Quentin Laporte-Fauret
email: quentin.laporte-fauret@u-bordeaux.fr

Ana Teresa

Alonso Ayuso

Tiffany Rodolfo-Damiano

Vincent Marieu
email: vincent.marieu@u-bordeaux.fr

Bruno Castelle
email: bruno.castelle@u-bordeaux.fr

Stéphane Bujan
email: stephane.bujan@u-bordeaux.fr

David Rosebery
email: david.rosebery@onf.fr

Richard Michalet
email: richard.michalet@u-bordeaux.fr

C ; Investigation

R M ; Methodology

R.M., S.B D R ; Validation

The role of physical disturbance for litter decomposition and nutrient cycling in coastal sand dunes

Keywords: Coastal dunes, Drone surveys, Ecosystem functioning, Litter decomposition, Physical disturbance, Sand deposition, Wind abrasion

Disturbance increases ecosystem functioning in productive habitats but its effect in stressful conditions is less documented, although this is crucial for understanding the resilience of disturbed systems to natural and anthropogenic disturbances. Our goal is to assess the influence of physical disturbance for ecosystem functioning in coastal sand dunes. We set up an experimental design, including two treatments in four blocks, in a four km-long dune site from South West France. We simulated physical disturbance from marine and wind origin, digging Experimental Notches (EN), in the incipient (West EN treatment) and established foredunes (East EN treatment), respectively and compared the effects of EN to controls along transects including 13 positions from the beach to the transition dune behind ENs. We sampled litter decomposition rate, elevation variation, wind abrasion, sand grain size and vegetation composition. We also used drones to quantify sand deposition sheets during severe winter storms. Litter decomposition rate was the highest where sand accumulated the most, at the ecotone between the established foredune and transition dune and in the East EN treatment. This increase of ecosystem functioning was correlated to wind patterns. However, there was also a strong alongshore variability, with important sand deposition sheets occurring in some blocks depending on dune geomorphology. Vegetation composition was mainly influenced by shoreline distance, but also by the block and EN treatment, with a strong interaction between these three effects. We conclude that physical disturbance increase ecosystem functioning in the stressful conditions of the Atlantic sand dunes, only when sand accumulates, whereas excessive disturbances enhancing sand erosion are not favorable for ecosystem functioning.

Introduction

The role of disturbance for community structure and ecosystem functioning has fascinated ecologists for decades (McArthur and Wilson 1963;[START_REF] Grime | Competitive exclusion in herbaceous vegetation[END_REF][START_REF] Connell | Diversity in tropical rain forests and coral reefs[END_REF][START_REF] Huston | A general hypothesis of species diversity[END_REF]Paine and Levine 1981;Pickett and White 1985). [START_REF] Grime | Competitive exclusion in herbaceous vegetation[END_REF] proposed to separate disturbance from environmental stress to straightforwardly assessing plant species functional strategies, community richness and composition, and ecosystem functioning in natural and managed ecosystems. Disturbance can be defined as a decrease in plant biomass due to either biotic or abiotic factors (herbivory, trampling, ploughing, fire, flooding, soil erosion, sediment accumulation) and stress as a decrease in plant growth and community productivity due to mainly four environmental constraints, shade, drought, oligotrophy and sub-optimal temperatures [START_REF] Grime | Competitive exclusion in herbaceous vegetation[END_REF][START_REF] Grime | Vegetation classification by reference to strategies[END_REF]. This distinction is crucial to understand the role of disturbance for ecosystem functioning since stress is always related to a decrease in community productivity, whereas disturbance can show different relationships with productivity.

Concerning plant species richness, [START_REF] Huston | A general hypothesis of species diversity[END_REF] proposed that physical disturbance decreases species richness in stressful environments but promotes it in favorable ones due to decreasing competition. Thus, [START_REF] Grime | Competitive exclusion in herbaceous vegetation[END_REF], [START_REF] Huston | A general hypothesis of species diversity[END_REF] (see Huston, 2014 for a synthesis) proposed the occurrence of a unimodal (i.e., highest diversity at mid-position along the gradient) relationship between plant species richness and stress, disturbance, and community biomass. This hypothesis has been supported by a number of empirical studies, for example [START_REF] Ross | Can mowing restore boreal rich-fen vegetation in the face of climate change?[END_REF] and [START_REF] Maalouf | Integrating climate change into calcareous grassland management[END_REF] for productive herbaceous communities and stressful calcareous grasslands, respectively. However, this relationship has been recently debated [START_REF] Adler | Productivity is a poor predictor of plant species richness[END_REF][START_REF] Fraser | Response to comment on "worldwide evidence of a unimodal relationship between productivity and plant species richness[END_REF].

Ecosystem functioning refers to the ecological processes that control the fluxes of energy, nutrients and organic matter through an environment (e.g., decomposition, primary production and nutrient cycling) [START_REF] Cardinale | Biodiversity loss and its impact on humanity[END_REF]. It is also widely acknowledged that the absence of disturbance in low stress conditions leads to a progressive decline in ecosystem functioning. For example, [START_REF] Wardle | Ecosystem properties and forest decline in contrasting long-term chronosequences[END_REF] studying the long-term evolution of six forest ecosystems from tropical, temperate, and boreal biomes, showed that litter decomposition, biomass, and decomposer microbe activity decreased over time in absence of major disturbance, mainly due to increasing phosphorus limitation. [START_REF] Fortunel | Leaf traits capture the effects of land use changes and climate on litter decomposability of grasslands across Europe[END_REF] also found an overall decrease in ecosystem functioning, measured by litter quality and decomposition, with decreasing disturbance for herbaceous systems from ten European sites exhibiting moderate to low stress conditions. Finally, [START_REF] Peltzer | Understanding ecosystem retrogression[END_REF] reviewed the findings from studies of longterm chronosequences assessing nutrient cycling for systems spanning the boreal, temperate, and subtropical zones. Although they also found an overall decrease in ecosystem functioning through time, they stressed the occurrence of exceptions in arid climates with no ecosystem retrogression or slower depletion in nutrients. The depletion of nutrients through time is highly dependent on climate drainage [START_REF] Huston | Precipitation, soils, NPP, and biodiversity: Resurrection of Albrecht's curve[END_REF][START_REF] Delgado-Baquerizo | Decoupling of soil nutrient cycles as a function of aridity in global drylands[END_REF], with increasing phosphorus limitation and decreasing productivity during succession in wet climates only. This explains why nutrient cycling is improved by disturbance in wet climates.

