# Using Different Grazing Practices for Increasing Plant Biodiversity in the Dykes and Embankments Along the Rhône River (Southern France)

Cannelle Moinardeau, François Mesléard, Thierry Dutoit

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Cannelle Moinardeau, François Mesléard, Thierry Dutoit. Using Different Grazing Practices for Increasing Plant Biodiversity in the Dykes and Embankments Along the Rhône River (Southern France). Environmental Management, 2016, 58 (6), pp.984-997. 10.1007/s00267-016-0744-9. hal-01473126

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# Using Different Grazing Practices for Increasing Plant Biodiversity in the Dykes and Embankments Along the Rhône River (Southern France)

Cannelle Moinardeau<fnref n="1" /> · François Mesléard<fnref n="1" />,<fnref n="2" /> · Thierry Dutoit<fnref n="1" />

**Abstract** Extensive grazing by domestic herbivores is a widespread management practice used since the 80s in many European agro-ecosystems such as semi-natural grasslands to maintain open habitats and to enhance biodiversity. Such grazing systems have principally been tested in cultural ecosystems of high nature value threatened by grazing abandonment. However, there have been few case studies of grazing management in very anthropized ecosystems, such as the new ecosystems created by urban or industrial conversions. In Southern France, the Rhône channeling for navigation and electricity production generated in the 1950s the construction of thousands of hectares of dams and dykes which were colonized naturally by diverse plant communities. Yet shrub encroachment and the consequent recourse to mechanical cutting to facilitate control and maintenance, raise the question of how best to maintain and manage these new habitats. Consequently, since 1999, different low-intensity grazing management systems using rustic breeds of cattle, horses and goats have been tested on a protected reserve of 1454 ha located in the lower part of the Rhône river. Extensive grazing, more than cutting or no management, positively modified vegetation

heterogeneity (beta-diversity), the target open grassland species, but not plant species richness (alpha-diversity). However, the current monitoring shows that these benefits of grazing will be confirmed only if low-intensity grazing systems are sustained and if new adaptations can be also made, such as the use of mixed stocking and the establishment of multiyear contracts with breeders.

**Keywords** Adaptive management · Extensive grazing · Anthropized ecosystems · Alpha-diversity · Beta-diversity

## Introduction

Since the beginning of the nineteenth century, the biodiversity of European river valleys has been heavily modified by human activities such as the construction of dams and dykes (Jensen et al. 2006). The straightening of these rivers led to extensive loss of habitat characteristics that supported an exceptionally high biodiversity. Then conservation of river valleys therefore needs sometimes to focus more on the restoration of the species-rich habitats previously destroyed by the construction of artificial dykes and embankments than just the conservation of relict pieces of ecosystems that have existed previously (Van Diggelen et al. 2006).

However, the restoration of natural or semi-natural vegetation to its original state is not possible on pre-dyke areas, as habitat conditions on these artificial constructions are strongly different to the initial ones. Moreover, the need, for safety reasons, to detect any anomaly such as potential water infiltrations more often requires the maintenance of open vegetation by recurrent mowing. In contrast, on

<footnote>Thierry Dutoit
 thierry.dutoit@imbe.fr</footnote>

<footnote n="1">¹ Institut Méditerranéen de Biodiversité et Ecologie, UMR CNRS-IRD, Avignon Université, Aix-Marseille Université, IUT d'Avignon, 337 chemin des Meinajariés, Site Agroparc BP 61207, 84911 Avignon Cedex 09, France</footnote>

<footnote n="2">² Institut de Recherche de la Tour du Valat, Le Sambuc, 13200 Arles, France</footnote>
embankments where safety is not a major concern, the spontaneous succession can lead to shrub encroachment. In this case, the establishment of open vegetation requires appropriate management measures as mowing at low frequencies or grazing at low stocking densities. So far, such sustainable forms of grazing have mainly been tested on cultural ecosystems of high nature value threatened by grazing abandonment (Van Wieren and Bakker 1998), the experiments often being funded under agri-environmental schemes (Rosen and Bakker 2005). Only a few case studies have examined grazing management in altered ecosystems such as recent fallows in former agricultural areas (Gibson et al. 1987; Török et al. 2014; Schrautzer et al. 2016), or in new ecosystems arising from urban or industrial conversions.

