Gabrielle Rudi
email: gabrielle.rudi@gmail.com

Jean-Stéphane Bailly

Gilles Belaud

Cécile Dagès

Philippe Lagacherie

Fabrice Vinatier

Cécile Dages

Multifunctionality of agricultural channel vegetation : A review based on community functional parameters and properties to support ecosystem function modeling

Keywords: Ditch, irrigation channel, plant community properties, plant community functional parameters, ecosystem function, management practices

ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

Networks of agricultural channels (irrigation channels and drainage ditches) play a considerable role in the abiotic and biotic fluxes in agricultural landscapes. Irrigation channels are built to conduct water from a source to its place of use [START_REF] López-Pomares | Irrigation canals in a semi-arid agricultural landscape surrounded by wetlands: Their role as a habitat for birds during the breeding season[END_REF]. Ditches are built to transfer excess water from agricultural plots [START_REF] Levavasseur | Simulating the effects of spatial configurations of agricultural ditch drainage networks on surface runoff from agricultural catchments[END_REF]. They also play an antierosive role in places where high water velocities occur, such as in Mediterranean landscapes with steep slopes [START_REF] Ramos | Analysis of design criteria for vineyard terraces in the Mediterranean area of north east Spain[END_REF][START_REF] Roose | Stratégies traditionnelles de conservation de l'eau et des sols dans le bassin méditerranéen : classification en vue d'un usage renouvellé[END_REF], and play a drainage role [START_REF] Carluer | Assessment and modelling of the influence of man-made networks on the hydrology of a small watershed: implications for fast flow components, water quality and landscape management[END_REF][START_REF] Krause | Modelling the impacts of land-use and drainage density on the water balance of a lowland-floodplain landscape in northeast Germany[END_REF] in places where the water table has to be lowered to enable cultivation. Their existence results from human decision making, and sometimes their establishment took place hundreds of years ago [START_REF] Berger | Les fossés bordiers historiques et l'histoire agraire rhodanienne[END_REF]. They are organized into networks; i.e., they are connected to each other hydraulically as well as ecologically. Their density can reach 200 m/ha in Mediterranean landscapes [START_REF] Levavasseur | Spatial modeling of man-made drainage density of agricultural landscapes[END_REF]. Agricultural channels cover a limited surface area of landscapes. For example, ditch networks in Mediterranean areas never represent more than 4 % of cultivated areas, based on data from [START_REF] Levavasseur | Spatial modeling of man-made drainage density of agricultural landscapes[END_REF]. Most of these agricultural channels are uncoated (earth channels), and especially the small ones (no more than 1.5 meters in width) that are located upstream of the network for drainage ditches and downstream of the network for irrigation channels. These small agricultural channels therefore can host flora and fauna species [START_REF] Herzon | Agricultural drainage ditches, their biological importance and functioning[END_REF]. They also are generally subject to intermittent flows associated with rainfall frequency and intensity, or irrigation.

The flora encountered in channels are not exclusive, i.e. they are encountered in other places of the catchment, although in some contexts, we can find plants of conservation interest [START_REF] Mountford | Aquatic plant diversity in arable ditches: scoping study[END_REF]. Agricultural networks are important habitats for floral species [START_REF] Herzon | Agricultural drainage ditches, their biological importance and functioning[END_REF] that were located in natural freshwater bodies and floodplains in the past [START_REF] Armitage | Ditch communities: a major contributor to floodplain biodiversity[END_REF] because they often represent the only sources of open water in agricultural landscapes. Channels and ditches generally have a high plant diversity [START_REF] Meier | Plant diversity in a water-meadow landscape: the role of irrigation ditches[END_REF]. Plants living in agricultural channels are generally terrestrial or semiaquatic, composed mainly of forbs and graminoids [START_REF] Bouldin | Vegetative and structural characteristics of agricultural drainages in the Mississippi Delta landscapes[END_REF][START_REF] Clarke | Conserving freshwater biodiversity: The value, status and management of high quality ditch systems[END_REF]Levavasseur et al., 2014a;[START_REF] Maheu-Giroux | Landscape ecology of Phragmites australis invasion in networks of linear wetlands[END_REF][START_REF] Rudi | Using geomorphological variables to predict the spatial distribution of plant species in agricultural drainage networks[END_REF][START_REF] Szymura | Grasses (Poaceae) in riparian vegetation of watercourses in agriculture landscape[END_REF], and sometimes strictly aquatic [START_REF] Clarke | Conserving freshwater biodiversity: The value, status and management of high quality ditch systems[END_REF][START_REF] Janse | A model of ditch vegetation in relation to eutrophication[END_REF][START_REF] Milsom | Dynamics and management of plant communities in ditches bordering arable fenland in eastern England[END_REF], according to the topographic, climatic, and pedological contexts, as well as location in the channels' network.

The vegetation must be managed regularly by users (farmers and irrigation managers) to restore its hydraulic capacity for transport [START_REF] Dollinger | Managing ditches for agroecological engineering of landscape. A review[END_REF].

In Mediterranean agricultural areas, one or two practices such as mowing, burning, clearing, and chemical weeding are conducted every year (Levavasseur et al., 2014a;[START_REF] Sánchez Martín | Effects of vegetation management on plant diversity in traditional irrigation systems[END_REF]. Dredging is carried out every 5 to 10 years to remove the deposited sediments. These frequencies can vary according to pedoclimatic context [START_REF] Milsom | Dynamics and management of plant communities in ditches bordering arable fenland in eastern England[END_REF][START_REF] Twisk | The nature value of the ditch vegetation in peat areas in relation to farm management[END_REF]. These management practices, according to their typology, frequency, and location in the channel, can alter the ditch bed material [START_REF] Dollinger | Managing ditches for agroecological engineering of landscape. A review[END_REF] and vegetation. In the short term, they directly affect the shape of the plant cover [START_REF] Dollinger | Impact of maintenance operations on the seasonal evolution of ditch properties and functions[END_REF]Levavasseur et al., 2014a), but in the medium and long term, management practices have the additional effect of modifying the composition of plant communities and their dynamics [START_REF] Manhoudt | Management regimes and farming practices enhancing plant species richness on ditch banks[END_REF][START_REF] Milsom | Dynamics and management of plant communities in ditches bordering arable fenland in eastern England[END_REF][START_REF] Leng | Spatiotemporal variation of plant diversity on ditch banks under different management regimes[END_REF][START_REF] Rudi | Modélisation et analyse de services éco-hydrauliques des réseaux de canaux et fossés des agrosystèmes méditerranéens[END_REF][START_REF] Sánchez Martín | Effects of vegetation management on plant diversity in traditional irrigation systems[END_REF][START_REF] Twisk | The nature value of the ditch vegetation in peat areas in relation to farm management[END_REF].

Nowadays, studies have highlighted the multifunctionality of agricultural channels' vegetation [START_REF] Dollinger | Managing ditches for agroecological engineering of landscape. A review[END_REF][START_REF] Herzon | Agricultural drainage ditches, their biological importance and functioning[END_REF][START_REF] Pierce | Managing Artificially Drained Low-Gradient Agricultural Headwaters for Enhanced Ecosystem Functions[END_REF], which is a significant provider of functions such as water flow regulation and bank strengthening as well as sediment, agricultural pollutant and plant propagule retention (a propagule is any structure for dispersal or reproduction, i.e., seeds, spores, and rhizomes). This multifunctionality of vegetation living in agricultural channels explains why these infrastructures are often referred to as "agroecological". Relying properly on these ecosystem functions could decrease intervention time and costs for farmers by reducing soil loss from farmed areas, decreasing the frequency of bank failure and reducing weed spreading while limiting overflows. It could also help decrease the concentrations of sediments and agricultural pollutants in receiving ecosystems, which could result in lower water treatment costs for society. These functions are mainly influenced by physical (and to a lesser extent chemical/biological) interactions between vegetation and water flow fields in the channel. As a consequence, the dynamics of the functions result mainly from (i) the spatiotemporal dynamics of vegetation in the agricultural channels and (ii) the spatiotemporal components of water flow fields, which especially depend on the frequency and intensity of rainfalls and irrigations, as well as the topological, pedological and morphological characteristics of the channel [START_REF] Dollinger | Managing ditches for agroecological engineering of landscape. A review[END_REF]. It seems reasonable to think that vegetation would be a powerful lever with which to optimize channels' ecosystem functions because (i) vegetation is already managed by farmers regularly and (ii) the management of vegetation produces few negative impacts on the ecosystem if properly conducted.

