreases sharply due to the exclusion of many
opportunistic herbaceous species. Within the cycle of fluvial landform
construction and plant succession, this stage represents a
biogeomorphological ``attractor''. It coincides with the formation of
post-pioneer and mature fluvial islands (\emph{sensu} Gurnell et al.,
2018a, 2002) and alluvial floodplains supporting softwood species and,
possibly at the end of the cycle, hardwood species.

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\verb`image5.emf`  available at \url{https://authorea.com/users/701832/articles/688065-fluvial-biogeomorphological-feedbacks-from-plant-traits-to-the-landscape-lessons-from-french-rivers-in-line-with-a-m-gurnell-s-influential-contribution}

\textbf{Figure 5:} Observed feedback at the community level. (a)
Herbaceous and arboreous communities in the active tract of the Tech
River and (b) the Allier River. Photographs: Dov Corenblit (a) and
Franck Vautier (b).

\textbf{Landscape-level feedbacks: biological \emph{versus} physical
forcing in contrasted hydrogeomorphological contexts}

At the scale of the fluvial landscape, our research was carried out
primarily on the Tech, Allier and Isère Rivers, using two and
three-dimensional spatiotemporal analyses with GIS, stereo and
multi-image photogrammetry and LiDAR. These three rivers differ in their
morphodynamics and the level of anthropogenic pressure. The Tech is a
Mediterranean torrential river with a very irregular
hydrogeomorphological regime. Anthropogenic pressure has led to channel
bed vertical incision, lateral stabilization of its banks and therefore
to a certain loss of dynamics despite the virulence of flash floods. The
Allier is a high-energy river that is, in its lower section (RNNVA)
still relatively free to move laterally in a wide meander belt. The
Isère River is a high-energy river that has was laterally confined with
huge embankments. This resulted in a channel mostly straightened and
migrating alternate bars progressively stabilizing and heavily colonized
by vegetation.

Spatial analyses of the feedback between colonization/vegetation
succession dynamics and landscape/fluvial style dynamics on these three
rivers have made it possible to highlight different types of responses
and effects of riparian vegetation at the landscape scale and thus to
estimate the impact of local feedbacks (at the scale of traits and
population) on the organization of the habitat mosaic over few decades
(Fig. 6).

\textbf{5.1. The Tech River landscape: dominance of biotic forcing in an
extremely irregular torrent context}

A spatially explicit retrospective GIS analysis of the landscape
dynamics of the Tech River, from a situation of complete abiotic
reinitialization at the scale of the fluvial corridor -- following the
catastrophic flood of October 1940 -- made it possible to specify the
intensity and spatial modalities of responses and effects of vegetation
on fluvial morphodynamics in a hydrologically irregular torrential
Mediterranean context with moderate anthropogenic pressure (Corenblit et
al., 2010).

The hydrological configuration (no major floods for ten years) that
followed immediately after the devastating flood of 1940 allowed to
clearly distinguish two modalities of plant succession specifically in
relation to the effect of close location to the water resource on the
growth rate and density of plant cover. Distance and relative altitude
from main and secondary channels acted in this Mediterranean context as
key parameters controlling vegetation growth rates. Indeed, (i) close to
channels and the water resource, in low-lying areas, plant
growth/succession is strongly stimulated. Dense tree formations develop
in less than ten years; the stimulation and success of the recruitment
of pioneer herbaceous and woody species that were measured at the local
scale on alluvial bars in the highly exposed zones (Corenblit et al.,
2009a) corroborate and clarify the processes underlying these dynamics.
(ii) Far from wet channels, plant growth/succession is slower and
follows a more progressive successional trajectory through a shrubby and
sparse tree stage that lasts about thirty years. A dendrochronological
analysis of adult \emph{P.~nigra} individuals growing close to and far
from the active channel showed a contrast in the growth rate of the
rings (on average 1.3 cm of annual growth at the edge of the channel
against 0.4 cm far from it). During this period, avulsions favoured by
the dense tree band at the edge of the channel led to the rejuvenation
of succession over a significant area of the corridor (about 40\%). When
the floodplain was covered with a dense forest after thirty years, the
system switched to a new domain of stability because the rejuvenation
process was strongly constrained at the scale of the entire corridor,
even for morphogenic floods with a return period of between twenty and
thirty years. The active tract (active channel and bare alluvial bars)
then contracted and the channel incised, potentially irreversibly in the
absence of an exceptional flood with a frequency \textgreater{}100
years.