However, if disturbance (e.g., wildfires, grazing) is essential to maintain ecosystem functioning in low stress conditions, its role in stressful systems is less known, with contrasting results, either increases or decreases of ecosystem functioning, depending on disturbance type [START_REF] Deng | Grassland responses to grazing disturbance: plant diversity changes with grazing intensity in a desert steppe[END_REF][START_REF] Dimitrakopoulos | The interactive effects of fire and diversity on short-term responses of ecosystem processes in experimental Mediterranean grasslands[END_REF][START_REF] Wardle | The influence of island area on ecosystem properties[END_REF]. Additionally, even in wet climates, changing and intensifying disturbance regimes has been shown to drive positive feedback loops between vegetation composition, structure, and ecosystem function that could reduce ecosystem resilience and trigger ecosystem collapse [START_REF] Lindenmayer | Avoiding ecosystem collapse in managed forest ecosystems[END_REF][START_REF] Bowd | Long-term impacts of wildfire and logging on forest soils[END_REF]. Such effects are even more likely to occur in stressful systems with lower resilience to natural and anthropogenic disturbances [START_REF] Kéfi | Spatial vegetation patterns and imminent desertification in Mediterranean arid ecosystems[END_REF]. Thus, the role of disturbance for ecosystem functioning is not clearly established in stressful systems and its occurrence might be more dependent on the frequency and intensity of disturbances than in wet climates. The resilience of ecosystems to natural and anthropogenic disturbances is highly dependent on the ability of plant species to recolonize disturbed habitats and, thus, to plant growth and ecosystem functioning. If disturbance is decreasing ecosystem functioning and, thus, plant growth and community biomass in stressful systems, this may induce a collapse of ecological communities and ecosystem services [START_REF] Michalet | Is facilitation in arid environments the result of direct or complex interactions?[END_REF]Kefi et al. 2007). This will support the CSR model of [START_REF] Grime | Vegetation classification by reference to strategies[END_REF] that consider there is no viable strategy for plant in highly stressed and disturbed systems. Therefore, increasing our knowledge on the role of physical disturbance for ecosystem functioning in stressful systems is key for managing and restoring disturbed stressful ecosystems, such as arid ecosystems or coastal sand dunes.

We choose to assess in coastal dunes the role of physical disturbance for ecosystem functioning in stressful environments since community structure and ecosystem functioning are known to be mainly driven by these two environmental constraints in these systems [START_REF] Maun | Adaptations of plants to burial in coastal sand dunes[END_REF][START_REF] Forey | The relative importance of disturbance and environmental stress at local and regional scales in French coastal sand dunes[END_REF]. Coastal dunes are generally occurring along coasts having sufficient sand supply, prevailing onshore winds and the presence of vegetation, or other obstructions, to trap the sand transported by the wind (Nordstorm, 2015). They are subjected to wave action, likely to induce drastic coastal erosion during storms [START_REF] Castelle | Impact of the winter 2013-2014 series of severe Western Europe storms on a double-barred sandy coast: Beach and dune erosion and megacusp embayments[END_REF]Guisado-Pintado and Jackson 2017). Additionally, coastal dunes are highly influenced by wind processes, transporting sand from the beach to the dune [START_REF] Hesp | Aeolian environments: coastal dunes[END_REF], and the presence of plants limiting wind erosion by trapping sand with their aerial parts [START_REF] Zarnetske | Biophysical feedback mediates effects of invasive grasses on coastal dune shape[END_REF].

Interactions between these physical and biotic processes can lead to the formation of blowouts in coastal dunes [START_REF] Hesp | Foredunes and blowouts: initiation, geomorphology and dynamics[END_REF], promoting sediment transfers from the beach to the back dune by wind acceleration in the blowout [START_REF] Hesp | Aeolian environments: coastal dunes[END_REF].

Plant communities in costal dunes are subjected to a large range of environmental stresses such as saline spray, swash inundation (principally in the beach and incipient foredune), drought, nutrient deficiency [START_REF] Hesp | Ecological processes and plant adaptations on coastal dunes[END_REF][START_REF] Hesp | Disturbance in coastal dune ecosystems[END_REF][START_REF] Martinez | Coastal dunes: Ecology and conservation[END_REF].