Grazing can be a key tool in conservation management (Weber et al. 1998), and extensive grazing by domestic herbivores (Allen et al. 2011) is a widespread management practice used in protected areas to maintain open habitats and to enhance biodiversity (Van Wieren and Bakker 1998; Rambo and Faeth 1999; Rosenthal et al. 2012). Grazing modifies the competitive hierarchies within plant communities (Dorrough et al. 2007). It can alter or favor the establishment of specific seedlings (Gomez Sal et al. 1999), halt succession to woodlands (Hill et al. 1995), or induce spatial heterogeneity of vegetation (Lin et al. 2010). The pros and cons of extensive grazing have been reviewed and discussed by various authors (Danell et al. 2006; Plachter and Hampicke 2010; Rosenthal et al. 2012).

The impact of domestic grazing can vary according to the herbivores used (Davidson 1993; Gordon 2003), to grazing intensity and to stocking cycles (Savory 1983; Salihi and Norton 1987; Ralphs et al. 1990; Hart and Ashby 1998; Fynn and O'Connor 2000; Dumont et al. 2009). Short but intensive grazing periods appear the most efficient way to control recruitment and development of species neglected by herbivores (Savory 1988; Perevolotsky and Seligman 1998). Nevertheless, there is ongoing debate over the type of extensive grazing to apply, especially in conservation management according to environmental conditions (Briske et al. 2008). Short rotation grazing systems are considered more favorable to homogeneity, whereas low-intensity grazing systems appear to generate a mosaic of successional stages (Olff et al. 1999; Wallis De Vries et al. 2002). Low-intensity grazing has been found to improve species richness at the plot (alpha-diversity) and at the landscape scale (gamma-diversity), as well as heterogeneity between plots (beta-diversity) (Plachter and Hampicke 2010; Rosenthal et al. 2012). It is therefore a convincing alternative to mowing by heavy machinery such as mechanical cutting (Bakker 1989). Through the consequences of the animals' behavior, low-intensity grazing enhances the mosaic of habitats (short grasslands, tall grasslands, shrubs,

etc.) and encourages the concomitant presence of species of early and late successional stages (Plachter and Hampicke 2010; Rosenthal et al. 2012). However, it can be difficult to fit grazing pressure and animal behavior to the specific application context, leading to failure to meet conservation objectives (Rosenthal et al. 2012). In addition, low grazing pressure or its unsuitable application where forage availability fluctuates (Peco et al. 1998) has been known to lead to irreversible situations, such as complete colonization by unpalatable woody species (Mesléard et al. 2011).

For decades, extensive grazing of various forms has been applied in Europe in numerous protected areas and former cultivated lands (Van Wieren and Bakker 1998), but its appropriateness as an alternative to mechanical management tools is still under debate (Bakker et al. 2002), especially in restoration contexts (Kolos and Banaszuk 2013). Grazing is often seen as socially acceptable, because it mimics traditional land use in pastoral regions (Vera 2000; Bobiec et al. 2011). However, this argument may not suffice to promote its use, insofar as the consequences of inappropriate management can threaten biodiversity and be costly to repair (e.g. uncontrolled colonization by ligneous and/or unpalatable invasive species). It is therefore essential to assess the impact of grazing on local vegetation dynamics, examining its capacity to support diversity at different scales in comparison with other management tools.

In this two-year study (2014–2015), we investigated the impacts both of four different low-intensity grazing regimes (cattle, horses, goats) and of mowing at the rate of one cut/year on the spontaneous vegetation of artificial dykes and embankments located in the Lower Rhône valley (South-eastern France). The use of such management systems to enhance the conservation value of habitat is original in the context of new ecosystems developing in an artificial environment. Thereby, references on plant community dynamics and on the impact of management at short and long times are missing. Then, the impacts of the low intensive grazing systems applied were assessed by comparing the resulting vegetation obtained when managed without grazing (negative control) or with mechanical cutting.

Our aim was then to answer the following questions:

* How does low-density cattle grazing affect grassland composition, species richness (alpha-diversity) and heterogeneity (beta-diversity) in comparison with a one cut/year regime in the dykes of a power canal?

* Does low-density horse grazing allow the creation and maintenance of species-rich open grasslands and heterogeneous stages of secondary plant successions on the embankments of a tailrace?

* Can goat grazing create and/or maintain open herbaceous plant communities on afforested tailrace embankments
after mechanical forest clearing?

How can these sustainable conservation management
systems be maintained or improved in the future?