It appears that the relationship between the speed, quality, and
location of vegetation closure and the frequency/amplitude of floods
determines the conditions of geomorphological equilibrium at the scale
of the corridor. The combined effect of the two modalities of plant
succession favours the progression of the riparian forest within the
entire fluvial corridor (from the outside to the inside and vice versa)
and the contraction and confinement of the active tract. The spatial
configuration which results from biotic-abiotic feedbacks after thirty
years on the Tech represents a biogeomorphological attractor state. The
shift of the geomorphological equilibrium system towards stabilization
is explained by the fact that, although plant growth is slower on the
floodplain, a dense tree formation eventually develops at the medium
term (30 years on the Tech), thus reducing the probability of
destruction by floods. The areas near the channel are frequently
flooded, but the growth of vegetation well adapted to hydraulic
constraints is stimulated there. The very rapid and efficient
development of pioneer vegetation in the backwater along the active
channel maintains the continuity of the coupling with sediment accretion
processes and thus minimizes the width of the active band while the
floodplain becomes progressively more densely wooded. This type of
phenomenon is involved when a river evolves from an unstable braiding
style to a more simplified and entrenched stable style.

\textbf{5.2. Isère River landscape: dominance of biological forcing
in-between dikes in a highly anthropized context}

The intricate interplay between hydrogeomorphological processes and
riparian vegetation can play a pivotal role in shaping the fluvial
landscapes within highly-confined rivers. Corenblit et al. (2020a)
studied the reciprocal coupling between river morphodynamics and
vegetation succession in the Isère River, France, where human
intervention in the form of lateral confinement with embankments and
straightening had a transformative impact on riverine landscape dynamics
(see also Vautier, 2000; Vautier et al., 2002).

Using GIS analysis of historical aerial photographs spanning nearly five
decades, from 1948 to 1996, the authors showed that a remarkable
biogeomorphological transformation unfolded as the once mobile
transverse gravel bars began a gradual but significant colonization by a
diverse array of vegetation types. This situation presented an
opportunity to understand the temporal adjustments between fluvial
landforms and vegetation succession stages, ultimately culminating in
the establishment of mature riparian forests on stabilized alternate
bars. This biogeomorphological trajectory of change was a direct
consequence of feedbacks between hydraulics, sediment transport, and
vegetation dynamics in a highly impacted river reach. The mosaic of
vegetated bars that emerged was an outcome of the strong bioconstruction
and biostabilization effects exerted by pioneer vegetation on bars of
differing ages, sizes, and mobility within the riverine landscape.

Field measurements undertaken in 1996 provided tangible evidence of a
robust positive feedback loop between sediment dynamics and the ongoing
riparian vegetation succession (Vautier, 2000). In the complex balance
between physical and biotic adjustments, the vegetated bars evolved as
emergent biogeomorphological entities specifically resulting from the
biogeomorphological feedbacks of sediment accretion, topographic raise,
and vegetation growth and succession. Co-inertia analysis (CoIA), a type
of ordination statistical technique used for exploring and understanding
bi-directional relationships between two or more data tables or
matrices, permitted the quantification of the intricate relationships. A
highly significant statistical association between geomorphological and
vegetation variables (as exemplified by an RV value of 0.41, p
\textless{} 0.001) was found. This association explained a staggering
95\% of the variability along just one axis, demonstrating the existence
of an exceedingly robust feedback loop between geomorphological changes
and vegetation succession.

To appreciate the full significance of these findings, we must turn our
gaze to the broader context of the theoretical model of fluvial
biogeomorphological succession based on positive feedback loops between
fluvial morphodynamics and vegetation succession (FBS model; Corenblit
et al., 2007). The empirical observations from the Isère and Tech Rivers
both allowed the validation of the FBS model, suggesting that this model
is applicable to natural but also channelized rivers.

\textbf{5.3. Allier River landscape: dominance of physical forcing in a
context with a more regular disturbance regime}

The biogeomorphological dynamics of the Allier River at the landscape
scale contrast with those of the Tech and Isère Rivers, where vegetation
act as an important factor of stabilisation in the active tract. On the
Allier River, hydrogeomorphological processes still seem to be dominant
within the active tract over the effect of vegetation on the structuring
of the habitat mosaic, due to a more regular high disturbance regime and
strong lateral bank erosion dynamics ensured by the translations of the
main channel during bankfull floods (Garófano-Gómez et al., 2017).
However, the hydrogeomorphological conditions in the Allier River are
presently undergoing changes.

