Léa Kervroëdan
email: lea.kervroedan@unilasalle.fr

Romain Armand

Mathieu Saunier

Michel-Pierre Faucon

Effects of plant traits and their divergence on runoff and sediment retention in herbaceous vegetation

Keywords: Aboveground functional traits, Plant-soil-runoff interactions, Sediment transport, Sediment trapping and deposition, Soil erosion control

Plant species diversity impacts ecosystem processes, but its effects on runoff and soil erosion are not fully understood. Herbaceous vegetation control concentrated runoff and soil erosion through the increase of hydraulic roughness which leads to sediment retention. This study aims to investigate the effect of aboveground traits and their divergence (i.e., stem diameter and density, and leaf area and density) on hydraulic roughness and sediment retention in plant communities.

Methods

Runoff experimentations were performed, using a sediment input, on three mono-specific and one multi-specific conditions composed of plant species with contrasting traits furthering hydraulic roughness and sediment retention.

Results

No effect of trait diversity was found on the hydraulic roughness and sediment retention; which would be explained by the absence of an optimal space use by the traits involved in the increase of the hydraulic roughness. The dominant effect of the community-weighted traits in the plant community drives the effects of the vegetation on the hydraulic roughness and sediment retention.

Conclusion

This absence of positive effects of the trait diversity on hydraulic roughness and sediment retention constitutes an important knowledge to model runoff and soil erosion processes and to design herbaceous infrastructures for soil erosion control.

Introduction

Soil-plant interactions play a key role in the reduction of runoff and soil erosion [START_REF] Styczen | Engineering properties of vegetation[END_REF]. The effects of plant functional traits (i.e., 'morpho-physio-phenological traits which indirectly impact fitness via their effects on growth, reproduction and survival' [START_REF] Violle | Let the concept of trait be functional![END_REF])) in herbaceous vegetation have been studied on different processes generating and favouring runoff and soil erosion [START_REF] Morgan | Vegetative-Based Technologies for Erosion Control[END_REF][START_REF] Hussein | Prediction of surface flow hydrology and sediment retention upslope of a vetiver buffer strip[END_REF][START_REF] Isselin-Nondedeu | Influence of alpine plants growing on steep slopes on sediment trapping and transport by runoff[END_REF][START_REF] Burylo | Plant functional traits and species ability for sediment retention during concentrated flow erosion[END_REF][START_REF] Stokes | Ecological mitigation of hillslope instability: ten key issues facing researchers and practitioners[END_REF][START_REF] Liu | Effects of vegetation cover and road-concentrated flow on hillslope erosion in rainfall and scouring simulation tests in the Three Gorges Reservoir Area, China[END_REF][START_REF] Mekonnen | Sediment trapping with indigenous grass species showing differences in plant traits in northwest Ethiopia[END_REF].