For studying and modeling channels' ecosystem functions, a relevant spatial scale has to be chosen [START_REF] Nepf | Hydrodynamics of vegetated channels[END_REF]. When estimating channels' functions at the landscape scale, the resolution at which the channels need to be studied is the reach scale [START_REF] Rudi | Modélisation et analyse de services éco-hydrauliques des réseaux de canaux et fossés des agrosystèmes méditerranéens[END_REF] (in this paper, the term "reach" refers to a channel section with homogeneous properties, as defined in [START_REF] Lagacherie | An indicator approach for describing the spatial variability of artificial stream networks with regard to herbicide pollution in cultivated watersheds[END_REF]), which generally corresponds to a description of vegetation at the communitylevel. Community functional parameters are plant functional traits aggregated at the level of the community (for example, mean height), and a community property is a feature estimated at the community level (for example, density) [START_REF] Violle | Let the concept of trait be functional![END_REF]. Better consideration of these plant community functional parameters and properties in the modeling of ecosystems functioning should enhance our ability to predict the spatiotemporal dynamics of the ecosystem functions that they provide [START_REF] Diaz | Functional diversity revealed by removal experiments[END_REF][START_REF] Lavorel | Pathways to bridge the biophysical realism gap in ecosystem services mapping approaches[END_REF][START_REF] Mori | Response diversity determines the resilience of ecosystems to environmental change[END_REF], and it would provide us with information on the parameters and properties that we need to measure and monitor in the field. Additionally, it would allow us to know which type of vegetation is desirable in channels, in terms of parameters/properties rather than species, which would provide more generic rules to managers and stakeholders for managing ecosystems functions [START_REF] Rudi | Modélisation et analyse de services éco-hydrauliques des réseaux de canaux et fossés des agrosystèmes méditerranéens[END_REF]. Although these types of approaches are currently encouraged, several challenges remain, as the community functional parameters and properties responsible for the dynamics of ecosystem functions have not been clearly identified. Moreover, if management practices (anthropogenic drivers) represent a lever with which to manage the dynamics of these parameters and properties, and, in turn, ecosystem functions [START_REF] Dollinger | Variability of glyphosate and diuron sorption capacities of ditch beds determined using new indicator-based methods[END_REF][START_REF] Dollinger | Impact of maintenance operations on the seasonal evolution of ditch properties and functions[END_REF][START_REF] Vinatier | The Use of Photogrammetry to Construct Time Series of Vegetation Permeability to Water and Seed Transport in Agricultural Waterways[END_REF], it would be useful to collect data on the effects of these practices on plant community parameters and properties.

The first objective of the review was to determine the plant community functional parameters and community properties involved in three main functions of agricultural channels : (i) water flow regulation, (ii) retention, and (iii) bank strengthening. We focused on parameters and properties that have a clearly established relationship with the studied functions that has been documented in research papers. The second objective was to collect data on the effect of management practices on plant community functional parameters and properties. The review first presents an overview of ecosystem functions provided by vegetation in agricultural channels. Then, it presents an up-to-date overview of plant community functional parameters and properties generally associated with the studied ecosystem functions. In the last section, the review presents what we currently know about the effect of management practices on the dynamics of plant community functional parameters and community properties in agricultural channels. The review focuses on uncoated and therefore vegetated channels, located in temperate climatic contexts, with a focus on Mediterranean agricultural areas.

Material and Methods : Methodology

The review focuses on vegetated channels that can generally be found in agricultural areas under temperate climatic contexts. Their width is generally between 50 to 120 cm and depth between 30 and 80 cm in Mediterranean agricultural landscapes [START_REF] Levavasseur | Simulating the effects of spatial configurations of agricultural ditch drainage networks on surface runoff from agricultural catchments[END_REF]. As underlined in the introduction, they can be intermittent or permanent, according to the climatic context and location according to the network. Regarding irrigation channels, the discharges transported to the plots are generally between 30 to 150 L.s -1 downstrean of the network [START_REF] Rudi | Modélisation et analyse de services éco-hydrauliques des réseaux de canaux et fossés des agrosystèmes méditerranéens[END_REF]. Regarding agricultural ditches, the mean discharge was estimated from 2 to 11 L.s -1 for an agricultural Mediterranean ditch for rainfall events with return times respectively of 1 and 12 months [START_REF] Dollinger | Analyse et modélisation des transferts et de la rétention de pesticides dans les fossés agricoles infiltrants en lien avec les stratégies d'entretien[END_REF].

However, these values are temporally-averaged and peak values can be drastically higher (occasionnally above 100 L.s -1 for ditches bordering plots). In this study, we will focus on agricultural channels with slopes under 5 ‰because channels with steep slopes were poorly studied to this day. Turbulent flow conditions are often observed in agricultural channels. The networks under study often transport water on distances from tens to hundreds of kilometers across the agricultural landscape.

To gather data on plant community functional parameters and properties involved in water flow regulation, retention, and bank strengthening, as well as on the effect of management practices on plant communities in agricultural channels, we relied on four scientific databases : Science Direct, Wiley Online Library, Springer Link and Google Scholar. We then collected all papers that contained the keyword "ditch", "irrigation channel" and "irrigation canal", with "vegetation" or "plant", resulting in 5087 papers. We then sorted the relevant papers, resulting in 146 papers dealing with the interactions between vegetation and the studied ecosystem functions, and with their management practices, in agricultural channels or flumes. These 146 papers were read and studied carefully. Note that papers relying on flume experiments, although not conducted in agricultural areas, were considered relevant for understanding some of the studied functions due to the physical similarity and comparable morphologies and hydraulic conditions of flumes and small agricultural channels.

The relationships between individual plant traits, community functional parameters and properties, and ecosystem functions are presented in Figure 1.

Overview of the functions of vegetation in agricultural channels

Agricultural channel vegetation is generally composed of terrestrial and semiaquatic species [START_REF] Bouldin | Vegetative and structural characteristics of agricultural drainages in the Mississippi Delta landscapes[END_REF][START_REF] Clarke | Conserving freshwater biodiversity: The value, status and management of high quality ditch systems[END_REF][START_REF] Maheu-Giroux | Landscape ecology of Phragmites australis invasion in networks of linear wetlands[END_REF][START_REF] Rudi | Using geomorphological variables to predict the spatial distribution of plant species in agricultural drainage networks[END_REF][START_REF] Szymura | Grasses (Poaceae) in riparian vegetation of watercourses in agriculture landscape[END_REF], and sometimes of strictly aquatic species [START_REF] Cassan | Velocity profiles in a real vegetated channel[END_REF][START_REF] Clarke | Conserving freshwater biodiversity: The value, status and management of high quality ditch systems[END_REF][START_REF] Janse | A model of ditch vegetation in relation to eutrophication[END_REF][START_REF] Milsom | Dynamics and management of plant communities in ditches bordering arable fenland in eastern England[END_REF][START_REF] Sabbatini | Vegetation-environment relationships in irrigation channel systems of southern Argentina[END_REF] not), and other conditions (topographic, climatic, and pedological). They form sparse or dense cover that is homogeneously distributed in the channel or forms patches, as illustrated in Figure 2. Vegetation is involved in ecosystem functions that have already been highlighted in other research works focusing on agricultural channels [START_REF] Aspe | Irrigation canals as tools for climate change adaptation and fish biodiversity management in Southern France[END_REF][START_REF] Dollinger | Managing ditches for agroecological engineering of landscape. A review[END_REF][START_REF] Herzon | Agricultural drainage ditches, their biological importance and functioning[END_REF][START_REF] Needelman | The role of ditch soil formation and redox biogeochemistry in mitigating nutrient and 48 pollutant losses from agriculture[END_REF][START_REF] Pierce | Managing Artificially Drained Low-Gradient Agricultural Headwaters for Enhanced Ecosystem Functions[END_REF]. Three main types of functions provided by agricultural channels vegetation have been identified (Figure 3). tural channels can contribute to mitigating the peak of floods according to the topological conditions and organization of the channels' network [START_REF] Moussa | Effects of the spatial organization of agricultural management on the hydrological behaviour of a farmed catchment during flood events[END_REF].