At the scale of the fluvial corridor, Hortobágyi et al. (2018b) showed
that the vegetation classes with the lowest height are concentrated
hundred meters away from the main channel and at higher elevations,
about 3-4~m. These results do not apparently match with the theoretical
FBS model (Corenblit et al., 2007), in which the age of the vegetation
increases with increasing distance from the main channel and relative
elevation. On the Allier River, the development of low shrub vegetation
hundred meters away from the main channel is the result of a secondary
``dry'' succession dominated by \emph{Prunus} spp. on floodplain
surfaces today disconnected from frequent floods that are also subject
to grazing. The fact that mature large-sized tree vegetation is located
near the main channel suggests that the preferential areas for
rejuvenation of the plant succession and associated fluvial forms are
located along the point bars in the meander concavities.

The tallest vegetation classes are located further upstream on the bars,
near the main channel. This is explained by the fact that the channel is
migrating very actively and progressively towards the opposite bank and
the bars are migrating downstream. Surprisingly, the difference between
the average elevation of bare and vegetated bar surfaces is not
significant. The vegetation cover is concentrated around 0.5 m above the
low water level, while the bare substrate is concentrated below 0.5 m
and at around 1 m of elevation. The occurrence of these open bare
surfaces at higher elevations relative to the low water level seems to
reflect an unsuccessful recruitment of pioneer riparian woody
vegetation, including \emph{P.~nigra} , on these surfaces too much
disconnected from the surface and alluvial water pools.

At the bar scale, on the Allier River, the biogeomorphological
succession model was rather supported by these results. The classes with
highest vegetation reflecting the mature stage within the vegetation
succession were indeed located at a higher altitude. However, the
average elevation of the vegetation height classes of 5-10 m and
\textgreater{} 10 m do not vary and therefore do not have a very
specific topographic signature. The maximum sediment trapping effect is
probably reached when the height of the vegetation is around 5-10~m, or
about 4-8 years after recruitment. These observations are in good
agreement with the observations on the Garonne River for
\emph{P.~nigra}cohorts. The most important physiognomic changes in
\emph{P.~nigra}(loss of the multi-stemmed shrub architecture in favour
of a single large trunk) occur during this rather early growth phase.
From 15 m in height, the density of \emph{P.~nigra} stems decreases
drastically and their diameter increases very significantly. These
physiognomic changes result from the reduction of the exposure of
individual plants to hydrogeomorphological constraints, partly thanks to
biotically controlled sediment accretion and the protective effect of
individuals against each other. The maximum occurrence of tall arboreous
structures at the scale of the bar is currently located on the Allier
from 0.5 to 1 m above the low water level. The observed elevation of the
classe with the highest vegetation on the Allier River is consistent
with observations on other high-energy rivers, such as the Tagliamento
River in Italy (Bertoldi et al., 2011) and the Santa Maria River in the
United States (Kui et al., 2014; Manners et al., 2015). In these rivers,
the highest topographic levels are associated with intermediate canopy
height classes (1-5 m and 5-10 m). However, on the Allier River, the
highest topographic levels are associated with canopy heights of 5-10 m
and \textgreater{} 10 m. A possible explanation of this distributional
pattern of vegetation height classes is that the rate of sediment
accumulation by the pioneer vegetation varies depending on the type of
river, sediment load, and hydrogeomorphological regime. The Allier River
has a substantial sediment load, which could result in a significant
sediment trapping effect caused by pioneer vegetation in the form of
successive parallel lines along the water channel in the Allier River.
This phenomenon could potentially account for the increased elevation in
the 5-10 m and \textgreater{} 10 m height classes.

However, Garófano-Gómez et al. (2017) analysed the vegetation succession
based on eight sets of aerial images of the riparian corridor spanning
from 1967 to 2014 and its relation with the Allier River flow regime.
The results indicated that between 1967 and 2005, the study area
exhibited a shifting habitat pattern. After 2005, there was a
significant decrease in progression and retrogression processes, leading
to increased stability, with no more channel migration and patches no
longer experiencing either progression or retrogression. This study
highlights a distinct turning point in succession processes at the
beginning of the 21st century with a contraction of the active tract
resulting from a decrease in flood frequency and magnitude. The
biogeomorphological interaction between vegetation and geomorphology is
likely to adapt in the near future, with the increasing influence of
plants on landform stabilization and development.