In vegetation patches, notably under temperate climates, concentrated runoff and soil erosion are mainly controlled by the hydraulic roughness of herbaceous vegetation (i.e. frictional resistance of the vegetation in contact with the flow) which induces sediment retention by slowing the flow velocity [START_REF] Meyer | Sediment-trapping effectiveness of stiff-grass hedges[END_REF][START_REF] Akram | The links between water profile, net deposition and erosion in the design and performance of stiff grass hedges[END_REF]). More the flow velocity will be reduced, more the quantity of sediments which will be retained by the vegetation will comprise a wider variety of particle sizes, and especially smaller particles [START_REF] Haan | Design Hydrology and Sedimentology for Small Catchments[END_REF][START_REF] Syversen | Retention of soil particle fractions and phosphorus in cold-climate buffer zones[END_REF]. This indirect effect of the plant traits on the fractions of sediment particles retained constitutes an important feature to understand the vegetation effects on soil erosion, notably regarding the environmental consequences as the smaller sediment particles are the most impacting ones regarding pollutant dispersion and water quality degradation [START_REF] Syversen | Retention of soil particle fractions and phosphorus in cold-climate buffer zones[END_REF][START_REF] Liu | Major Factors Influencing the Efficacy of Vegetated Buffers on Sediment Trapping: A Review and Analysis[END_REF][START_REF] Lambrechts | Can vegetative filter strips efficiently trap trace elements during water erosion events? A flume experiment with contaminated sediments[END_REF]. The effects of functional traits on hydraulic roughness and sediment retention have been studied over the past decades, showing the effects of aboveground biomass, projected stem area (i.e. stem area towards the flow direction), stem density and diameter as well as leaf area and density [START_REF] Burylo | Plant functional traits and species ability for sediment retention during concentrated flow erosion[END_REF][START_REF] Zhu | Reducing soil erosion by improving community functional diversity in semi-arid grasslands[END_REF][START_REF] Mekonnen | Sediment trapping with indigenous grass species showing differences in plant traits in northwest Ethiopia[END_REF][START_REF] Kervroëdan | Plant functional trait effects on runoff to design herbaceous hedges for soil erosion control[END_REF]. Some of these traits are found negatively correlated (e.g. leaf area and density), implying that trait diversity and divergence could have a better impact on hydraulic roughness and sediment retention by exerting complementary effects of the traits [START_REF] Kervroëdan | Plant functional trait effects on runoff to design herbaceous hedges for soil erosion control[END_REF]. Positive effects of the trait aggregation, due to trait divergenceor functional diversityare driven by the complementarity of the traits which leads to a better space occupation inducing greater effects of the community structure and distribution on soil erosion reduction [START_REF] Zhu | Reducing soil erosion by improving community functional diversity in semi-arid grasslands[END_REF]. On the other hand, when no effects of the trait diversity are observed, it is the community-weighted traits (i.e. the mean of the trait values in the plant community) that are leading the effects on the hydraulic roughness and sediment retention [START_REF] Erktan | Morphological diversity of plant barriers does not increase sediment retention in eroded marly gullies under ecological restoration[END_REF][START_REF] Zhu | Reducing soil erosion by improving community functional diversity in semi-arid grasslands[END_REF]. As for several other ecosystem processes, plant species diversity integrating contrasting plant functional traits can influence the processes involved in soil erosion [START_REF] Naeem | Declining biodiversity can alter the performance of ecosystems[END_REF]Tilman 2001a; [START_REF] Martin | Interrill erosion at disturbed alpine sites: Effects of plant functional diversity and vegetation cover[END_REF][START_REF] Zhu | Reducing soil erosion by improving community functional diversity in semi-arid grasslands[END_REF]. The mechanisms driving the effects of plant species diversity on ecosystem processes can follow two non-exclusive hypotheses [START_REF] Loreau | Biodiversity and ecosystem functioning: recent theoretical advances[END_REF][START_REF] Garnier | Plant Functional Diversity: Organism Traits, Community Structure, and Ecosystem Properties[END_REF]. The selection hypothesis stipulates that one or more dominant species in the community would increase the overall productivity and efficiency of the community. On the other hand, the complementarity hypothesis specifies that plant species diversity effects on ecosystem processes result from a more complete space and resource uses by the trait divergence composing the plant community [START_REF] Loreau | Partitioning selection and complementarity in biodiversity experiments[END_REF][START_REF] Fargione | From selection to complementarity: shifts in the causes of biodiversity-productivity relationships in a long-term biodiversity experiment[END_REF]).

The relationships between plant trait diversity and soil erosion processes have been studied, mostly focussing on the effects of non-herbaceous plant roots on soil stabilisation and vegetation patch pattern impact on erosion rates [START_REF] Gyssels | Impact of plant roots on the resistance of soils to erosion by water: A review[END_REF][START_REF] Bautista | Plant Spatial Pattern Predicts Hillslope Runoff and Erosion in a Semiarid Mediterranean Landscape[END_REF][START_REF] Pohl | Higher plant diversity enhances soil stability in disturbed alpine ecosystems[END_REF][START_REF] Martin | Interrill erosion at disturbed alpine sites: Effects of plant functional diversity and vegetation cover[END_REF][START_REF] Berendse | Loss of plant species diversity reduces soil erosion resistance[END_REF][START_REF] Hou | Effects of plant diversity on soil erosion for different vegetation patterns[END_REF]. However, the effects of trait diversity on sediment retention are not well known and can be contrasting [START_REF] Martin | Interrill erosion at disturbed alpine sites: Effects of plant functional diversity and vegetation cover[END_REF][START_REF] Erktan | Morphological diversity of plant barriers does not increase sediment retention in eroded marly gullies under ecological restoration[END_REF][START_REF] Zhu | Reducing soil erosion by improving community functional diversity in semi-arid grasslands[END_REF][START_REF] Hou | Effects of plant diversity on soil erosion for different vegetation patterns[END_REF]. Studies focussing on the effects of functional diversity on sediment retention and erosion rates showed controversial effects on the functional diversity (i.e., 'the value, range, and relative abundance of plant functional traits in a given ecosystem' (Tilman 2001b;[START_REF] Díaz | Functional Diversityat the Crossroads between Ecosystem Functioning and Environmental Filters[END_REF])) [START_REF] Erktan | Morphological diversity of plant barriers does not increase sediment retention in eroded marly gullies under ecological restoration[END_REF][START_REF] Zhu | Reducing soil erosion by improving community functional diversity in semi-arid grasslands[END_REF].

Influenced by neighbouring, the interspecific differences in functional traits among diverse plant communities have been found to impact positively soil erosion reduction [START_REF] Zhu | Reducing soil erosion by improving community functional diversity in semi-arid grasslands[END_REF].