However, in dunes from wet temperate climates where salt can be washed from plants and through soils during heavy rain events, disturbance by sand deposition has been shown to be the crucial factor driving plant community zonation before salinity (Moreno-Cassola, 1986;[START_REF] Maun | Adaptations of plants to burial in coastal sand dunes[END_REF][START_REF] Forey | The relative importance of disturbance and environmental stress at local and regional scales in French coastal sand dunes[END_REF]. [START_REF] Maun | Adaptations enhancing survival and establishment of seedlings on coastal dune systems[END_REF] also showed that partial sand burial, for some species, stimulated growth of ramets, stolon, roots and leaves leading to an increase of total leaf area, number of tillers and total dry biomass. Moreover, sand deposition also increases soil volume, soil resources and mycorrhizae activity promoting ecosystem functioning [START_REF] Maun | Adaptations of plants to burial in coastal sand dunes[END_REF][START_REF] Forey | The relative importance of disturbance and environmental stress at local and regional scales in French coastal sand dunes[END_REF]. However, in these studies, the effects of increasing disturbance on ecosystem functioning might have been confounded by a decrease in stress also occurring towards the ocean, as observed in salt marches by [START_REF] Proença | Intraspecific facilitation explains the spread of the invasive engineer Spartina anglica in Atlantic salt marshes[END_REF]. Indeed, [START_REF] Forey | The relative importance of disturbance and environmental stress at local and regional scales in French coastal sand dunes[END_REF][START_REF] Forey | Does disturbance drive the collapse of biotic interactions at the severe end of a diversity-biomass gradient?[END_REF] have documented a beneficial effect of the ocean spray for plant growth and survival in the incipient foredune, with lower vapor pressure deficit values and maximum temperatures than in the inland transition dunes protected by the incipient foredune and the established foredune. In order to assess the potential role of disturbance for ecosystem functioning in stressed environments, we have to control for variation in stress.

Recently, scientists and coastal dune stakeholders have been attempting to reintroduce a certain dynamic in stabilized coastal dunes characterized by high plant cover and biomass [START_REF] Creer | Is 're-mobilisation' nature conservation or nature destruction? A commentary[END_REF][START_REF] Pye | Is 're-mobilisation' nature restoration or nature destruction? A commentary. Discussion[END_REF][START_REF] Arens | Comment on 'Is 're-mobilisation' nature restoration or nature destruction? A commentary[END_REF]. This is particularly the case in Northern Europe where some remobilization projects are attempting to restore a dynamic of sediment transport from the beach to the back of the dune by removing vegetation or foredune notch excavation [START_REF] Eamer | Geomorphic and sediment volume responses of a coastal dune complex following invasive vegetation removal[END_REF][START_REF] Konlechner | Evolution of foredune texture following dynamic restoration, Doughboy Bay, Stewart Island, New Zealand[END_REF][START_REF] Arens | Dynamic dune management, integrating objectives of nature development and coastal safety: examples from the Netherlands[END_REF][START_REF] Kuipers | Coastal dunes management strategies and practices: perspectives and case studies[END_REF]. In these low-stress coastal dunes, these methods have been shown to be successful in increasing sand transport and thus disturbance in the dunes [START_REF] Ruessink | Coastal dune dynamics in response to excavated foredune notches[END_REF], leading to an increase in diversity, by reducing competition, and a rejuvenation of the soils [START_REF] Brunbjerg | Disturbance in dry coastal dunes in Denmark promotes diversity of plants and arthropods[END_REF][START_REF] Nordstrom | Increasing the dynamism of coastal landforms by modifying shore protection methods: Examples from the eastern German Baltic Sea Coast[END_REF], confirming the CSR model [START_REF] Grime | Vegetation classification by reference to strategies[END_REF]. In contrast, sand remobilization in dunes from southern latitudes with lower summer rainfall and, thus, higher stress may reduce ecosystem functioning and diversity, consistent to [START_REF] Huston | A general hypothesis of species diversity[END_REF]. To the best of our knowledge, there has been no experimental study assessing the impact of sand remobilization on ecosystem functioning in more stressed coastal dunes. These impacts are crucial to assess in the context of changing management paradigms.

We designed an original experiment in the coastal sand dunes of South West France, where we applied experimental disturbances simulating blowout effects from marine and aeolian origins at two positions along the complex stress/disturbance gradient. Indeed, in northern Europe, some studies have attempted to reintroduce sedimentary dynamics within the coastal dunes by blowout restoration or notches excavation (Van Boxel et al., 1997;Pye et Boltt, 2016;[START_REF] Ruessink | Coastal dune dynamics in response to excavated foredune notches[END_REF]. However, these studies have only focused on morphological changes without assessing the impact on plant communities and ecosystems. As interactions between physical and biological processes play a major role in coastal dunes evolution while evolving across many spatial and temporal scales, it is important to set up experiments able to understand these interactions across different dune habitats. We choose to quantify litter decay as good proxy of nutrient cycling and availability along transects delineated from the beach to the back dunes, along the experimental blowouts and in controls. We also measured in all treatments environmental variables (i.e., wind abrasion, elevation variation, and grain size, [START_REF] Forey | The relative importance of disturbance and environmental stress at local and regional scales in French coastal sand dunes[END_REF] and plant species composition as indicator of community response to environmental disturbances and changes in ecosystem functioning. Additionally, in order to take into account the context-dependency of the effect of disturbance [START_REF] Grime | Vegetation classification by reference to strategies[END_REF], we also assessed if community and ecosystem responses were affected by the beach dune system geomorphology [START_REF] Davidson-Arnott | Introduction to coastal process-es and geomorphology[END_REF]. We aim to answer the following questions: (1) do contrasting disturbance types (e.g., marine and aeolian) have different effects on ecosystem functioning and (2) does dune geomorphology (e.g., foredune elevation, oceanward slope and width, transition dune elevation) alter the effect of disturbance on ecosystem functioning.