\textbf{Hosted file}

\verb`image6.emf`  available at \url{https://authorea.com/users/701832/articles/688065-fluvial-biogeomorphological-feedbacks-from-plant-traits-to-the-landscape-lessons-from-french-rivers-in-line-with-a-m-gurnell-s-influential-contribution}

\textbf{Figure 6:} Observed feedback at the community mosaic level. (a)
Community mosaic of the Allier River with a predominant physical
signature and (b) the Isère River with a predominant biotic signature on
the alternate bars in the active tract.

\textbf{Conclusion}

The here presented results from various biogeomorphological
investigations on French rivers underpin the important role played by
riparian vegetation in such riverine ecosystems of the temperate zone in
modulating hydrogeomorphological processes and fluvial landforms, as
early advocated by Angela M. Gurnell. Her essential suggestions, having
led at the time to some controversies, are now widely accepted and, as
shown here, refined at different spatiotemporal scales as well as
different ecosystem levels, from the individual/micro-form to the
community mosaic/fluvial style.

Today, the fluvial biogeomorphological lessons learned from the French
rivers, in addition to other studies in France, Europe and overseas,
open new perspectives in the comprehensive study of riverine systems and
their management considering feedback mechanisms between water, sediment
and riparian plants. The here presented results, often obtained in close
collaboration with Angela M. Gurnell, point to the need to better
quantify, understand and model feedbacks between river morphodynamics
and vegetation at nested spatiotemporal scales as well as its resilience
aptitudes to disturbances. Indeed, the resilience of rivers may largely
dependent on feedbacks between hydrogeomorphology and the dynamics of
engineer populations, such as black poplar. Its comprehension, based on
the biogeomorphological approach advocated, is needed to better predict
future river evolutionary trajectories in the light of human pressures
as well as restoration measures, climate and global change.

In addition, the different methods and approaches employed here and
observations made, clearly helped, over the last twenty years, to
contribute to the enlargement of the discipline of geomorphology to
ecology and also to evolutionary ecology, endorsed by the
biogeomorphological approach.

\textbf{Conflict of Interest Statement}

No conflict of interest.

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\textbf{Figure legends}

\textbf{Figure 1.} Number of peer reviewed journal articles per year
using the combined terms ''riparian vegetation'' + ''geomorphology'' +
''river*'', as archived on Clarivate Web of Knowledge (all databases)
(https://www-webofscience-com.inee.bib.cnrs.fr/). A.M.~Gurnell's first
article using these terms was published in 1997 (see vertical black
arrow) and her name comes out first with more than 12\% of all articles
authored or co-authored. Accessed on 11\textsuperscript{th} November
2023.

\textbf{Figure 2:} Multi-scale theoretical model of the feedback between
riparian vegetation dynamics and river morphodynamics.

\textbf{Figure 3:} Observed feedback at the individual level. (a)
PopTrait \emph{ex situ} experiment in Clermont-Ferrand for
understanding\emph{P. nigra} responses to biomechanical constraints; (b)
sediment tail downstream of a multi-stemmed black poplar on an alluvial
bar of the Tech River. The morphology of the sediment tail is correlated
with morphological traits of the black poplar individual. Photographs:
Virginia Garófano-Gómez (a) and Aurélien Chabanon (b).

\textbf{Figure 4:} Observed feedback at the population level. (a)
Established dense pioneer black poplar cohort with a very high growth
rate on a gravel bar of the Garonne River; (b) same cohort in winter,
with well-distinguishable massive sediment trapping in the vegetation
unit. Photographs: Dov Corenblit.

\textbf{Figure 5:} Observed feedback at the community level. (a)
Herbaceous and arboreous communities in the active tract of the Tech
River and (b) the Allier River. Photographs: Dov Corenblit (a) and
Franck Vautier (b).

\textbf{Figure 6:} Observed feedback at the community mosaic level. (a)
Community mosaic of the Allier River with a predominant physical
signature and (b) the Isère River with a predominant biotic signature on
the alternate bars in the active tract.

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