These debated results stress the need to deepen the knowledge on the effects of the trait diversity in herbaceous vegetation on concentrated runoff and soil erosion to increase hydraulic roughness and sediment retention. This trait-based plant-soil study aimed to examine the effects of the traits and their divergence on the hydraulic roughness and sediment retention by characterising the quantity of retained sediments and their sizes. We hypothesised that the aggregation of contrasting traits involved in the increase of the hydraulic roughness would induce a better aboveground space use and a better sediment retention. The traits and their divergence, which would have a greater effect on the hydraulic roughness, would lead to a wider variety of the size of the retained sediments by the multi-specific plant communities.

Materials and methods

Plant material

Three herbaceous species presenting contrasting aboveground morphological traits furthering the hydraulic roughness were selected from a list of 76 potential candidate species of indigenous plants from north-west Europe. This list was obtained by applying six criteria to the 3500 spermatophyte species of north-west Europe [START_REF] Lambinon | Nouvelle flore de la Belgique, du G.-D. de Luxembourg, du nord de la France et des régions voisines[END_REF] based on the identified functional types influencing runoff and soil erosion in this phytogeographical territory [START_REF] Kervroëdan | Plant functional trait effects on runoff to design herbaceous hedges for soil erosion control[END_REF]): (1) the Raunkiaer's life-form categories "herbaceous chamaephytes", "hemicryptophytes" and "geophytes" which select perennial herbaceous species with an effective soil cover all year; (2) the presence of biomass during winter, when soil erosion is observed at its highest in north-west Europe (Boardman and Poesen 2006), either dry (i.e., noncaespitose hemicryptophytes and geophytes) or fresh (i.e., herbaceous chamaephytes and caespitose hemicryptophytes); (3) the presence of stolon or rhizomes which ensure a lateral spreading growth pattern forming a homogenous cover and burying tolerance towards sediment deposition; (4) a vegetative height ≥ 20 cm to avoid any submergence of the vegetation by the water flow and ensure the efficiency towards concentrated flows [START_REF] Dillaha | Vegetative filter strips for agricultural nonpoint source pollution control[END_REF]), as the water maximal level is 20 cm in north-west European catchments; (5) a broad ecological niche which guarantee the plants development within a wide gradient of soil conditions and (6) the non-invasiveness of the plants which select non-weed species and avoid the plant spreading into agricultural fields.

To restrict the competition for light, the species were chosen considering their vegetative height range with a minimal height comprised between 40 and 70 cm. The species Carex pendula Huds., Tanacetum vulgare L. and Festuca arundinacea Schreb. were tested under three monospecific and one multi-specific communities (Fig. 1), each condition comprising three replicates. Only one multi-specific condition (comprising the three species) was tested as the aim of this study was to examine the effect of the trait diversity on the hydraulic roughness and sediment retention using contrasting traits. The multi-specific communities were composed of the three species and presented a high functional diversity, with a functional divergence (FDis, with a range between 0 and 1) of 0.88 (±0.04) [START_REF] Villéger | New multidimensional functional diversity indices for a multifaceted framework in functional ecology[END_REF].

Experimental site design

The plots were implanted at the bottom edge of a loamy field cultivated with winter wheat located in the municipality of Drosay, Normandy, France (Fig 2 .A). The soil, defined as Luvisol, was characterised by 12.5% (± 0.66%) clay, 64.3% (± 0.84%) silt and 23.2% (± 0.64%) sand with a pH of 5.3 (±0.1), 2.1% (±0.19%) of organic matter content and a crusting index of 1.8 (± 0.08).

In October 2016, one year prior experiments, the plants were planted in 1m 2 plots with a density of 53 young plants.m -2 (Fig. 1). The multi-specific plots were planted with the same proportion of each species and the young plants were placed so young plants of the same species would not be next to each other. A control condition, with no vegetation, was also tested with three replicates to compare the effects of the traits between each species. Each of the 15 plots was randomly distributed along a 50m strip and 2m were left between each plot to allow movement and limit plant spreading in the next plot (Fig. 2

.B).

Hydraulic measurements

A runoff simulator (Fig. 3) generating a flow at set discharges in controlled conditions was used to performed the experiments in October and November 2017 [START_REF] Richet | The role of vegetative barriers such as fascines and dense shrub hedges in catchment management to reduce runoff and erosion effects: Experimental evidence of efficiency, and conditions of use[END_REF][START_REF] Kervroëdan | Plant functional trait effects on runoff to design herbaceous hedges for soil erosion control[END_REF]. The simulator was composed of three parts, two Venturi channels with ultrasound probes measuring the water level in the channels (± 1.26 mm) on both ends of a channel where measurements were performed. The central channel was a 5.2 m by 0.90 m stainless steel channel with a 1.2 m bottomless part where the vegetation was located. The channel slope was measured for each plot using an automatic rotating laser (Geo Fennel EL 515plus) and was adjusted to 4.90% (± 0.13%), using wooden planks to ensure stability during experiments. The backwater levels in front of the plots were measured using six spacers placed upstream the plants. The closest spacer from the plants was 13 cm away, the next two were spaced by 25 cm, the forth spacer was 50 cm away from the third and the last two were spaced by 75 cm. Each spacer was levelled and their elevations were measured and used as elevation-known baselines for the water level measurements.