Material and methods

Study area

The study site, Truc Vert beach, is a representative beach of the 110 km long Gironde sandy coast, in SW France (Fig 1.a). The climate is temperate oceanic with mean annual precipitations of 900 mm mainly during winter period (data were extracted from the website of METEO FRANCE, see supp mat 1 & 2). This open sandy beach of high energy is meso-to macro-tidal with an average tidal range of 3.7 m during spring tides [START_REF] Castelle | Impact of the winter 2013-2014 series of severe Western Europe storms on a double-barred sandy coast: Beach and dune erosion and megacusp embayments[END_REF]. Due to its remoteness, this beach represents an ideal natural laboratory that has been extensively studied for sediment transport and beach morphodynamics [START_REF] Castelle | Foredune morphological changes and beach recovery from the extreme 2013/2014 winter at a high-energy sandy coast[END_REF], plant-plant interaction experiments and community composition [START_REF] Forey | The relative importance of disturbance and environmental stress at local and regional scales in French coastal sand dunes[END_REF][START_REF] Forey | Spatial patterns of association at local and regional scales in coastal sand dune communities[END_REF][START_REF] Forey | Does disturbance drive the collapse of biotic interactions at the severe end of a diversity-biomass gradient?[END_REF][START_REF] Bagousse-Pinguet | Disentangling the effects of water and nutrients for studying the outcome of plant interactions in sand dune ecosystems[END_REF]. The Truc Vert beach coastline has remained stable over the past 65 years, even after being exposed to major storm events [START_REF] Castelle | Spatial and temporal patterns of shoreline change of a 280-km high-energy disrupted sandy coast from 1950 to 2014: SW France[END_REF]. It is bordered by a large coastal barrier system with significant foredune system (250 m wide and 20-25 m height) part of which was re-contoured since the 60s, after severe storms in 20s and World War II. The National Forest Office in charge of the management of the dunes has undertaken extensive dune profiling and plant species planting work to renovate the dune system until the 80s. Since then, the dune has always been softly managed by vegetation planting, particularly to prevent the development of blowouts. In terms of vegetation, three dune types from the beach to the forest are distinguished, each representing a particular habitat with (1) the incipient foredune, in direct contact with the upper beach and dominated by pioneer species as Elymus farctus and Euphorbia paralias, (2) the established foredune, dominated by Ammophila

Experimental design

The experiment was carried out on a 4 km-long alongshore and 500 m cross-shore dune system.

The site was subdivided into four 500 m-long alongshore blocks, guaranteeing spatial replication. Blocks were numerated from one to four from the south to the north (Fig. 1 The quadrat number always represented the same habitat through the different transects, even if the shoreline distance can slightly differ between transects. This approach was chosen in order to better capture vegetation changes, which should preferentially occur at the ecotones. Thus, our design included four West ENs, four East ENs and height control transects and 208 sampling quadrats (13 positions X four transects X four blocks), for which the spatial coordinates were monitored using a differential GPS.

Environmental variables and litter decomposition

Several environmental variables were monitored in 2018 in order to quantify the disturbance inducted by the ENs, together with the quantification of litter decomposition that was chosen as indicator of ecosystem functioning. Elevation monitoring was carried out on the 208 sampling quadrats (13 quadrats X 4 transects X 4 blocks), whereas the monitoring of other environmental variables (i.e., wind abrasive potential and grain size), and litter decomposition were carried out at five specific positions in each transect, at the ecotones (positions 7 and 11) and at the center of each dune habitats (positions 4, 9 and 13), providing a total of 80 measurement points (5 positions X 4 transects X 4 blocks) (see grey plots on Fig. 1.c). Sand deposition, the major factor of disturbance in coastal dunes [START_REF] Maun | Adaptations of plants to burial in coastal sand dunes[END_REF][START_REF] Forey | The relative importance of disturbance and environmental stress at local and regional scales in French coastal sand dunes[END_REF], was quantified with elevation measurements at the 208 points. We used a Trimble® differential GPS and a known base, resulting in vertical and horizontal accuracy of 0.015 m and 0.008 m, respectively. This monitoring was carried out just after the EN set up and during the next 3 seasons (i.e., Winter: March 2018, Spring: May 2018 and Summer: August 2018).

Comparison between the elevations of each season allowed to identify the quadrats that have been subjected to erosion (negative Dz) or accretion (positive Dz) processes according to equation 1:

𝐷𝑧 = 𝑧 𝑆𝑢𝑚𝑚𝑒𝑟 2018 -𝑧 𝐴𝑓𝑡𝑒𝑟 𝐸𝑇𝑁 (1)
where Dz represents the elevation variation between two seasons and z the quadrat elevation at the chosen season (a Dz equal to 0 indicates that there has been no elevation change).

Wind abrasive potential was measured indirectly by quantifying the loss of flag material due to wind action [START_REF] Rutter | Tattering of Flags under Controlled Conditions[END_REF][START_REF] Forey | The relative importance of disturbance and environmental stress at local and regional scales in French coastal sand dunes[END_REF]. Indeed, the cotton surface of the flag, sensitive to the sand abrasion transported by the wind, loses surface area by abrasion depending on wind intensity and frequency. The

𝑆𝑙 = 100 - 𝑆𝑟 × 100 𝑆𝑖 (2)
where Sr is the surface remaining after exposure to the wind (m²) and Si the initial surface (m²).

Sand grains were collected in sampling tubes (0,5 x 8 cm) on the ground surface at the specific positions in each transect in mid-March 2018. They have been dried before measuring their size by a laser diffraction microgranulometer (from Malvern Panalytical®). As the data had a unimodal distribution, the median diameter of the sand grains of each sample was chosen to characterize the grain size at each position.

The rate of organic matter decomposition, which indirectly refers to soil fertility and therefore ecosystem functioning [START_REF] Fortunel | Leaf traits capture the effects of land use changes and climate on litter decomposability of grasslands across Europe[END_REF], was assessed using the litterbag method, In order to assess whether geomorphology played a role in these sand depositions, the cross-shore profiles of the 16 transect were extracted from the DSM in order to calculate geomorphological parameters of the foredune (i.e., oceanward slope, elevation and cross-shore wide), the transition dune (i.e., elevation) and the elevation difference between the foredune and the transition dune (see App1.a for a four km alongshore averaged profile of Truc Vert with precision about geomorphological parameters). Then the large sand deposition sheets were digitized from the DSMs and a surface and volume value were assigned to each transect when they were present within 25m alongshore on either side of the transect.