The second ecosystem function provided by agricultural channel vegetation is retention (sediments, agricultural pollutants, and plant propagules). The presence of vegetation can enhance retention by three main means : (i) it can affect the components of the water flow and then favor sedimentation of particles transported in the water column, (ii) it can mechanically retain particles, and (iii) dissolved pollutants can be absorbed or adsorbed by vegetation. The retention of sediments can reduce the turbidity of water in receiving ecosystems (at the outlet of the ditch networks) [START_REF] Fiener | Effectiveness of grassed waterways in reducing runoff and sediment delivery from agricultural watersheds[END_REF][START_REF] Flora | Use of vegetated drainage ditches and low-grade weirs for aquaculture effluent mitigation: II. Suspended sediment[END_REF]. Vegetation also slows the transfer of pesticides to receiving ecosystems [START_REF] Dollinger | Managing ditches for agroecological engineering of landscape. A review[END_REF][START_REF] Dollinger | Variability of glyphosate and diuron sorption capacities of ditch beds determined using new indicator-based methods[END_REF][START_REF] Margoum | Rétention des produits phytosanitaires dans les fossés de connexion parcelle-cours d'eau[END_REF][START_REF] Lagacherie | An indicator approach for describing the spatial variability of artificial stream networks with regard to herbicide pollution in cultivated watersheds[END_REF], and therefore favors their degradation in the channels. Last, standing vegetation influences the dispersal of species through agricultural channels by retaining plant propagules (seeds and other propagules such as rhizomes) transported by water (Rudi et al., 2018a;[START_REF] Soomers | The dispersal and deposition of hydrochorous plant seeds in drainage ditches[END_REF]. This limits the spread of adventitious propagules in cultivated areas (Rudi et al., 2018a) but consequently also limits the dispersal of non-adventitious or protected species.

The last type of function is bank strengthening. Terrestrial and semiaquatic vegetation plays a part in maintaining the banks of waterways through the dissipation of hydraulic energy due to the presence of the aboveground part of the plants, but also through the stabilization, and limitation of soil bank loss, due to the development of the root system [START_REF] De Baets | Effects of grass roots on the erodibility of topsoils during concentrated flow[END_REF][START_REF] Ghestem | The Influence of Plant Root Systems on Subsurface Flow: Implications for Slope Stability[END_REF][START_REF] Mamo | Detachment rate, soil erodibility, and soil strength as influenced by living plant roots. part ii : Field study[END_REF][START_REF] Pollen-Bankhead | Hydrologic and hydraulic effects of riparian root networks on streambank stability: Is mechanical rootreinforcement the whole story?[END_REF]. We therefore assume that the effects of vegetation are similar between agricultural channels and natural waterways.

In the following paragraphs, we aim to characterize plant functional community parameters and properties involved in the ecosystem functions described above.

Characterization of plant community functional parameters and properties involved in ecosystem functions of vegetation

Plant community functional parameters and properties involved in water flow regulation

The function of water regulation is generally assessed through the velocity of the water flow and the height of the water column. Water velocity and height mainly rely on imposed hydraulic conditions (inflow, downstream conditions), as well as on a physical quantity called hydraulic resistance. In agricultural channels, hydraulic resistance is largely driven by vegetation when it is present in the channels. Hydraulic resistance induced by vegetation in channels can be studied at different spatial resolutions : (i) at the plant resolution (or lower resolution such as the stem or leaf scale when relevant), but this approach is not practicable when estimating ecosystem functions in a whole network of channels, (ii) at the resolution of small homogeneous plant covers (iii) at the reach resolution : in this case the total hydraulic resistance is generally encompassed in a global resistance coefficient.

Characterization of the hydraulic resistance of homogeneous plant covers in small sections of channels

Plant cover in agricultural channels exerts a drag force opposite to the motion of the water. A quadratic drag law can be used to assess this spatially averaged drag force f (Equation 1):

f = 1 2 × C d × A × ρ × V 2 (1)
where

C d is the drag coefficient [-], A is the area of plants projected in the flow direction [L 2 ], ρ is the density of water [M.L -1 ] and V is the local flow velocity [L.T -1 ].
The coefficient C d is dependent on community parameters and properties such as the projected area A (Nepf, 2012a), and, therefore, on density D [START_REF] Li | Effect of Tall Vegetations on Flow and Sediment[END_REF], mean diameter d [START_REF] Li | Effect of Tall Vegetations on Flow and Sediment[END_REF] and morphology [START_REF] James | Flow resistance of emergent vegetation[END_REF]Nepf, 2012a), as well as flexibility [START_REF] James | Flow resistance of emergent vegetation[END_REF]. [START_REF] Nepf | Hydrodynamics of vegetated channels[END_REF] explains that for large Reynolds numbers (calculated at the stem scale), it is reasonable to choose a constant drag coefficient C d between 0.7 and 1.5. This drag coefficient is often fixed at 1 when no data are available for estimating it more accurately. In contrast, for small stem Reynolds' numbers, C d can take larger values and increases as the Reynolds number decreases [START_REF] Tanino | Laboratory investigation of mean drag in a random array of rigid, emergent cylinders[END_REF].

Velocity fields are heterogeneous within plant cover (Nepf, 2012a). Generally, studies differentiate emergent and submerged plant covers because the mean velocity and turbulence fields can be truly different in these two conditions [START_REF] Defina | Mean flow and turbulence in vegetated open channel flow[END_REF][START_REF] Finnigan | Turbulence in Plant Canopies[END_REF][START_REF] Nepf | Flow structure in depth-limited, vegetated flow[END_REF] (Figure 4).

The scale of turbulence is also dependent on the density of the cover, which is generally defined as sparse or dense (Table 1). Sparse covers are covers with approximately C d ah p < 0.04 and dense covers are covers with approximately C d ah p > 0.1 [START_REF] Belcher | Adjustment of a turbulent boundary layer to a canopy of roughness elements[END_REF][START_REF] Nepf | Retention time and dispersion associated with sub-merged aquatic canopies[END_REF], with a representing the frontal area of the plant cover by volume and h p the height of the plant cover.

Note that all conditions (sparse and dense plant covers, as well as submerged and emergent covers) are commonly found in agricultural channels.

In emergent conditions, the mean velocity inside the vegetation is always lower than that for a bare-bed channel. However, the vegetation concomitantly creates an increase in turbulence. The turbulence inside the plant cover is greater than that due to bed friction, and therefore, the total turbulence cannot be predicted from only the bed shear velocity [START_REF] Nepf | Hydrodynamics of vegetated channels[END_REF], as is commonly done in studies on bare beds.

In submerged conditions, two main configurations are observed [START_REF] Nepf | Hydrodynamics of vegetated channels[END_REF] (Figure 4). If the plant cover is sparse, the velocity profile has the shape of a turbulent boundary layer. In this case, turbulence intensity increases with vegetation density (Nepf, 2012a). If the plant cover is dense, the velocity profile has an inflection point as observed in Figure 4. When the vegetation is dense and thus an inflection point occurs in the velocity profile, plant cover-scale vortices are expected to appear [START_REF] Brown | On density effects and large structure in turbulent mixing layers[END_REF]. It is possible to predict the length of penetration δe of these Kelvin-Helmholtz vortices in the plant cover [START_REF] Nepf | Retention time and dispersion associated with sub-merged aquatic canopies[END_REF]. This length of penetration decreases with an increasing density of stems. For 0.2 < C d ah p < 1, δe ∼(C d a) -1 [START_REF] Nepf | Retention time and dispersion associated with sub-merged aquatic canopies[END_REF]. When C d ah p < 0.2, δe = h p and the vortex penetrates to the bed. Lastly, when (C d a) -1 < 2d, δe ≈2d(W hiteandN epf , 2008).

In both emergent and submerged conditions, flexible vegetation can bend under the effect of water velocity. This phenomenon is called reconfiguration [START_REF] Vogel | Life in Moving Fluids: The Physical Biology of Flow[END_REF] and is dependent on water velocity. The streamlining of the vegetation with increasing water velocity might also change the friction coefficient C * at the interface of the top of the plant cover and the free layer of water [START_REF] Aberle | Flow resistance of emergent rigid and flexible floodplain vegetation[END_REF]. [START_REF] Luhar | Flow-induced reconfiguration of buoyant and flexible aquatic vegetation[END_REF] then [START_REF] Chapman | Drag force parameters of rigid and flexible vegetal elements[END_REF] proposed the prediction of drag coefficients for plants or flexible elements with different flexibilities. In real conditions in agricultural channels, most of the vegetation is flexible because it is composed mainly of nonligneous species. To the best of our knowledge, no estimation of flexibility for a whole plant community has been conducted, and research has generally considered individual plants, despite the importance of this property for understanding flow patterns.

The flexible cover can move according to the mean and turbulent fields [START_REF] Nepf | Flow and transport in channels with submerged vegetation[END_REF]. The progressive waving of the vegetation due to the passage of the Kelvin-Helmholtz vortices is named "monami" and was first observed for seagrasses [START_REF] Fonseca | Effects of current on photosynthesis and distribution of seagrasses[END_REF][START_REF] Grizzle | Hydrodynamically induced synchronous waving of seagrasses: 'monami' and its possible effects on larval mussel settlement[END_REF], and then experimentally characterized in flumes (for example see [START_REF] Okamoto | Flow-vegetation interactions: length-scale of the "monami" phenomenon[END_REF]). In their study, [START_REF] Okamoto | Flow-vegetation interactions: length-scale of the "monami" phenomenon[END_REF] revealed that the quantity of vegetation waving at the same time was associated with the lenght-scale of the turbulence in the mixing-layer zone.