The hydraulic parameters which were measured during the experiments were the flow discharge and the water levels in the backwater area. The discharge used for the experiments was 3.6 L.s - 1 .m -1 (± 0.06 L.s -1 .m -1 ), and was continuously monitored using the upstream and downstream flowmeters. The water level measurements were carried out when the upstream and downstream discharges were equivalent. The perpendicular distance between the bottom of the spacer and the top of the water flow was measured with an aluminium ruler. Ten water level measurements were done on each spacer, every 10 cm along the spacer from one channel edge to the other. All the water level determinations were undertaken using clear water flows (Table 1).

The unit stream power (USP, m.s -1 ) was used to characterise the hydraulic roughness [START_REF] Yang | Unit stream power and sediment transport[END_REF][START_REF] Govers | Relationship between discharge, velocity and flow area for rills eroding loose, non-layered materials[END_REF]. Often used as a sediment transport capacity index, it defines the energy dissipation of the flow, per unit of time and per unit of weight, and depends on the mean flow velocity (V, m.s -1 ) and the slope (S, m.m -1 ) [START_REF] Govers | Relationship between discharge, velocity and flow area for rills eroding loose, non-layered materials[END_REF]):

𝑈𝑆𝑃 = 𝑉𝑆 (1)
The USP was calculated based on the water levels measured on the closest spacer to the vegetation. The USP is negatively related to the hydraulic roughness: the lower it is, the higher the hydraulic roughness will be.

Sediment retention

In June 2017, soil in the experimental site was taken off, sifted at 2 mm and stored until October 2017. Prior the experiments, the sediments were dried at 40°C for 72h and stored in sealed containers. After conducting the water level measurements, sediments were integrated into the flow in order to characterise the trait diversity effect on sediment retention capacity. The sediment concentration was at 11 g.L -1 (± 0.36 g.L -1 ) at a discharge of 3.6 L.s -1 .m -1 (± 0.09 L.s - 1 .m -1 ). The sediment concentration and discharge used were chosen in order to analyse the effects of traits and their divergence in plant communities on frequent erosional episodes (e.g.

annually) observed in the upper parts of catchments located in the European loess belt. Indeed, the sediment concentration of 11g.l -1 represents more than 75% of the sediment concentrations measured in small catchments and in the upper parts of the catchments in the European loess belt [START_REF] Van Dijk | Retention of water and sediment by grass strips[END_REF][START_REF] Cerdan | Sediment concentration in interrill flow: interactions between soil surface conditions, vegetation and rainfall: SEDIMENT CONCENTRATION IN INTERRILL FLOW[END_REF][START_REF] Bissonnais | Grass strip effects on runoff and soil loss[END_REF][START_REF] Delmas | Erosion and sediment budget across scale: A case study in a catchment of the European loess belt[END_REF][START_REF] Richet | The role of vegetative barriers such as fascines and dense shrub hedges in catchment management to reduce runoff and erosion effects: Experimental evidence of efficiency, and conditions of use[END_REF]).

The experiment lasted 3 min of sediment input and was followed by 30 sec of clear water to allow the last sediments to be transported. The extra 30 sec of clear water did not significantly influence the sediment particles detachment and their transport in the flow downstream the vegetation (Supplementary material). In order to estimate the quantity of sediment passing through the vegetation, a 250 ml flow sample was taken every 15 sec, from the sediment input to the end of the experiment. After the test run, the sediments deposited in the channel were collected. All the sampled sediments were oven-dried at 105°C for 48h and weighted. The quantity of trapped sediments in the vegetation was estimated from calculation. Particle granulometry was analysed using a LS 13 320 Beckman Coulter particle sizing analyser for the sediments deposited in the channel (upstream) and the ones downstream the vegetation.

The total sediment quantity downstream the vegetation was calculated using a sequential calculation method, from each sample collected. The quantity trapped within the vegetation (Swithin) was estimated as followed:

Swithin = Sinput -(Supstream + Sdownstream) (2)
where Sinput is the total quantity of sediments integrated into the flow, Supstream is the quantity deposited in the channel and Sdownstream the quantity which passed through the vegetation.

The sediment retention capacity represented the quantity of sediments upstream and within the vegetation:

Sretention = Supstream + Swithin (3)

Leaf and stem trait measurements

Six aboveground plant morphological traits (stemdensity, diameter, projected area; leafdensity, area and specific area), known to influence the hydraulic roughness and sediment retention [START_REF] Kervroëdan | Plant functional trait effects on runoff to design herbaceous hedges for soil erosion control[END_REF]), were measured on three levels along the stem: between 0 and 5 cm, 0 and 10 cm and 0 and 20 cm. Sampling collection, samples conservation and analyses methods were conducted following the standardised guidelines from [START_REF] Pérez-Harguindeguy | New handbook for standardised measurement of plant functional traits worldwide[END_REF]). The leaves were collected in sealed bags with moist tissue until measurements were performed and then, dried at 70°C for 72h. The traits were all measured within one quadrat (10 by 10 cm) per monospecific community plot and one quadrat per species within the multi-specific community plots, due to the homogeneous plant cover within the plots.