Vegetation sampling

Plant community composition was monitored, with a seasonal frequency (Winter: mid March 2018, Spring: mid May 2018 and Summer: late August 2018) at each of the 208 positions. We quantified the cover of each species using a 1 m² quadrat with a mesh size of 0.2 m (Goodall 1952). Plants or their vertical projections located at the 36-line intersections were recorded to calculate their percent cover. An abundance of 0.5% was assigned to species present in the quadrat but only outside these intersections.

Statistical analysis

Elevation variation, wind abrasive potential and grain size were analyzed with mixed models with Treatment, Position and their interaction as fixed effects; and Treatment nested within block as random effects. Litter decay was also analyzed with mixed models with Treatment, Position, Species and their interactions as fixed effects; and Species nested within Treatment, which is nested within block as random effect. Variance analyses (ANOVA) were performed on theses mixed models. Least Significant Difference (LSD) tests were used when there was a significant position effect, and sample t-tests to test for significant differences from zero values.

A PCA was performed to search for relationships between, litter decay of the two species, and four environmental variables (wind intensity, elevation variation, sand grain size and distance to the shoreline). We then used, a two-way ANOVA model to assess the effects of the position, EN treatments and their interaction on plot scores along the two first axes of the PCA. Vegetation data was analyzed with a Correspondence Analysis. Plots without species and species with a frequency lower than four were removed before the CA. A three-way ANCOVA was conducted on plot scores along the two first axes of the CA, in order to assess the effects of the block (qualitative data), the position (quantitative data), the EN (qualitative data) and the season (qualitative data) treatments and their interactions on species composition. For the position treatment, we used the shoreline distance as a continuous variable with also a quadratic expression to detect concave or convex shapes.

Finally, in order to detect if differences in geomorphology among blocks may have affected our results and in particular sand deposition, we assessed geomorphological block effects, the EN treatments and their interaction on the sand deposits surfaces and volumes observed behind the established foredune with a two-way ANOVA. We also search for relationships between the sand deposits surface and the volume and the dune morphological parameters (i.e., oceanward foredune slope, foredune elevation, foredune width, transition dune elevation and difference between foredune and transition dune elevation) at the level of each transect by independent linear regressions. Finally, we assessed the geomorphological block effects and the EN treatments and their interaction on the foredune oceanward slope and the established foredune width with a two-way ANOVA. All statistical analyses were performed using the software R (version 1.2.5033).

Results

Mixed models and PCA

PCA axis 1 was primarily correlated to litter decomposition rates and transect positions and secondarily to elevation variation, wind intensity and the EN treatment (Fig. 2). The fastest rate of decomposition for the two species (with Elymus farctus litter decomposing almost twice faster than Helichrysum stoechas one, as shown by the highly significant species effect in Table 1 and see App. 2) occurred at positions 9 (and then 7 and 11) of the East EN treatment (positive side of axis 1 in Fig. 2 and highly significant position and marginally significant treatment effects in Table 2). The highest elevation increase due to sand deposition and highest wind intensity were also observed at these positions (Table 1 and see results of LSD and sample ttests in Fig. 3 for elevation and App. 4 for wind). Axis 2 was explained by shoreline distance and grain size, the latter decreasing with increasing distance to the shoreline (Fig. 2 and Tables 1 &2 and see LSD test for the position effect in App. 3). Wind intensity contributed to the two axes with the highest values in the established foredune and for the East EN treatment (Table 1 and see LSD test for the position treatment in App. 4). All together, these results highlight that the most important variation in environmental conditions affecting decomposition rates occurred during our experiment at the crest and behind the established foredune, and in particular for the East EN treatment.

Large-scale dune geomorphology

Sand deposition sheets mainly occurred at three transects in block 2 and four in block 3 (App.

5), and extended from the established foredune crest up to a hundred meters within the transition dune. They reached a surface and volume of 130 354 m² and 1827 m 3 for block 2 and 130 821 m 2 and 1878 m 3 for block 3. The surface and volume of these sand deposition sheets were not significantly different between blocks nor EN treatments (Table 3.a). However, there were highly significant correlations between these sand deposition sheets parameters and both the oceanward slope and width of the foredune (Table 3.b). Finally, only the oceanward foredune slope was significantly different among blocks with a mean slope of 0.24, 0.17, 0.15 and 0.23 for the blocks 1 to 4, respectively (Table 3.

c).

Vegetation

There was a strong arc effect on CA 1-2 diagram showing that one main factor was driving differences in species composition along the two axes (Fig. 4.a). Indeed, the quadratic distance (to the shoreline) was highly significantly correlated to both axes in the ANCOVA on plot scores, with the incipient foredune community at the extreme upper left of the cloud and the transition dune community at its extreme upper right (App. 6). However, two other treatments also significantly explained the CA axes, the EN treatment for axis 1 and the season for axis 2.

Additionally, there was a highly significant block effect on both axes, as well as interactions between block and distance and between block and quadratic distance on both axes. Thus, CA axis 1 appeared to be primarily correlated to species tolerance to disturbance by sand deposition (see also Fig. 4.b) and CA axis 2 to species phenology. The effect of EN treatment on axis 1 can be easily understood with the results displayed in Fig. 4.b, with lower scores on axis 1 for the East EN treatment than for the two other EN treatments, highlighting that EN dug in the established foredune induced an increase disturbance by sand deposition, in particular behind the foredune crest at the ecotone with the transition dune. Additionally, the interaction between the block, EN treatment and distance to the shoreline on axis 1 can be understood in App. 7, with the scores on axis 1 of the East EN treatment decreasing faster than those of the West and Control EN treatments with increasing distance to the shoreline, in particular for blocks 3 and 2. Overall, blocks 1 and 4 appeared to be less disturbed, which is consistent with the results of App. 5 showing that the sand deposition sheets occurred preferentially in blocks 2 and 3.