Characterization of hydraulic resistance in real plant cover at the reach scale

Hydraulic resistance at the reach scale is generally described by a resistance coefficient, which is commonly the Manning n, Darcy-Weisbach or Chezy coefficient, but some authors have proposed other coefficients (for example [START_REF] James | Flow resistance of emergent vegetation[END_REF]). In traditional hydraulic engineering, resistance coefficients were assessed through visual aspect of channel, then refering to tables linking channels' features (including vegetation) to resistance coefficient values (see [START_REF] Chow | Open-channel hydraulics[END_REF] for example). Manning's coefficient n is widely used and is associated to flow characteristics by Manning's equation [START_REF] Manning | On the flow of water in open channels and pipes[END_REF] (Equation 2) :

n = R 2/3 × i 1/2 u (2)
However, vegetation presenting very different morphologies and characteristics according to the context, this approach was considered as limiting, and led to inaccurate values of resistance coefficients [START_REF] Green | Modelling flow resistance in vegetated streams: review and development of new theory[END_REF]. This led many authors to propose formulas allowing to predict resistance from measurable plant community parameters and properties. Note that as underlined by Folkard properties have been developed (some of them are presented in Table 2).

In practice, at the network scale, few hydraulic or hydrological models consider the resistance created by vegetation, especially its spatialization at the resolution of reaches. When vegetative resistance is considered, its relationship with vegetation properties is highly simplified as in [START_REF] Doncker | Determination of the Manning roughness coefficient influenced by vegetation in the river Aa and Biebrza river[END_REF] or [START_REF] Bertoldi | Modeling vegetation controls on fluvial morphological trajectories[END_REF], who considered a really simplified relationship between vegetation biomass and the resistance coefficient. These last two examples have been found for natural waterways: no studies on this subject were found for agricultural channels.

Plant community functional parameters and properties involved in retention

of sediments, agricultural pollutants and plant propagules

Sediment transport and deposition

It is generally considered that the presence of vegetation is associated with increased sedimentation of suspended particles in rivers, flumes and agricultural channels [START_REF] Abt | Sediment Deposition and Entrapment in Vegetated Streambeds[END_REF][START_REF] Cotton | The effects of seasonal changes to in-stream vegetation cover on patterns of flow and accumulation of sediment[END_REF][START_REF] López | Open-channel flow through simulated vegetation: Suspended sediment transport modeling[END_REF] and decreased bed-load transport [START_REF] Kothyari | Effect of tall vegetation on sediment transport by channel flows[END_REF]. However, some researchers with:

1 F f = 1-N πd 2 4 F dw 8 +C d 1 2 N Hd gH and u = 1 F f √ S Emergent vegeta- tion • Both
• Cu = 1 with D ≤ 5m -1 • Cu = 2 with D > 5m -1

Submerged vegetation

Parameters and properties : hp, D, flexibility Green, 2005b,a;[START_REF] Nepf | Hydrodynamics of vegetated channels[END_REF] For B = 1:

n g 1/2 KH 1/6 = C d aH 2
For B < 1:

n g 1/2 KH 1/6 = C * 2 (1 -B) -3/2
Submerged vege- [START_REF] Fauria | Suspended particle capture by synthetic vegetation in a laboratory flume[END_REF] showed the significant role of the biofilms developing in vegetation in small particles capture (range 1.25 -250 µm) in some aquatic systems. Note that these biofilms developing on freshwater vegetation have been mainly characterized in streams [START_REF] Besemer | Biodiversity, community structure and function of biofilms in stream ecosystems[END_REF] and marshes [START_REF] Buesing | Microbial communities in contrasting freshwater marsh microhabitats[END_REF] and have an important diversity affected by several factors, although abundance and community assemblage of biofilms seems specifically affected by the presence of inorganic nutrients and dissolved organic matter [START_REF] Olapade | Seasonal Response of Stream Biofilm Communities to Dissolved Organic Matter and Nutrient Enrichments[END_REF]. Relying on plant density conditions relevant to the ones found in agricultural channels (2724 and 7209 stems.m -2 ) and on a natural biofilm devloped in eutrophic conditions, [START_REF] Fauria | Suspended particle capture by synthetic vegetation in a laboratory flume[END_REF] have determined new coefficients for the formulation of the particle capture efficiency (especially with b a negative exposant, highlighting the fact that particle retention is decreased with increased flow velocity).

For bare beds, the settling velocity of the particles and the minimal shear stress for entrainment are generally used to model entrainment and transport.

To understand the interactions between sediment transport and vegetation in channels more deeply, Nepf (2012a) reported that sediment transport was linked not only to the mean bed stress (as for bare-bed sediment transport) but also to the profile of turbulence. Indeed, [START_REF] Luhar | Interaction between flow, transport and vegetation spatial structure[END_REF] found that in sparse plant cover, the turbulent stress remained elevated close to the bed, whereas it was reduced in dense plant cover. For plant cover for which C d ah p < 0.1, the resuspension is of the same order of magnitude as that for bare-bed channels.

In other words, plant cover below this threshold has a limited influence on sediment retention. [START_REF] Moore | Influence of Seagrasses on Water Quality in Shallow Regions of the Lower Chesapeake Bay[END_REF] even noted enhanced erosion under these conditions due to the augmentation of stem-scale turbulence. For plant cover above this threshold, we observed in paragraph 4.1.1 that the penetration scale of the Kevin-Helmholtz vortices was dependent on the density of the stems. In this case, the higher the density, the more important the sheltering between adjacent stems. As a consequence, the bed is protected from the turbulent stress caused by Kevin-Helmholtz vortices, and the momentum transfer is therefore extremely reduced. Additionally, the mean bed stress is generally reduced in the presence of vegetation (Nepf, 2012a), but the methods used to characterize it are still controversial (Nepf, 2012a;[START_REF] Yang | Estimation of the bed shear stress in vegetated and bare channels with smooth beds[END_REF]. Recently, [START_REF] Yang | Estimation of the bed shear stress in vegetated and bare channels with smooth beds[END_REF] presented a formula (Equation 4) for estimating this bed shear stress in the presence of vegetation, but only for plant covers with frontal areas per volumetric unit a up to 4.3 m -1 .

U ef f * = max C f < V 0 >, 2 ν < V 0 > d (4)
with V 0 the local time-averaged velocity in the uniform layer (m.s -1 ), C f the drag coefficient for the bare-bed (-), ν the kinematic viscosity of water (m 2 .s -1 ) and d the diameter of the stems (m).

Under these conditions, stem diameter d is considered a determining parameter for bed shear stress, independent of density.

Other studies focused on the effects of patches of vegetation on sediment transport and deposition [START_REF] Zong | Spatial distribution of deposition within a patch of vegetation[END_REF][START_REF] Ortiz | Mean and turbulent velocity fields near rigid and flexible plants and the implications for deposition[END_REF]. In the wake behind a patch, it seems that sedimentation is increased, but this appears to be true only for emergent and rigid patches [START_REF] Ortiz | Mean and turbulent velocity fields near rigid and flexible plants and the implications for deposition[END_REF]. However, at the leading edge of the patch, where the flow is generally deflected, the net deposition is reduced compared to that of the bare-bed [START_REF] Zong | Spatial distribution of deposition within a patch of vegetation[END_REF]. [START_REF] Bouma | Spatial flow and sedimentation patterns within patches of epibenthic structures: Combining field, flume and modelling experiments[END_REF] and Sand-Jensen and Vindbaek Madsen (1992) reported that these two mechanisms explained why patches generally grow in length but rarely grow in width along open channels.

Nepf (2012a) and [START_REF] Solari | Advances on Modelling Riparian Vegetation-Hydromorphology Interactions[END_REF] reported that no general model cur-rently exists for sediment transport in vegetated channels. First, most of the models are based on the estimation of bed shear stress. However, as stated before, the bed shear stress in vegetated flows is very difficult to calculate (Nepf, 2012a;[START_REF] Yang | Estimation of the bed shear stress in vegetated and bare channels with smooth beds[END_REF]. Second, as turbulence is reported as an influential parameter for resuspension and transport under some conditions, bed shear stress might not be sufficient to take into account the effect of vegetation on sediment transport [START_REF] Vollmer | Predicting incipient motion, including the effect of turbulent pressure fluctuations in the bed[END_REF].