The leaf density (leaves.m -2 ) was measured for each level along the stems by counting fresh and dry leaves. The leaf area (mm 2 ) and specific area (mm 2 .mg -1 ) were determined using six representative leaves, scanned at 600dpi resolution, and the images were analysed using Gimp 2.8. Pseudoculms for sedges species (Cyperaceae) and tillers for grass species were counted with the stems to measure the stem density (stems.m -2 ). The stem diameter (mm) was measured for each level on three representative stems and was used to calculate the projected stem area (mm 2 ), using the rectangle area formula. The aboveground biomass (g) was estimated by collecting a 50 x 50 cm quadrat in each plot and dried during 72h at 70°C. The species biomass in the multi-specific plots were separated in order to determine the biomass productivity of each species.

Moreover, a density-weighted trait was calculated for each trait by multiplying the (1) leaf traits with the leaf density and (2) stem traits with the stem density; which corresponded to the density value of the traits and were dissociated from the traits with a "D" at the front of the trait appellations.

Characterisation of the trait diversity effects on the hydraulic roughness and sediment retention

All multi-specific plots were planted with an equal abundance of each species, although these abundances changed through the year to experiments. The abundance (%) of each species was estimated for each multi-specific plot.

The community-weighted traits (CWT) were calculated for each trait in the multi-specific plots by weighting each trait with the species abundance [START_REF] Díaz | Functional Diversityat the Crossroads between Ecosystem Functioning and Environmental Filters[END_REF][START_REF] Violle | Let the concept of trait be functional![END_REF]:

𝐶𝑊𝑇 𝑖 = ∑ 𝐴 𝑘,𝑖 𝑇 𝑘,𝑖 𝑛 𝑖 𝑘=1
(5)

with CWTi being the community-weighted value of the trait in the community i, Ak,i and Tk,i being, respectively, the relative abundance and the trait value of the species k in the community i and ni being the number of species in the community i.

The complementarity effect on the traits was determined using the land equivalent ratio (LER); which is the relative area required for monoculture to grow with the same productivity as when the species are in mixture [START_REF] Mead | The Concept of a 'Land Equivalent Ratio' and Advantages in Yields from Intercropping[END_REF]. The LER was calculated for each species trait in the multi-specific plots as follows:

𝐿𝐸𝑅 = 𝑝𝐿𝐸𝑅 𝐴 + 𝑝𝐿𝐸𝑅 𝐵 = 𝑇 𝐴,𝑚𝑢𝑙𝑡𝑖-𝑠𝑝𝑒𝑐𝑖𝑓𝑖𝑐 𝑇 𝐴,𝑚𝑜𝑛𝑜𝑠𝑝𝑒𝑐𝑖𝑓𝑖𝑐 + 𝑇 𝐵,𝑚𝑢𝑙𝑡𝑖-𝑠𝑝𝑒𝑐𝑖𝑓𝑖𝑐 𝑇 𝐵,𝑚𝑜𝑛𝑜𝑠𝑝𝑒𝑐𝑖𝑓𝑖𝑐 (4)
with pLERA and pLERB being the partial LERs for the species A and B; TA,multi-specific and TB,multi-specific being the trait values of the species A and B in the multi-specific conditions; and TA,monospecific and TB,monospecific being the trait values of the species A and B in their respective monospecific conditions. If LER > 1, the multi-specific condition presents an advantage for the trait value compared when the species is in monospecific condition. In the contrary, if LER < 1, there is a detrimental effect on the trait of the multi-specific condition compare to the monospecific. The pLER allows to indicate competitive effects between the species, within the multi-specific condition, using pLER < (1/n) or pLER > (1/n) as an indices, with n the species number [START_REF] Mead | The Concept of a 'Land Equivalent Ratio' and Advantages in Yields from Intercropping[END_REF][START_REF] Hector | The Effect of Diversity on Productivity: Detecting the Role of Species Complementarity[END_REF][START_REF] Darch | Inter-and intra-species intercropping of barley cultivars and legume species, as affected by soil phosphorus availability[END_REF].

Data analyses

Comparison analyses highlighting the differences between the community-weighted traits of each tested condition were carried out using ANOVA and Tukey post-hoc tests on the community-weighted trait values of the three monospecific and the multi-specific conditions.

The effects of each vegetated condition in comparison to the control condition on the unit stream power and sediment retention were tested using Kruskal-Wallis and Dunn's tests.

Comparative analyses using Mann-Whitney tests were performed on the USP and the sediments (quantities and granulometries) data, gathered under "monospecific" and "multi-specific" categories, in order to identify if there was a complementarity or a selection effect of the trait diversity on these processes.