**Methods**

Study Site

The Rhône is one of the major rivers of Europe (813 kilo-
meters long), rising in Switzerland, passing through Lake
Geneva and running through southeastern France. The local
climate of the study site is Mediterranean, with a mean
yearly temperature of 15 °C and a mean yearly accumulated
precipitation between 500 and 700 mm. The site (coord.
North 44°18′13.3″ East 4°44′32.9″) is constituted by the
artificial dykes and embankments of the dam and hydro-
electric power station of Donzère-Mondragon located ca.
180 km south of Lyon and 55 km north of Avignon in the
lower course of the river (Fig. 1).

Built between 1948 and 1952 and managed up to now by
the “*Compagnie Nationale du Rhône*” (CNR), the floodway
of Donzère-Mondragon transports a substantial part of the
water of the Rhône necessary to the power station, on 33 km
between the towns of Donzère and Mondragon in the
alluvial plain of Tricastin (Fig. 1).

The dykes and embankments were built with millions of
tons of alluvial sediments (pebbles, sands and loams)
extracted from digging the canals, and were planted with

ornamental trees for reasons of esthetics. In February 1954,
a hunting and wildlife reserve was created, to date managed
by the French national agency for wildlife (*Office National
de la Chasse et de la Faune Sauvage*) in an area of 1,454 ha,
covering a band 28 km long and about 300-850 m wide
between river kilometer points 170 and 200.5. Its objective
was to enhance game species as well as habitat diversity for
flora and fauna (Roux et al. 2007).

In 2014, we selected three areas where different forms of
management by grazing or mechanical cutting were already
applied (Fig. 1). These areas differed in management history
(cattle, horse or goat grazing, mechanical cutting), vegeta-
tion and grazing regime over 2014–2015.

Site 1: The management of this site started in 1999, with a herd of rustic Galloway cattle (105 cows, 30 bulls and 40 calves) introduced over 130 ha on the dykes of the supply channel and surrounded by electric fences (Fig. 1). The grazing season extends from mid-November to mid-July. The site is characterized by herbaceous vegetation dominated by a tussock grass (*Brachypodium phoenicoides*) and some annual species. Some trees were also planted in this site immediately after the construction, to prevent soil erosion (*Robinia pseudoacacia*) or for ornamental reasons (*Platanus* sp. *Populus* sp., *Cupressus* sp., *Pinus* sp., etc.) (Roux et al. 2007). Some shrubs (*Crataegus monogyna*, *Prunus spinosa*, *Rubus* sp.,...) have also naturally colonized this area.

On a small part of this area (4 ha), then, with the same
initial construction conditions and soil physico-chemical
parameters as the remaining grazing part, (Fig. 1), the

<figure><img src="image_2.png" /><figcaption>**Fig. 1** Localization of the study site of Donzère-Mondragon in Southeastern France and in the lower Rhône valley. The dam, hydro-electric power station, power canal, tailrace, and the three study sites in dykes and embankments are indicated: (1GG) cattle grazing, (1MG) mechanical cutting, (2) horse grazing and (3) forest mechanical clearing and goat grazing</figcaption></figure>
grazing system was replaced by an annual summer (August) mowing system in 2012. The grazing pressure previously applied did not favor enough of the short herbaceous vegetation essential to immediately locate any water leak in this area where safety is a major concern because of the proximity of the Tricastin nuclear power station.

Site 2: This embankment of 27 ha was created by 50 million tons of sediment accumulated from the construction of the station and digging channels, and is located 900 m south of the hydroelectric power station (Fig. 1) along the tailrace. In 2008, a small group of Konik-Polski horses (four mares) was introduced to control natural shrub encroachment and maintain short grasslands. Animals are grazing on the site all the year round.

Site 3: This 20 ha site is located 4 km south of the hydroelectric power station on an embankment of the tailrace (Fig. 1). Vegetation was mechanically cleared in 2012 to restrain the proliferation of wild boar in this area. Between July and October 2013, 300 goats of a rustic breed (*Chèvres du Rove*) were present; however, there were only 100 during the same period in 2014 and 2015 for reasons specific to the breeder. A shepherd is present during the grazing period (May–October), situation is the same as site 1 and 2 but more dominated by a dense canopy (95 %) of planted trees (*R. pseudoacaccia*) or trees which have naturally colonized the site since the fifties (*Populus* sp., etc.).