Agricultural pollutants (pesticides and nutrients) retention

Sorption is a major mechanism involved in pesticides retention in agricultural areas [START_REF] Dousset | Effect of grass cover on water and pesticide transport through undisturbed soil columns, comparison with field study (Morcille watershed, Beaujolais)[END_REF][START_REF] Stehle | Pesticide risk mitigation by vegetated treatment systems: a meta-analysis[END_REF], in particular in farmed ditches [START_REF] Dollinger | Managing ditches for agroecological engineering of landscape. A review[END_REF]. This has been demonstrated for an insecticide (pyrethroid) [START_REF] Bennett | Vegetated agricultural drainage ditches for the mitigation of pyrethroid-associated runoff[END_REF] and a herbicide (diuron) [START_REF] Dollinger | Variability of glyphosate and diuron sorption capacities of ditch beds determined using new indicator-based methods[END_REF][START_REF] Margoum | Investigation of various physicochemical and environmental parameter influence on pesticide sorption to ditch bed substratum by means of experimental design[END_REF]. Sorption increases with increasing organic content for most pesticides, namely hydrophobic ones [START_REF] Margoum | Investigation of various physicochemical and environmental parameter influence on pesticide sorption to ditch bed substratum by means of experimental design[END_REF]. This explains that an effective retention has been observed not only in the presence of living vegetation but also in the presence of litter (dead vegetation) at the bottom of agricultural channels. Based on these principles, [START_REF] Margoum | Rétention des produits phytosanitaires dans les fossés de connexion parcelle-cours d'eau[END_REF] proposed a retention index based on the relative cover (in the wetted section of channel) of living vegetation, dead vegetation and sediments, reported as Herbicide Retention Power (HRP ) in [START_REF] Lagacherie | An indicator approach for describing the spatial variability of artificial stream networks with regard to herbicide pollution in cultivated watersheds[END_REF] (Equation 5) :

HRP = LV + 2DV + 0.5S (5)
with LV the relative cover of living vegetation (%), DV the relative cover of dead vegetation (%) and S the relative cover of sediments (%).

More recently, [START_REF] Dollinger | Variability of glyphosate and diuron sorption capacities of ditch beds determined using new indicator-based methods[END_REF] proposed a "Sorption induced Pesticide Retention Indicator" (SPRI) directly based on masses of the different elements 22 composing the bottom and banks of the channel, including living and dead vegetation 6) :

SP RI(%) = n i M i Kd i n i M i Kd i + V 100 ( 6
)
with i being material i composing the channel (ie living vegetation, dead vegetation, soil, ash), M i the corresponding mass of material i (g), Kd i the linear adsorption coefficient (cm 3 .g -1 ) and V the volume of water flowing during a flow-event (cm 3 ).

such as [START_REF] Dollinger | Variability of glyphosate and diuron sorption capacities of ditch beds determined using new indicator-based methods[END_REF]; [START_REF] Margoum | Rétention des produits phytosanitaires dans les fossés de connexion parcelle-cours d'eau[END_REF]; [START_REF] Lagacherie | An indicator approach for describing the spatial variability of artificial stream networks with regard to herbicide pollution in cultivated watersheds[END_REF] proposed retention indicators partly based on living and dead biomasses of vegetation in agricultural channels. Additionally, absorption of pesticides by vegetation is also expected to play a role in retention, but it is not easy to differentiate between absorption and adsorption in field experiments [START_REF] Moore | Use of vegetated agricultural drainage ditches to decrease pesticide transport from tomato and alfalfa fields in California, USA[END_REF]. Vegetation can also indirectly drive the retention of pesticides prone to adsorption on sediments by influencing sedimentation rates under some conditions (see paragraph 4.2.1). Vegetation can also increase water retention times and therefore favor sorption [START_REF] Dollinger | Analyse et modélisation des transferts et de la rétention de pesticides dans les fossés agricoles infiltrants en lien avec les stratégies d'entretien[END_REF] and biodegradation mechanisms [START_REF] Liu | Research on the Influencing Factors of Hydraulic Efficiency in Ditch Wetlands[END_REF].

Regarding nutrients, [START_REF] Castaldelli | Vegetated canals mitigate nitrogen surplus in agricultural watersheds[END_REF] showed that N uptake by agricultural channel vegetation was low but that vegetation played a role in microbial denitrification by providing interfaces favoring the activity of bacteria. To the best of our knowledge, only Janse (1998) proposed a model assessing nutrient uptake by vegetation in agricultural channels based on the biomasses of plant functional groups. As for pesticides, nutrients that tend to be transported bounded on sediments (for example P), might also be retained by vegetation through increased sedimentation [START_REF] Bouldin | Vegetative and structural characteristics of agricultural drainages in the Mississippi Delta landscapes[END_REF][START_REF] Liu | Major factors influencing the efficacy of vegetated buffers on sediment trapping: a review and analysis[END_REF].

Plant propagule retention

Traditionally, studies on hydrochorous plant propagule dispersal distinguishes nonfloating and floating propagules (usually seeds). Indeed, seeds can be transported in the bedload, suspended in the flow or at the water surface [START_REF] Carthey | How seed traits predict floating times: a biophysical process model for hydrochorous seed transport behaviour in fluvial systems[END_REF][START_REF] Chambert | Sorting of seeds by hydrochory[END_REF][START_REF] Jager | Seed size regulates plant dispersal distances in flowing water[END_REF]. For nonfloating seeds, [START_REF] Gurnell | Analogies between mineral sediment and vegetative particle dynamics in fluvial systems[END_REF] established an analogy between sediment and seed transport. This was confirmed by an experiment led by [START_REF] Chambert | Sorting of seeds by hydrochory[END_REF] who concluded that the main factors explaining the entrainment and deposition of nonfloating seeds were minimum critical shear stress necessary for their entrainment and the settling velocity. Therefore, the plant community functional parameters and properties of the standing vegetation involved in the retention of nonfloating seeds are the same as those described in paragraph 4.2.1.

For floating seeds, [START_REF] Defina | Floating particle trapping and diffusion in vegetated open channel flow[END_REF] reported the four main mechanisms governing the seed-plant interactions in an artificial flume : the Cheerios effect, when a seed is attracted by the meniscus around the stem due to surface tension forces [START_REF] Vella | The "Cheerios effect[END_REF]; inertial impaction, when a collision between a seed and plant results in a change in the direction of the seed [START_REF] Palmer | Observations of particle capture on a cylindrical collector: Implications for particle accumulation and removal in aquatic systems[END_REF]; wake trapping, when the seed is temporarily trapped in the recirculation zone behind a stem [START_REF] White | Scalar transport in random cylinder arrays at moderate Reynolds number[END_REF]; and net trapping, when plants form a net that stops the seeds [START_REF] Defina | Floating particle trapping and diffusion in vegetated open channel flow[END_REF]. Many studies, mainly focused on the Cheerios effect, highlighted the role of stem density (more precisely the mean centre-to-centre spacing) in the rate of propagules' retention [START_REF] Defina | Diffusion of floating particles in flow through emergent vegetation: Further experimental investigation[END_REF][START_REF] White | Scalar transport in random cylinder arrays at moderate Reynolds number[END_REF][START_REF] Liu | Floating seed dispersal in open channel flow with emergent vegetation[END_REF].

Net trapping depends on the density, height, and spatial distribution of plants in the reach (homogeneous distribution or existence of patches), as well as on plant architectures at the water surface (Rudi and Vinatier, personnal observation). At low velocities, the Cheerios effect is important but it disappears progressively as turbulent diffusion phenomena become too important [START_REF] Chambert | Sorting of seeds by hydrochory[END_REF][START_REF] Defina | Diffusion of floating particles in flow through emergent vegetation: Further experimental investigation[END_REF][START_REF] Peruzzo | Capillary trapping of buoyant particles within regions of emergent vegetation[END_REF][START_REF] Peruzzo | A semi-empirical model to predict the probability of capture of buoyant particles by a cylindrical collector through capillarity[END_REF].

Net trapping is generally predominant when stem density and water turbulence are important [START_REF] Defina | Floating particle trapping and diffusion in vegetated open channel flow[END_REF].

In addition, for floating seeds, the velocity fields at the water surface significantly influence the deposition of seeds [START_REF] Soomers | The dispersal and deposition of hydrochorous plant seeds in drainage ditches[END_REF]. In this context, emergent vegetation can be considered a porous obstacle modifying the velocity fields [START_REF] Defina | Floating particle trapping and diffusion in vegetated open channel flow[END_REF], and the presence of vegetation is susceptible to creating recessional flows, recirculating flows, or eddies, which are generally associated with high rates of seed deposition [START_REF] Merritt | Processes Governing Hydrochory along Rivers: Hydraulics, Hydrology, and Dispersal Phenology[END_REF][START_REF] Nilsson | Importance of hydrochory in structuring plant communities along rivers[END_REF].

Due to the inherent difficulty of taking into account some plant parameters such as the specific architecture of plants at the scale of the community in heterogeneous plant covers, Rudi et al. (2018a); [START_REF] Vinatier | The Use of Photogrammetry to Construct Time Series of Vegetation Permeability to Water and Seed Transport in Agricultural Waterways[END_REF] relied on a measure of the area of vegetation at the water surface (at the scale of a section of approximately one meter), accounting for the porosity of vegetation, because this property was assumed to explain seed deposition with good performance.