To identify differences between the communities, ANOVA and Kruskal-Wallis analyses were computed accordingly to the normality of the data on the USP, the sediments (quantities and granulometries) and the LER data. The analyses' respective post-hoc tests Tukey and Dunn's were also performed. As the aim of the analyses was to understand the effects of the trait diversity on the tested variables, control data were excluded. All the statistical analyses were carried out using the "Stats" and "dunn.test" packages in the statistical software R (version

3.3.2).

Results

The traits in the monospecific conditions and their effects on the unit stream power and sediment retention

The comparison analyses of the trait values between each monospecific condition emphasised the contrasting leaf densities found between each species (Table 2). C. pendula presented significantly lower CW density-weighted projected stem area and CW density-weighted stem diameter with F. arundinacea and no difference with T. vulgare for the cited traits. The CW density-weighted leaf area of C. pendula was not different with F. arundinacea and larger than T. vulgare's. The CW leaf area of C. pendula presented a significant difference with the leaf area of F. arundinacea and T. vulgare. Regarding the effects on the unit stream power and sediment retention, the analyses showed no significant differences between the control and T.

vulgare, while C. pendula and F. arundinacea had significant lower unit stream power and higher quantity of retained sediments than the control. C. pendula had higher hydraulic roughness (thus, a lower unit stream power) and quantity of sediments retained than T. vulgare.

Effects of the trait diversity on the community-weighted traits involved in the increase of the hydraulic roughness and sediment retention

The comparison analyses between the CW traits in the monospecific and the multi-specific conditions showed (1) a higher CW leaf area in the C. pendula monoculture than in the multispecific condition;

(2) more important CW leaf densities (0 -5cm, 0 -10cm and 0 -20cm) in the multi-specific condition than in the T. vulgare monoculture; and (3) more important CW density-weighted leaf areas densities (0 -5cm, 0 -10cm and 0 -20cm) in the multi-specific condition than in the T. vulgare monoculture (Table 2). Regarding the LER, the biomass productivity and the leaf density (0 -5 cm; 0 -10 cm and 0 -20 cm) were higher in the multispecific communities, with a LER > 1 (Table 3). The density-weighted projected stem area (0 -5 cm) and density-weighted stem diameter (0 -5 cm) did not show differences between monospecific and multi-specific communities, with a LER = 1. Higher traits values were found for F. arundinacea, C. pendula (only the leaf area, the specific leaf area and the leaf density (0 -5 cm, 0 -10 cm and 0 -20 cm)) and T. vulgare (only the leaf density (0 -5 cm, 0 -10 cm and 0 -20 cm)) in the multi-specific communities, with partial LER > 0.33. The biomass productivity was favoured in the multi-specific condition for F. arundinacea and T. vulgare (LER > 0.33) but was not higher for C. pendula.

Trait diversity effects on the unit stream power, the sediment retention and the quantity of each size category of sediment particle retained

When averaging the three monospecific conditions under the category "monospecific", the comparison analysis of the unit steam power showed no significant difference between the monospecific and the multi-specific conditions. There were no differences between the mean of the monospecific and the multi-specific conditions for the sediment quantities upstream, within and downstream the vegetation. Similar results were also found for the sediment particle sizes.

The comparison analysis between each condition regarding the unit stream power showed that T. vulgare had a significantly lower hydraulic roughness than C. pendula and the multi-specific condition (Fig. 4). Regarding the sediment retention (quantities upstream and within the vegetation), the results showed differences between the multi-specific condition and T. vulgare, which was also different from C. pendula (Fig. 5.A). When focussing on the size categories of the sediment particles retained by the vegetation, the multi-specific condition was not different from the monospecific conditions for each category; while C. pendula had a bigger amount of each size category than T. vulgare (Fig. 5.A). F. arundinacea did not present any difference with any of the other conditions for each sediment size fraction (Fig. 5.A). The quantity of particles between 100 and 2000 µm showed no difference between each condition. The quantity of sediments downstream the vegetation was significantly higher for T. vulgare than for C. pendula and the multi-specific conditions (Fig. 5.B). The quantity of particles between 50 and 100 µm and between 100 and 2000 µm was smaller for the multi-specific and C. pendula than for T. vulgare (Fig. 5.B).