Discussion

In response to our first question, we found that only the EN positioned close to the foredune crest (East EN), simulating wind-driven blowouts, induced an increase in litter decomposition at the ecotone between the established foredune and the transition dune, where sand deposition was also the highest. Additionally, across the four km of our study site, we found, in response to our second question, strong spatial variation in sand deposition, correlated to dune geomorphology and contributing to explain changes in vegetation in interaction with our treatments. Our results suggest that physical disturbance is likely to increase ecosystem functioning in stressed coastal sand dunes as documented in conditions of low stress, but only in particular conditions of sediment deposition, which are highly variable across time and space.

Our knowledge of the effects of disturbance on nutrient cycling and productivity in stressful environments is limited. Additionally, previous studies that have shown positive effects of disturbance on ecosystem functioning in stressful systems such as coastal dunes and salt marshes were conducted along complex environmental gradients. Variation in disturbance were likely to be confounded by variation in stress because both environmental constraints varied in opposite directions along a gradient of distance to the ocean or of inundation (e.g., [START_REF] Forey | The relative importance of disturbance and environmental stress at local and regional scales in French coastal sand dunes[END_REF][START_REF] Forey | Does disturbance drive the collapse of biotic interactions at the severe end of a diversity-biomass gradient?[END_REF][START_REF] Proença | Intraspecific facilitation explains the spread of the invasive engineer Spartina anglica in Atlantic salt marshes[END_REF].

In our study, we controlled stress variation by applying EN at two positions along the complex disturbance/stress gradient and comparing community and ecosystem responses to controls. We showed that only the East EN, positioned in the foredune crest and simulating natural blowouts with a wind origin, induced a positive effect on litter decomposition and, thus, ecosystem functioning. The West EN positioned in a less stressful position along the gradient, closer to the ocean spray in the incipient foredune and simulating natural blowouts with a marine origin, had no effect on both vegetation composition, maybe because the pioneer species in incipient foredune are already highly adapted to disturbance. These differences were not likely to be explained by different positions along the stress gradient, but rather by patterns of sand deposition. Indeed, the PCA results showed that there was along the first axis a highly significant correlation between litter decomposition, flag abrasion by wind and changes in elevation due to sand accretion. Additionally, variation in these direct variables were highly explained by both the position along the transect and the East EN. Finally, changes in vegetation composition along the first CA axis were also significantly explained by the position along the gradient and the East EN treatment, with more disturbed vegetation behind the East experimental blowout.

Results of flag surface degradation measurements allowed to indirectly highlighting the wind speed profiles from the beach to the transition dune [START_REF] Rutter | Tattering of Flags under Controlled Conditions[END_REF]. The presence of coastal dunes generates a wind flow acceleration, promoting sand transport on the upwind dune side, then deceleration, promoting sand deposition on the leeward dune side, respectively by streamlines compressing and decompressing [START_REF] Walker | Fundamentals of Aeolian sediment transport: airflow overdunes[END_REF]. This flag surface degradation was even more marked in the East EN, in particular at the dune crest position (position 9). Indeed, the EN was set up slightly West of this position and the blowouts are known to be depressions causing wind flow acceleration with a greater sand transport [START_REF] Hesp | Foredunes and blowouts: initiation, geomorphology and dynamics[END_REF]. This interaction between the EN treatment and wind intensity may explain the greater sand deposition observed behind the crest dune in the East treatment.

Concerning sand erosion and accretion, Truc Vert beach is subject to the impact of winter storms [START_REF] Castelle | Impact of the winter 2013-2014 series of severe Western Europe storms on a double-barred sandy coast: Beach and dune erosion and megacusp embayments[END_REF], which caused significant beach erosion with the formation of localized erosion cliffs on the incipient foredune during winter [START_REF] Castelle | Foredune morphological changes and beach recovery from the extreme 2013/2014 winter at a high-energy sandy coast[END_REF]-2018(Laporte-Fauret et al., 2019 and Fig. 2). In contrast, significant accretion rates were monitored from the established foredune crest to the transition dune, with a significantly higher deposition for the East EN (Fig. 3). This gradient of elevation variation on the Truc Vert beach system during our study contrasts with the study conducted by [START_REF] Forey | The relative importance of disturbance and environmental stress at local and regional scales in French coastal sand dunes[END_REF] along the whole Aquitaine coast, which showed that the maximum sand deposition occurred in the incipient foredune with a regular decrease towards the transition dune. This divergence can be explained by likely both the higher precision of measurements of sand deposition in our study than in that of [START_REF] Forey | The relative importance of disturbance and environmental stress at local and regional scales in French coastal sand dunes[END_REF] and the absence of major storm events favoring sand erosion and deposition during their study [START_REF] Dodet | Beach recovery from extreme storm activity during the 2013-14 winter along the Atlantic coast of Europe[END_REF]. This latter difference might also explain why the West EN had no effect in our study. It is very likely that the dominance of erosion downstream the West EN, due to a year with important storm events, impeded sand deposition and any increase in litter decomposition. The sand deposition occurred more easily downstream East EN, because of its close proximity to the foredune crest and, thus, to the strong wind flow deceleration occurring after the dune crest on its leeward side.