These metrics were assessed through image analysis (Structure-from-Motion using Multi-View Stereo algorithm). Note however that this approach is less performing in case of abundant vegetation because the highest strata of vegetation might hide the water surface in some images. for a mixture of Lolium sp and Festuca sp. The difference in the values of the threshold could arise not only from differences in root properties (especially diameter distributions in the soil profile) but also from differences in experimental conditions. The negative exponential shape observed between RLD (and RD)

and the volume of eroded soil led De [START_REF] De Baets | Effects of grass roots on the erodibility of topsoils during concentrated flow[END_REF] to propose a power and a hill curve functions for this relationship, for which they obtained good correlation coefficients (equations 7 and 8).

RSD = 16.38RLD -0.78 (7) RSD = RLD -2.22 RLD -2.22 + 0.000026 (8)
with RSD being the Relative Soil Detachment rate.

However, it seems difficult to experimentally differentiate the influence of the aboveground and belowground parts of the plant [START_REF] Mamo | Detachment rate, soil erodibility, and soil strength as influenced by living plant roots. part ii : Field study[END_REF][START_REF] Zhang | Impact of Grass Root Mass Density on Soil Detachment Capacity by Concentrated Flow on Steep Slopes[END_REF]. Indeed, aboveground part of the plant community also tend to dissipate hydraulic energy and therefore protect the banks' soils from hydraulic erosion. Some aboveground plant architectures, such as the presence of rosettes, can also protect the soil [START_REF] Kramer | Relative Efficiency of Roots and Tops of Plants in Protecting . the Soil from Erosion[END_REF].

In addition, some parameters, such as the distribution of root diameters (in a soil unit) and the number of roots, were reported to be related to the fullroot tensile strength, which is frequently used in models evaluating the stability of stream banks and based on mechanical reinforcement [START_REF] Pollen-Bankhead | Enhanced application of rootreinforcement algorithms for bank-stability modeling[END_REF]. Fibrous roots [START_REF] Operstein | The influence of vegetation on soil strength[END_REF], and fibrils [START_REF] Li | Effectiveness of plant roots to increase the anti-scourability of soil on the Loess Plateau[END_REF], are considered more efficient than taproots [START_REF] Zhang | Impact of Grass Root Mass Density on Soil Detachment Capacity by Concentrated Flow on Steep Slopes[END_REF] , 2015) or Tripsacum dactyloides [START_REF] Ghestem | The Influence of Plant Root Systems on Subsurface Flow: Implications for Slope Stability[END_REF]. However, according to [START_REF] Reubens | The role of fine and coarse roots in shallow slope stability and soil erosion control with a focus on root system architecture: a review[END_REF], it seems that for slope stabilization, plants with deep anchorage and many lateral roots, for example dicotyledons, would be more effective than plants with only fibrous roots. [START_REF] Ghestem | The Influence of Plant Root Systems on Subsurface Flow: Implications for Slope Stability[END_REF] also stressed the importance of considering the network of macropores (pores up to 30 µm), generally of biotic origin, in the mechanisms governing slope stability and therefore bank strengthening. Indeed, high convergence of macropores or dead-end macropores can generate high pore water pressure, in turn destabilizing the banks. Root orientation (upslope or downslope) is also of great importance. Deep-rooted plants generally perform better than shallow-rooted plants in terms of driving water away from unstable areas.

However, well-developed fibrous and shallow root systems can help dissipate pore water pressures that could otherwise concentrate in one location [START_REF] Ghestem | The Influence of Plant Root Systems on Subsurface Flow: Implications for Slope Stability[END_REF].

Finally, Pollen-Bankhead and Simon (2010) highlighted the hydrological effect of root systems on bank stability. Total evapotranspiration, driven by the composition of the plant community, removes water from the soil and therefore influences soil matrix suction. This is assumed to influence apparent soil cohesion, but this hydrological effect of plants on bank stability is still difficult to quantify (Pollen-Bankhead and Simon, 2010).

Effects of channel management practices on plant community functional parameters and properties

The main aim of managing a ditch or a channel is to restore its hydraulic capacity [START_REF] Dollinger | Managing ditches for agroecological engineering of landscape. A review[END_REF][START_REF] Sánchez Martín | Effects of vegetation management on plant diversity in traditional irrigation systems[END_REF]. Different management practices (mowing, dredging, burning, chemical weeding, and clearing) affect the communities in the short term (less than one year) and medium (between one and five years) and long term (more than five years). In the short term, the practices of mowing, dredging, burning and chemical weeding affect the dynamics of height, density and porosity of vegetation, as was illustrated by [START_REF] Dollinger | Impact of maintenance operations on the seasonal evolution of ditch properties and functions[END_REF]; [START_REF] Vinatier | The Use of Photogrammetry to Construct Time Series of Vegetation Permeability to Water and Seed Transport in Agricultural Waterways[END_REF] (see Figure 5). These two studies related the dynamics of functional parameters and properties of plant communities driven by management practices throughout the year, to the dynamics of ecosystem functions in which vegetation is involved (especially water regulation and pollutant and propagule retention). The two papers suggested that both the temporality and type of practice are determinant of the resulting ecosystem functions.

In the medium term, dredging allows a return to the early stages of veg- etation succession [START_REF] Clarke | Conserving freshwater biodiversity: The value, status and management of high quality ditch systems[END_REF] and therefore allows more replacements of plant species [START_REF] Teurlincx | Managing Successional Stage Heterogeneity to Maximize Landscape-52 Wide Biodiversity of Aquatic Vegetation in Ditch Networks[END_REF]. Mowing and burning generally maintain or improve richness and diversity [START_REF] Chaudron | Influence of management practices on plant assemblages of road-field boundaries in an agricultural landscape[END_REF][START_REF] Milsom | Dynamics and management of plant communities in ditches bordering arable fenland in eastern England[END_REF][START_REF] Sánchez Martín | Effects of vegetation management on plant diversity in traditional irrigation systems[END_REF]. These practices influence competitive interactions betweens species because the distribution of water, nutrients and light is modified. Indeed, frequently disturbed ecosystems have been reported to be less competitive than natural ones [START_REF] Connell | Diversity in tropical rain forests and coral reefs[END_REF][START_REF] Odum | The Strategy of Ecosystem Development[END_REF]. Successions at a multiyear scale after management are generally characterized by strictly aquatic plants in the early stages and emergent facultative plants and herbs in the late stages [START_REF] Clarke | Conserving freshwater biodiversity: The value, status and management of high quality ditch systems[END_REF][START_REF] Milsom | Dynamics and management of plant communities in ditches bordering arable fenland in eastern England[END_REF], which implies different parameters and properties. [START_REF] Janse | A model of ditch vegetation in relation to eutrophication[END_REF] showed that the level of nutrient (N and P), that depends on management practices in adjacent fields and inside the ditch, influenced the composition of the plant community : duckweed was mainly found in highly loaded ditches and submerged plants were found in poorly loaded ditches. They showed that there was a critical level in nutrient input that triggered the shift in plant community composition. Note that most studies on channel management practices focused on general indices of richness, diversity or productivity and did not clearly evaluate the response of vegetation in terms of community parameters or properties. Moreover, the long-term effect of management practices on community composition has been poorly studied and is difficult to predict [START_REF] Blomqvist | Seed and (micro)site limitation in ditch banks: Germination, establishment and survival under different management regimes[END_REF].

Discussion

In conclusion, the vegetation in agricultural channels is involved in three This review pointed out that knowledge of the effect of functional parameters and properties on channel ecosystem functions is far more developed for water regulation than for the other functions. Mechanisms underlying water regulation have been described both at the local scale and at the reach scale, using physical and semiempirical approaches, although research on the effect of heterogeneous vegetation cover on water flow fields remains to be conducted. Bank strengthening has been explored, especially for waterways, with models based on the number of roots and tensile strength, which do not explicitly take into account the complexity of the relationships between soil texture/structure, root systems and their effect on the hydrological status of the soil matrix. This can be partly explained by (i) the complexity of the relationships between soils and roots that involve physical, biological and chemical processes, (ii) the difficulty of measuring belowground properties and parameters, and (iii) the dynamic nature of the contributions of the different effects of roots on bank stability [START_REF] Pollen-Bankhead | Hydrologic and hydraulic effects of riparian root networks on streambank stability: Is mechanical rootreinforcement the whole story?[END_REF]. Moreover, regarding root systems, other properties/parameters such as root system architecture and orientation with respect to the banks of the channel, as well as the hydrological effect of root systems on soil reinforcement, have been poorly explored. Nonfloating propagules and sediment retention is still difficult to model at the reach scale due to the complexity of the processes involved, especially due to the effect of turbulent phenomena in vegetated channels. The retention of pesticides, as well as nutrients, is generally related to total plant biomass, which is an integrative parameter, due to the difficulty to disentangle the processes involved in the function (e.g. both dissolved and bound particle transport, quantification of the role of absorption, effect of water residence times on sorption processes,...).