Discussion

The aboveground traits of a vegetation constitute the main drivers for generating hydraulic roughness and sediment retention in an herbaceous community. The density-weighted leaf area (product of the leaf area and the leaf density) influenced the hydraulic roughness and the sediment retention in the present study, highlighted by the lower values of this trait in the T.

vulgare condition which showed the lowest hydraulic roughness and sediment retention. The density-weighted leaf area has been emphasised as a main trait influencing the hydraulic roughness, especially when combined with an important density-weighted stem diameter in herbaceous vegetation [START_REF] Kervroëdan | Plant functional trait effects on runoff to design herbaceous hedges for soil erosion control[END_REF]. This strong relationship between the densityweighted leaf area and the hydraulic roughness and sediment retention strengthen the hypothesis observed in the literature that vegetation composed of species with a good trade-off between leaf density and leaf area (i.e. presenting an important density of long leaves), such as graminoid species, would present a greater impact on the hydraulic roughness and a higher efficiency to retain sediments [START_REF] Morgan | Vegetative-Based Technologies for Erosion Control[END_REF][START_REF] Isselin-Nondedeu | Influence of alpine plants growing on steep slopes on sediment trapping and transport by runoff[END_REF]. Better effects of the traits of graminoid species were also highlighted in the present study with a higher hydraulic roughness and sediment retention for C. pendula and F. arundinacea (with 84% and 76.2% of the sediments retained, respectively) than T. vulgare (with 69.7% of the sediments being retained). The hypothesis that vegetation composed of traits inducing a high efficiency in sediment retention would present a wider diversity of sediment particle sizes was also emphasised in the graminoid conditions due to their higher density-weighted leaf area, especially for C. pendula. The effects of herbaceous vegetation, notably graminoid species, on sediment retention regarding the particle size fractions have been studied over the past decades, especially in the concern of pollutants dispersion with the finer particles [START_REF] Meyer | Sediment-trapping effectiveness of stiff-grass hedges[END_REF][START_REF] Syversen | Retention of soil particle fractions and phosphorus in cold-climate buffer zones[END_REF][START_REF] Liu | Major Factors Influencing the Efficacy of Vegetated Buffers on Sediment Trapping: A Review and Analysis[END_REF][START_REF] Lambrechts | Can vegetative filter strips efficiently trap trace elements during water erosion events? A flume experiment with contaminated sediments[END_REF]. It has been shown that the coarser particles tend to deposit upstream the vegetation while finer one deposit within or pass through the vegetation. In our study, most of the particles coarser than 50 µm were retained by all the conditions tested with vegetation while there was a decrease of the retention efficiency for finer particle sizes, which is the same pattern found in the literature [START_REF] Meyer | Sediment-trapping effectiveness of stiff-grass hedges[END_REF][START_REF] Syversen | Retention of soil particle fractions and phosphorus in cold-climate buffer zones[END_REF]. This was especially observed for T. vulgare (for particles <2 µm) which presented significantly lower density-weighted leaf areas and thus, a lower hydraulic roughness.

The effects of the trait diversity on runoff and soil erosion processes have been emphasised, with contentious results [START_REF] Pohl | Higher plant diversity enhances soil stability in disturbed alpine ecosystems[END_REF][START_REF] Martin | Interrill erosion at disturbed alpine sites: Effects of plant functional diversity and vegetation cover[END_REF][START_REF] Erktan | Morphological diversity of plant barriers does not increase sediment retention in eroded marly gullies under ecological restoration[END_REF][START_REF] Berendse | Loss of plant species diversity reduces soil erosion resistance[END_REF][START_REF] Zhu | Reducing soil erosion by improving community functional diversity in semi-arid grasslands[END_REF]. In this study, we focussed on the effects of the aggregation of contrasting traits involved in the increase of the hydraulic roughness and sediment retention which are negatively associated. We hypothesised that the aggregation of these contrasting traits would present a complementary effect on the hydraulic roughness and sediment retention, as well as would induce a greater quantity of sediment particle sizes retained by the vegetation. However, while the trait diversity should have led to a greater hydraulic roughness, the results showed no significant increase in hydraulic roughness nor sediment retention for the multi-specific communities. Indeed, the complementarity hypothesis stipulates that the trait divergence in plant communities influences ecosystem processes and leads to a more complete space and resources use [START_REF] Loreau | Partitioning selection and complementarity in biodiversity experiments[END_REF]. This absence of complementarity effects could not be explained by the biomass productivity of the plant community, as for a greater productivity in the multi-specific condition there was no greater effects on the hydraulic roughness nor on the sediment retention, emphasising that biomass productivity was not a major factor to increase sediment retention. The absence of differences between the mean of the monospecific and the multi-specific communities indicated a dominant effect of the traits of specific species in the community on runoff and sediment transport capacity [START_REF] Grime | Benefits of plant diversity to ecosystems: immediate, filter and founder effects[END_REF][START_REF] Lorentzen | Species richness and identity affect the use of aboveground space in experimental grasslands[END_REF]).