The highest decomposition rates were measured in the upper established foredune positions for both species, positions where sand deposition was also the highest. [START_REF] Forey | The relative importance of disturbance and environmental stress at local and regional scales in French coastal sand dunes[END_REF] also found that the highest decomposition occurred at the position where sand deposition was the highest. However, in her study both sand deposition and litter decomposition were the highest in the incipient foredune, likely due to a year with lower storm event. In our study, although the West EN treatment was positioned closer to the wind spray than the East EN, decomposition was lower in the former, which strongly suggests that sand deposition rather than ocean spray controlled litter decomposition. Increased disturbance increases soil fertility and litter decomposition through plant tissues rejuvenation and reduced nutrient uptake [START_REF] Grime | Vegetation classification by reference to strategies[END_REF][START_REF] Wardle | Ecosystem properties and forest decline in contrasting long-term chronosequences[END_REF][START_REF] Davis | Fluctuating resources in plant communities: a general theory of invisibility[END_REF]. However, our results and the study of [START_REF] Forey | The relative importance of disturbance and environmental stress at local and regional scales in French coastal sand dunes[END_REF] suggest that decomposition rates might not increase when disturbance is too high, with dominant sand erosion. Disturbance might increase ecosystem functioning in stressful environments only when soils or sediments accumulate but not when soil erosion dominates due to excessive disturbance. This might explain also why productivity decreases in alpine and dry systems on convex topographic positions when disturbance remove soils [START_REF] Michalet | Is facilitation in arid environments the result of direct or complex interactions?[END_REF], whereas productivity increases with disturbance in floodplains, salt marshes, coastal dunes and avalanche slopes in wet mountains where soils or sediments accumulate [START_REF] Forey | The relative importance of disturbance and environmental stress at local and regional scales in French coastal sand dunes[END_REF][START_REF] Proença | Intraspecific facilitation explains the spread of the invasive engineer Spartina anglica in Atlantic salt marshes[END_REF]. When disturbance is too high in those latter systems like in our West EN treatment, erosion dominates over sediment accretion and ecosystem functioning decreases, through plant and soil removal, and eventually collapses [START_REF] Kéfi | Spatial vegetation patterns and imminent desertification in Mediterranean arid ecosystems[END_REF][START_REF] Bowd | Long-term impacts of wildfire and logging on forest soils[END_REF].

The surface and volume of the important sand deposition sheets that we observed from the foredune to the transition dune [START_REF] Laporte-Fauret | Low-Cost UAV for High-Resolution and Large-Scale Coastal Dune Change Monitoring Using Photogrammetry[END_REF] were correlated to dune geomorphology, and in particular the oceanward slope and width of the foredune. Sand deposition sheets were only present at transects where the oceanward foredune slope was less than 20°. Indeed, gentle dune slopes act as a springboard and facilitate the transport of sand beyond the crest, whereas steeper dune slopes render sand transport beyond the crest more difficult (Iversen and Rasmussen, 1999). We also found that the oceanward foredune slope varied significantly across blocks. Indeed, each block measured 500 m alongshore and there were significant morphological differences at this spatial scale, as shown by the DSM in Fig. 1.a. We must be aware that these morphologic parameters alone cannot explain the location and surface of these sand deposition sheets. Other parameters should be taken into account, such as the beach width, which directly defines the sand stock available for transport, the surface moisture [START_REF] Nickling | Aeolian sediment transport on beaches and coastal sand dunes[END_REF], the wind intensity and direction affecting the fetch surface [START_REF] Walker | Fundamentals of Aeolian sediment transport: airflow overdunes[END_REF] and the feed-back effect of vegetation cover [START_REF] Hesp | Foredunes and blowouts: initiation, geomorphology and dynamics[END_REF].

Thus, geomorphological variations on the beach dune system affected sand transport and deposition through the system alongshore, as much as did our EN treatment and the West-East position along transects. Ultimately, these three complex factors affected vegetation composition, as shown by the occurrence of a significant interaction between the block, EN treatment and distance to the shoreline on CA axis 1 (see also App. 7).

Conclusion

We found that physical disturbance increased litter decomposition in the stressful environmental conditions of the coastal sand dunes. However, this positive effect for ecosystem functioning occurred and affected vegetation composition only when associated with sand deposition at the ecotone between the established foredune and the transition dune, as observed in our East EN transects or even in control transects in specific conditions of dune geomorphology. When disturbance was too high and sand erosion dominated there was no effect for ecosystem functioning, like in the West EN treatment. This suggests that the relationship between disturbance and ecosystem functioning in stressful conditions is unimodal and highly stochastic across time and space. Significant results are indicated in bold and marginally significant in italics.

a.

Factors

  arenaria and (3) the transition dune, dominated by Helichrysum stoechas. Together the incipient foredune and established foredune are forming the foredune sensu lato (App. 1.a, b).

  .a) and separated from each other by a 500 m-long alongshore distance. In each block, two large Experimental Notches (EN) were dug in December 2017, one in the incipient foredune (called the West EN, Fig. 1.b), to simulate the effect of wave erosion processes during severe storms, and one at the top of the established foredune (called the East EN, Fig. 1.c), to simulate the blowout initiation by wind erosion. These EN were dug with a horizontal dimension similar to the blowouts naturally present in the study area with alongshore and cross-shore distances of 7 m and 15 m, respectively. In order to monitor environmental parameters and plant community composition in our treatments, four cross-shore transects were placed in each block from the beach to the transition dune. Two transects were aligned along ENs, and two other transects were placed approximately 100 m further south of experimental ENs, as controls. Each transect was composed of 13 sampling quadrats distributed at the three ecotones and in the middle of each community type (e.g., incipient foredune, established foredune and transition dune) (Fig. 1.d).