Similarly, the retention of floating propagules in vegetated channels is influenced by the specific features of the propagules (e.g. roundness, density, and shape) [START_REF] Carthey | How seed traits predict floating times: a biophysical process model for hydrochorous seed transport behaviour in fluvial systems[END_REF], as well as by hydrodynamic conditions at the free surface of water and other environmental factors (such as the intensity and direction of wind [START_REF] Soomers | The dispersal and deposition of hydrochorous plant seeds in drainage ditches[END_REF]) that make it even more difficult to study. Therefore, research efforts still need to be made to be able to quantify accurately the effect of vegetation and of its dynamics on the three studied ecosystem functions.

Channel management practices (mowing, dredging, burning, clearing and chemical weeding) influence the dynamics of vegetation (composition and properties/parameters) in the short and long term. However, except for a few studies [START_REF] Dollinger | Impact of maintenance operations on the seasonal evolution of ditch properties and functions[END_REF][START_REF] Vinatier | The Use of Photogrammetry to Construct Time Series of Vegetation Permeability to Water and Seed Transport in Agricultural Waterways[END_REF], the effect of management practices on the dynamics of these properties and parameters has not been clearly addressed because research traditionally focused on the effect of management practices on preserving the richness and diversity of plant communities. This review therefore identifies a lack of knowledge of the potential for management practices to drive the properties and parameters of plant communities that are involved in the three ecosystem functions studied.

Conclusion

Water regulation; retention of sediments, agricultural pollutants and plant propagules; and bank strengthening are essential ecosystem functions that support the agroecological transition because they help limit the harmful effects of agriculture on ecosystems. It would be interesting to adopt an approach based on community parameters/properties to understand how vegetation influences the functioning of agricultural channels because doing so would allow generical conclusions about the effect of vegetation on these ecosystem functions to be drawn and support the subsequent modeling of these functions. This review provides insights into the parameters/properties that have been clearly associated with the studied ecosystem functions in the literature and that can be measured 34 in the field or modeled to study the dynamics of these ecosystem functions in space and time. Modifying plant community parameters and properties through agricultural channel management could be a powerful lever with which to manage ecosystem functions and optimize them in space and time. However, the potential role of agricultural channel vegetation remains largely underestimated and underexploited.

Figure 1 :

 1 Figure 1: Conceptual representation of the link between plant traits, plant community functional parameters and plant community properties, and ecosystem functions

DFigure 2 :

 2 Figure 2: Illustrations of vegetation living in agricultural channels

Figure 3 :

 3 Figure 3: Ecosystem functions provided by vegetation in agricultural channels : (i) water flow regulation (ii) retention of sediments, pollutants and plant propagules, (iii) bank strengthening.

Figure 4 :

 4 Figure 4: Vertical profiles of flow velocity in emergent (A), submerged and sparse (B), and submerged and dense (C) plant covers. In (A) and (B), there is a turbulent boundary layer profile. In (C), there is a region of shear at the top of the plant cover and an inflection point. The shear layer at the top of the plant cover is present when momentum absorption by the vegetation layer is sufficient. The characteristics of the three flow configurations (A), (B) and (C) are detailed in Table1.

(

  2011), it seems that the use of resistance coefficient n to account for vegetation resistance is only valid when the height of vegetation cover is low compared to the height of water. However, despite the lack of theoretical validity, n is still really used in practice.Research focusing on flow resistance caused by vegetation at the reach scale increased approximately around 15 years ago. First,[START_REF] Luhar | Interaction between flow, transport and vegetation spatial structure[END_REF] highlighted that in natural channels, the architecture of the submerged vegetation was of low importance for predicting the resistance coefficient.[START_REF] Luhar | From the blade scale to the reach scale: A characterization of aquatic vegetative drag[END_REF] showed that the individual description of all patches with their own properties did not give very different results for resistance coefficient predictions compared to those obtained with an average description at the reach scale. According to[START_REF] Green | Modelling flow resistance in vegetated streams: review and development of new theory[END_REF];[START_REF] Nikora | Hydraulic Resistance due to Aquatic Vegetation in Small Streams: Field Study[END_REF];[START_REF] Luhar | Interaction between flow, transport and vegetation spatial structure[END_REF], the blockage factor B had the best explanatory power for three resistance coefficients (Manning, Darcy-Weisbach and Chezy). The blockage factor B is the fraction of channel cross-section occupied by vegetation (B = wh p / W H). The relationship between the resistance coefficient and B has been reported as nonlinear by many authors(Nepf, 2012a;[START_REF] Nikora | Hydraulic Resistance due to Aquatic Vegetation in Small Streams: Field Study[END_REF][START_REF] Vinatier | From 3d grassy vegetation point cloud to hydraulic resistance: Application to close-range estimation of Manning coefficients for intermittent open channels[END_REF].[START_REF] Nikora | Hydraulic Resistance due to Aquatic Vegetation in Small Streams: Field Study[END_REF] interpreted this nonlinearity as the result of a decrease of bed contribution to total resistance with an increase in vegetation and suggested that linear formulas were thus poorly accurate. In emergent conditions, Fathi-Maghadam and[START_REF] Fathi-Maghadam | Nonrigid, Nonsubmerged, Vegetative Roughness on Floodplains[END_REF] showed that the shape and distribution of vegetation were poor descriptors of resistance equations and highlighted the role of vegetation density D.[START_REF] Aberle | Flow resistance of emergent rigid and flexible floodplain vegetation[END_REF] argued that the density and ability for reconfiguration were the most important properties for determining the resistance. This was empirically inferred with decreasing exponential laws in[START_REF] Bailly | Vegetation patch effects on flow resistance at channel scale[END_REF]. Recently,[START_REF] Rubol | Universal scaling-law for flow resistance over canopies with complex morphology[END_REF] successfully modeled the resistance by representing the vegetation as a porous medium. Many formulas predicting resistance coefficients based on plant community parameters and

  have observed the opposite, such as van[START_REF] Van Katwijk | Sediment modification by seagrass beds: Muddification and sandification induced by plant cover and environmental conditions[END_REF], who reported negative net sedimentation in patches of sparse vegetation (seagrass) compared with a bare bed.At the local scale,[START_REF] Palmer | Observations of particle capture on a cylindrical collector: Implications for particle accumulation and removal in aquatic systems[END_REF] described the capture by a plant stem of small suspended particles (range 177-210 µm) relying on a metrics named the particle capture efficiency η, using an empirical formula (Equation3) of the form :η = aRe b c R c (3)with a, b and c positive constants, Re c the "collector Reynolds number"

4. 3 .

 3 Plant community functional parameters and properties involved in bank strengtheningPlants growing in agricultural channels have root systems that develop on the channel bottom or bank soil substrates. Most monocotyledons have fibrous root systems, and dicotyledons generally have taproot systems, from which other roots sometimes grow laterally. Beyond this coarse differentiation, channels' plants exhibit a gradient of root systems, differing in length, deepness, diameter, architecture, junction and spatial distribution in soils. These root parameters and properties, in interaction with soil properties, channel morphology, and hydrologic/hydraulic regimes, greatly influence the stability of channels' banks. Some parameters and properties of root systems, such as root density (RD)[START_REF] De Baets | Effects of grass roots on the erodibility of topsoils during concentrated flow[END_REF]; root length density (RLD), corresponding to the cumulative length of root per soil unit volume[START_REF] De Baets | Effects of grass roots on the erodibility of topsoils during concentrated flow[END_REF][START_REF] Pollen-Bankhead | Hydrologic and hydraulic effects of riparian root networks on streambank stability: Is mechanical rootreinforcement the whole story?[END_REF]; root biomass and total root volume (Pollen-Bankhead and Simon, 2010), have been reported to have an effect on the limitation of the erosion rates of banks, and therefore on bank strengthening. It has been shown that the volume of eroded soil is a negative exponential of RLD or RD[START_REF] De Baets | Effects of grass roots on the erodibility of topsoils during concentrated flow[END_REF][START_REF] Pollen-Bankhead | Hydrologic and hydraulic effects of riparian root networks on streambank stability: Is mechanical rootreinforcement the whole story?[END_REF]. Due to this nonlinearity, there appears to be a threshold of RLD above which an increase in RLD has little effect on the eroded volume of soil. This threshold was found to be approximately 50 km roots .m -3 soil by Pollen-Bankhead and Simon (2010) for switchgrass (Panicum virgatum) and approximately 400 km roots .m -3 soil by De Baets et al. (2006)