This dominance effect would be led by the traits of F. arundinacea and C. pendula and more specifically by their CW density-weighted leaf area, emphasising the greater effect of graminoid species. These results on dominance effects of the community-weighted traits are not in accordance with the complementarity effects of plant species diversity found on runoff, soil erosion and soil aggregation capacity in other studies [START_REF] Pohl | Higher plant diversity enhances soil stability in disturbed alpine ecosystems[END_REF][START_REF] Martin | Interrill erosion at disturbed alpine sites: Effects of plant functional diversity and vegetation cover[END_REF][START_REF] Zhang | Effects of vegetation on runoff and soil erosion on reclaimed land in an opencast coal-mine dump in a loess area[END_REF]. However, these studies focussed on non-herbaceous spontaneous plant communities of mountainous or semi-arid environments, which usually comprise different (e.g. more intense) runoff and erosion episodes and adapted spontaneous vegetation towards these processes [START_REF] Guerrero-Campo | Effects of soil erosion on the floristic composition of plant communities on marl in northeast Spain[END_REF]. These opposing results could be explained by idiosyncratic effects (i.e. contrasting effects of the plant species diversity), impacted by species-traits and soil-plant interactions [START_REF] Loreau | Biodiversity and ecosystem functioning: recent theoretical advances[END_REF]. The idiosyncratic effects could be influenced by the species combination and structure in the community [START_REF] Parsons | Responses of interrill runoff and erosion rates to vegetation change in southern Arizona[END_REF][START_REF] Loreau | Biodiversity and ecosystem functioning: recent theoretical advances[END_REF][START_REF] Erktan | Morphological diversity of plant barriers does not increase sediment retention in eroded marly gullies under ecological restoration[END_REF], which is consistent with results showing the decrease of soil erosion resistance with the disappearance of species in the community [START_REF] Berendse | Loss of plant species diversity reduces soil erosion resistance[END_REF]. Moreover, an increasing proportion of species with larger stems diameters would create preferential flow paths and further runoff intensity, which would impact the effect of the trait diversity on the hydraulic roughness and sediment retention [START_REF] Parsons | Responses of interrill runoff and erosion rates to vegetation change in southern Arizona[END_REF][START_REF] Erktan | Morphological diversity of plant barriers does not increase sediment retention in eroded marly gullies under ecological restoration[END_REF]). This hypothesis is consistent with our results, showing the negative influence on the unit stream power of the traits of T. vulgare by reducing the hydraulic roughness and sediment retention within the multi-specific conditions, due to its larger stem diameters than the two other tested species. Negative effects of species with large stem diameters were also highlighted in the literature, highlighting their influence over the water path within the vegetation and the overall neutral effect of trait diversity [START_REF] Erktan | Morphological diversity of plant barriers does not increase sediment retention in eroded marly gullies under ecological restoration[END_REF]. This postulate coincides with the traits' effects found regarding the size of the sediment particles upstream the vegetation, as the preferential flow paths induced by the presence of larger stems would accelerate the flow velocity at the path's scale and further the transport of sediment particles. Indeed, while a wider variety of sediment particle sizes was expected within the retained sediments by the multispecific condition, there was no significant difference between the monospecific and the multispecific conditions for each particle size category. To understand the idiosyncratic effects on hydraulic roughness and sediment retention, perspective is to examine the effects of trait diversity on hydraulic roughness and sediment retention by using a wide gradient of functional diversity, with a variation in the abundance of the traits involved in hydraulic roughness increase, to unravel the relationship between trait diversity and sediment retention.

Conclusions

This trait-based ecohydrology study allowed to further the understanding of the effect of trait diversity on runoff and soil erosion processes. The results emphasised that there was not a better effect of trait diversity on the hydraulic roughness and sediment retention, as well as on the quantity of sediment fractions. The absence of negative effects on sediment retention suggests that multi-specific communities can be used to mitigate soil erosion and can be recommended because plant species and functional diversity could offer other ecosystem processes and services. Perspective is to examine the effects of functional diversity on several main ecosystem processes in herbaceous hedges created to mitigate soil erosion, to design multi-functional ecosystems.

Fig. 1 Experimental plots with three monospecific and one multi-specific communities.

The photos from October 2016 were taken just after the plantation and the ones from September 2017 were taken a month prior experiment.

Fig. 2 Fig. 3

 23 Fig. 2 Experimental site. A -Location of the experimental site in Drosay, France and B

Fig. 4

 4 Fig. 4 Differences between each community for the unit stream power. The bars represent

Fig. 5

 5 Fig. 5 Differences between each community for (A) the sediment quantities for each

Acknowledgements

The authors thank the funders of this study: Agence de l'Eau Seine-Normandie (Seine-Normandie Catchment Agency), Région Normandie (Normandy council) and ANRT (National Association for Technological Research). Special acknowledgements are given to Dr. Yves le Bissonnais and Dr. Freddy Rey for their scientific comments on the methods of our study and to Dr. Jean-François Ouvry and Mr Jean-Baptiste Richet for their insightful comments and technical advices.

Authors' Contributions

Léa Kervroëdan, Michel-Pierre Faucon and Romain Armand conceived the objectives and designed the methodology; Léa Kervroëdan and Mathieu Saunier collected the data; Léa Kervroëdan analysed the data; Léa Kervroëdan and Michel-Pierre Faucon led the writing of the manuscript. All authors contributed critically to the drafts and gave final approval for publication.