  80 flags were characterized by a length and width of 0.38 m and 0.16 m, respectively (0.006 m²), attached to a bamboo pole at 0.3 m height and placed at the specific positions in each transect from mid-March to late May 2018. The remaining area of each flag was measured after exposure to the wind using a planimeter and WinFolia software (from Regent Instruments Inc®). The percentage of lost surface Sl (%) was expressed by equation (2):

  with two dominant dune plant species of the study field. Half of the bags were filled with Elymus farctus, the species dominating the incipient foredune and the other half with Helichrysum stoechas, the species dominating the transition dune. Litterbags, 0.25 m by 0.15 m with a mesh size of 0.001 m, were initially filled with 10 g of leaf (harvested early March 2018) previously dried at 40°C during 48 hours (in order to control for moisture content without changing the palatability of the litter for decomposers). In mid-March 2018, just before the beginning of the spring growing season, 80 Elymus farctus litterbags and 80 Helichrysum stoechas litterbags were buried at 0.1 m depth at the specific positions of each transect.Litterbags were harvested late May after 10 weeks, washed to remove the sand, dried at 40°C for 48 hours and weighed.[START_REF] Forey | The relative importance of disturbance and environmental stress at local and regional scales in French coastal sand dunes[END_REF] have shown that this experiment duration and period of the year were adapted to assessing differences in litter decomposition rates among sand dune communities in these climatic conditions.Large-scale dune geomorphologyLarge scale dune geomorphology was obtained by Unmanned Aerial Vehicle (UAV) surveys covering the 4 km alongshore of the Truc Vert coastal dune. They were performed using a DJI Phantom 4 Pro quadricopter equipped with a 20 MPix camera and a set of 36 permanent ground point controls (250 m spaced pairs). The combination of UAV images and photogrammetry algorithms from the Agisoft Metashape ® software v1.5 allowed to generate and geo-rectified (using the permanent ground point controls) accurate Digital Surface Model (DSM) with a planimetric and altimetric error of 0.04 m and 0.05 m respectively and a 0.1 m resolution (Laporte-Fauret et al., 2019). During the 2017-2018 winter, Laporte-Fauret et al. (2019) highlighted the formation of 0.1-0.3 m thick sand deposition sheets over the transition dune, extending from the established foredune sometimes to the inland edge of the transition dune. Sand sheets have a typical width of 100-200 m, significantly affecting the vegetation (mortality by burial).

  (a) Results of the two-way ANOVAs on the effects of the block (B), EN (Experimental Notch) treatments and their interactions on sand field surface and volume, (b) Results of linear regressions between dune morphological parameters and sand field surface and volume and (c) Results of two-way ANOVAs on the effects of the geomorphological block (B), EN (Experimental Notch) treatments and their interactions on foredune elevation, oceanward slope, width, transition dune elevation and elevation difference between foredune and transition dune.

Fig. 1 .

 1 Fig. 1. (a) Truc Vert beach location map with a digital surface model showing the elevation.

Fig. 2 .

 2 Fig. 2. PCA results with mean quadrats scores (± SE, N = 8 for Control and 4 for East and West

Fig. 3 .

 3 Fig. 3. Mean elevation variation (± SE, N = 8 for Control and 4 for East and West positions)

Fig. 4 .

 4 Fig. 4. (a) CA 1-2 diagram for vegetation quadrats, (b) Mean CA axis 1 quadrat scores with

Table 1 .

 1 Our results provide important new knowledge for understanding and predicting responses of stressful systems to increased disturbances in a changing environment. They suggest that coastal dunes may keep an important resilience to increasing probability of storm events and sea-level rise in a changing climate, only in case of intermediate disturbances. Indeed, increasing sand deposition after storm events may trigger ecosystem functioning, vegetation growth and dune stabilization. In contrast, extremely high disturbances inducing intense sand erosion may induce a collapse of the whole system as observed for example at the emblematic Dune du Pilat, the highest European sand dune with almost no plant allowing a stabilization of the system. This stresses the importance, in dry stressful systems, to maintaining the soft control of sand movements in a climate change context. Results of the ANOVAs on the effects of the EN (Experimental Notch), position (P), species (Sp, only for litter decomposition rate) treatments and their interactions on environmental variables. Since interactions with the species treatment were not significant, they are not represented in the table. Significant results are indicated in bold and marginally significant in italics.

	Tables
						Litter
	Factors Elevation variation				Wind abrasion	Median grain size
					decomposition
		Df	F	P	Df	F	P	Df	F	P	Df	F	P
	EN	2	0.82	ns	2	2.08	0.06	2	1.83	ns	2	0.87	ns
	P	12 6.22 <0.001	4	5.64 <0.001	4	6.32 <0.001 4	5.95 <0.001
	EN x P 24 0.16	ns	7	0.41	ns	8	2.58	0.02	8	1.79	ns
	Sp				1	840 <0.001

Table 2 .

 2 Results of the ANOVAs on the effects of the EN (Experimental Notch), position (P) treatments and their interactions on PCA axes 1 and 2. Significant results are indicated in bold and marginally significant in italics.

	Factors		PCA Axis 1			PCA Axis 2
		Df	F	P	Df	F	P
	EN	2	2.46	0.09	2	1.50	ns
	P	4	38.2	<0.001	4	44.4	<0.001
	EN x P	8	0.50	ns	8	1.70	ns
	Residuals	65			65

Table 3 .

 3

Sand deposition sheet surface Sand deposition sheet volume

	709	B x EN	6	2.27	ns	6	0.11	ns
	710	Residuals	4			4
	707
	708
			Df	F	P	Df	F	P
		B	3	3.11	ns	3	3.24	ns
		EN	2	0.61	ns	2	0.57	ns
		B x EN	6	0.37	ns	6	0.40	ns
		Residuals	4			4
		b.
			F	P	R	F	P	R
		Foredune elevation	0.11	ns	-0.08	0.05	ns	-0.06
		Oceanward foredune slope	25.95	<0.001	-0.81	26.13	<0.001	-0.81
		Transition dune elevation	0.67	ns	0.21	0.70	ns	0.22
		Foredune width	9.02	0.009	0.63	10.42	0.006	0.65
		Dz grey dune and foredune	0.85	ns	0.24	0.72	ns	0.22
		c.
			Oceanward foredune slope		Foredune width
			Df	F	P	Df	F	P
		B	3	48.25	0.01	3	3.63	ns
		EN	2	0.93	ns	2	0.25	ns
								33 34

This research was funded by Agence Nationale de la Recherche (ANR) grant ANR-17-CE01-0014 (SONO project).

Highlights