  , in increasing soil resistance. The technical literature on waterway bank strengthening suggests sowing the banks with perennial and fibrous-rooted vegetation, such as Festuca sp, Lolium sp, Trifolium sp, Elymus sp, Carex sp, Potentilla sp, and Mentha sp (APEL

Figure 5 :

 5 Figure 5: Evolution of porosity (%) of an agricultural channel plant cover in a ditch from Spring 2015 to Summer 2017. The arrows report the temporality of the practices throughout the year. The lines report the evolution of the porosity. The four practices and the control are represented with different colors. The figure was made based on data from Vinatier et al. (2018).

  main ecosystem functions : (i) water regulation, (ii) retention of sediments, propagules and agricultural pollutants, (iii) bank strengthening. At the local scale, parameters and properties such as the drag coefficient, frontal area, density, height, stem diameter, and flexibility, are generally used to assess the effect of vegetation on water flow. At the reach scale, the parameters and properties used to assess water flow regulation are mainly the mean height of the plant cover, porosity, flexibility and the density of the plant community. These parameters and properties are sometimes integrated in a global property named the "blockage factor", which is the fraction of channel cross-section occupied by vegetation. The mean height and density of plant cover are easily measurable; it is far more complicated to measure the flexibility[START_REF] Rudi | Modélisation et analyse de services éco-hydrauliques des réseaux de canaux et fossés des agrosystèmes méditerranéens[END_REF], porosity[START_REF] Vinatier | The Use of Photogrammetry to Construct Time Series of Vegetation Permeability to Water and Seed Transport in Agricultural Waterways[END_REF] or blockage factor[START_REF] Vinatier | From 3d grassy vegetation point cloud to hydraulic resistance: Application to close-range estimation of Manning coefficients for intermittent open channels[END_REF] of vegetation cover. Regarding the retention function (sediments and nonfloating seeds), the parameters and properties generally used to assess the effect of vegetation on the function, are those affecting the mean velocity and the turbulence (especially the drag coefficient, density, diameters of stems and height), as well as the width of patches. However, this function has been generally studied in small sections of channels due to the complexity of the processes involved. Indeed, the calculation of bed shear stress in the presence of vegetation and the quantification of turbulence and its influence on particle transport are far from being elucidated. For floating propagules (especially floating seeds), the height of the plant cover is the most influencial parameter because only emergent vegetation is involved in retention. The architecture of individual plants and the spatial distribution of the plants play a significant role in net trapping. In practice, the total area of the plants at the water surface is generally associated with seed retention. For agricultural pollutants, the biomass of living or dead vegetation in contact with them is the plant community property generally used to assess the retention function. For N and P retention, biomass is also influential because vegetation can both absorb nutrients and create interfaces for biodegradation, but the quantification of N and P decay in agricultural channels has mainly been demonstrated experimentally. Only a few studies have proposed to model it to date (see for example[START_REF] Janse | A model of ditch vegetation in relation to eutrophication[END_REF]). For bank strengthening, aboveground community parameters and properties involved in the control of the average components of water flow are those associated with the dissipation of hydraulic energy. Aboveground parts of the plant community play a role in protecting the banks' soils from hydraulic erosion. Belowground community functional parameters, such as root density and root length density, are frequently used for the prediction of soil loss under different hydraulic conditions, and the number of roots per unit area of soil and the total tensile strength associated with these roots are often used in bank stability models. The plant parameters and properties involved in the studied ecosystem functions are summarized in

Table 1 :

 1 Characteristics of velocity-fields in submerged or emergent conditions

Table 2 :

 2 Examples of equations used in literature to link plant community parameters and properties to resistance described as Re c = ud / ν and R = d p / d with d p the particle diameter. Based on the same formalisms,

			Parameters and
	tation		properties	:
	• Flexible	a,B,C
	• Non	uni-
	form cover

* or C d

Table 3 .

 3

	Ecosystem function	Associated plant community functional pa-
		rameters or properties
	Water flow regulation
	-Local scale	Drag coefficient C d , frontal area A, stem diameter d,
		height h p , density of the plant cover D, flexibility
	-Reach scale	Height h p , density of the plant cover D, flexibility,
		blockage factor B
	Sediments' retention	Same parameters and properties as water flow regu-
		lation, and width of vegetation patch w
	Pollutants' retention
	-Adsorbed on sediments	Same as for sediments' retention
	-Dissolved	Living and dead biomass
	-Absorbed	Living biomass
	Propagules' retention
	-Non floating propagules	Same as for sediments' retention
	-Floating propagules	Density D, height h p , plant architectures, spatial dis-
		tribution of plants in the channel, area of plant cover
		at the water surface
	Bank strengthening
	-Due to aboveground parts Same parameters and properties as water flow regu-
		lation, and plant architectures (for example presence
		of rosettes protecting the soils from hydraulic ero-
		sion)
	-Due to belowground parts Hydraulic effect : Root Density, Root Length Den-
		sity, total root biomass, total root volume -Mechan-
		ical effect : Type of root systems (taproot or fibrous
		roots), number of roots, tensile strenth, distribution
		of diameters in the soil profile -Hydrological effect :
		Composition and stage of development of the plant
		community

Table 3 :

 3 Recapitulative table of plant community functional parameters and properties generally used to assess the studied ecosystem functions. The reported parameters and properties are those for which there is a clearly established relationship between the parameter/property and the function, established by research studies

This work was funded by Agropolis Fondation under the reference ID 1605-034 through the 'Investissements d'avenir' programme (Labex Agro:ANR-10-LABX-0001-01). The project was also supported by funds from a PhD grant provided by Montpellier Supagro and the INRA institution via the EcoServ metaprogram.


H Guet-Revillet

M Delage

É Riverain-Gillet

J-P Jais

M-N Ungeheueur

P-H Consigny

N Lemarchand

V De Parades

Aude Nassif

O Join-Lambert

To characterize the microbiology of suppurative PSD compared to HS gluteal lesions.

Bacterial culture results (Fig 1)

). Corynebacterium, Staphylococcus, Anaerococcus and Finegoldia were the most abundant genera in unaffected gluteal HS skinfolds and controls. The microbiome of PSDs and HS gluteal lesions was predominantly anaerobic, Prevotella, Porphyromonas, Fusobacterium, and Peptoniphilus being the main taxa. The relative abundance of skin commensals (Staphylococcus and Corynebacterium) was higher in HS lesions skin samples, probably due to the sampling method.

16S ribosomal gene amplicon sequencing sequencing results (Fig

The microbiology of suppurative PSD is typical of polymicrobial anaerobic abscesses and similar to that of gluteal HS lesions. As previously reported in HS 2-5 , Prevotella and Porphyromonas are the main anaerobic taxa, establishing a new link between the two diseases and suggesting common pathophysiological features.

The Microbiology of suppurative Pilonidal Sinus Disease:

another link with Hidradenitis Suppurativa

Intergluteal lesions are associated with Hidradenitis Suppurativa (HS) in up to 27% of cases and there are accumulating evidences suggesting that pilonidal sinus disease (PSD) may be a variant form of Hidradenitis suppurativa 1 . The microbiology of HS lesions has been recently characterized, showing that a specific anaerobic flora is associated with a vast majority of HS lesions

  Fig3-Beta-diversity : principal component analysis using Bray-Curtis distance

  We compared the microbiology of 12 suppurative PSD to that of 17 intergluteal HS lesions, of 44 clinically non affected HS gluteal skinfold and of 16 in healthy subjects. Patients with PSD had no other skin lesions and were recruited at the Saint-Joseph Hospital Leopold Bellan clinic, a surgical specialized center in Paris. HS subjects and controls were recruited at the Dermatology Unit of the Centre Médical and at the ICAReB Platform of the Institut Pasteur, respectively. All sample were obtained by pus or surface skin swabbing, except for PSD samples that were obtained during surgery. No patients received antibiotics during the month preceding inclusion. All samples were analysed by prolonged bacterial cultures. 16S ribosomal RNA gene amplicon sequencing was performed on 50 randomly selected samples including 6 PSD samples.

	Background
	2-5 .
	Aim of the study
	Methods
	Cross-sectional study realised in Paris, France.
	Results
	Discussion and conclusion
	References

maia.delage-toriel@pasteur.fr, aude.nassif@pasteur.fr, olivier.join-lambert@unicaen.fr 1-Benhadou et al, Br J Dermatol, 2019 2-Guet-revillet et al Emerging Infect Dis, 2014 3-Guet-revillet et al, Clin Infect Dis, 2017 4-Ring et al, JAMA Dermatol, 2017 5-Ring et al, JEADV, 2019