## Vegetation Analyses

Sampling measurements consisted of quadrats whose sizes were determined by the respective minimal areas (Guinochet 1973) for each plant community encountered: 4 m² (2 × 2 m) for the herbaceous plant communities of sites 1 and 2 and 25 m² (5 × 5 m) for the shrub and forested plant communities of site 3. The abundance of each plant species was defined by assignment to a cover class using a scale, as follows: 1 = species covering less than 5 % of the plot, 2 = species covering between 5 and 25 %, 3 = species covering between 26 and 50 %, 4 = species covering between 51 and 75 %, 5 = species covering more than 75 %, and 0.2 = species represented by a single individual in the quadrat. Vegetation cover (%), litter cover (%) and bare soil cover (%) of the plots were also recorded in 2014 and 2015. This two-year monitoring period was chosen to separate the long-term and the recent expected effects of the different grazing systems established at different periods (1999, 2008, 2012) from the inter-annual variations in species composition, richness and diversity that can occur in Mediterranean early successional grasslands dominated by annual species (Caldeira et al. 2005). During the year before the botanical inventories of May 2014, the mean yearly temperature was 15 °C and mean yearly accumulated precipitation 620 mm. In spring 2014, mean monthly

temperature was 14 °C and monthly accumulated precipitation 77 mm. During the year before the botanical inventories of May 2015, the mean yearly temperature was 15 °C again and mean yearly accumulated precipitation 828 mm. In spring 2015, mean monthly temperature was again 14 °C and monthly accumulated precipitation 113 mm.

Botanical inventories and plant productivity measurements were not realize monthly to take into account variation of defoliation following times especially in the grazed treatments as it was not possible to change the period of mowing (August 2014 and 2015). Nervertheless, the period of May–June 2014 and 2015 was choosen because it allows to identify most of the plants and it corresponds to a period when grazing as always started. Even if mowing and grazing include summer months, it is then not possible to recognize most of the species which are then totally dried.

On site 1, we sampled the mown grassland (MG) and an area of grassland grazed by cattle (GG). To minimize bias on the expected impacts of cattle grazing or mechanical cutting, ten quadrats were set up in the middle of the dyke slope in homogeneous herbaceous areas, thus avoiding spaces impacted by passing vehicles, such as the roads leading up and down the embankment, and the influence of planted ornamental trees (Nicaise 1996). The quadrats were more than 10 m apart, to avoid spatial autocorrelation (Table 1).

On site 2, the area grazed by horses, five quadrats were distributed randomly in each of the different vegetation patches identified previously via aerial photographs (Table 1). Open grasslands (OG) with bare zones corresponding to former silt and clay deposits; short grasslands (SG) totally covered by herbaceous vegetation of 10–20 cm

<table><caption>**Table 1** Treatment, abbreviation and number of sampling quadrats in the three study sites of the reserve of Donzère-Mondragon, southeastern France</caption><thead><tr><th>Site</th><th>Treatment</th><th>Abbreviation</th><th>Number of quadrats</th></tr></thead><tbody><tr><td rowspan="2">1</td><td>Mown grassland</td><td>MG</td><td>10</td></tr><tr><td>Grazed grasslands</td><td>GG</td><td>10</td></tr><tr><td rowspan="5">2</td><td>Open grasslands</td><td>OG</td><td>5</td></tr><tr><td>Short grasslands</td><td>SG</td><td>5</td></tr><tr><td>Tall grasslands</td><td>TG</td><td>5</td></tr><tr><td>Tall grasslands dotted with shrubs</td><td>TGS</td><td>5</td></tr><tr><td>Shrub thickets</td><td>ST</td><td>5</td></tr><tr><td rowspan="3">3</td><td>Non-grazed control plot</td><td>C</td><td>5</td></tr><tr><td>Cleared and grazed</td><td>CG</td><td>5</td></tr><tr><td>Cleared and fenced</td><td>CF</td><td>5</td></tr><tr><td rowspan="2">3</td><td>Grazed</td><td>G</td><td>5</td></tr><tr><td>Non-cleared and non-grazed control plot</td><td>C</td><td>5</td></tr></tbody></table>
height, tall grasslands (TG) dominated by *B. phoenicoides* of 20–30 cm height, tall grasslands dotted by shrubs (TGS), shrub thickets (ST) and non-grazed tall grasslands, outside the grazed areas, used as controls (C).

On site 3, grazed by goats, five quadrats were distributed at random in each of the different treatments used to clear the forest vegetation (Table 1).

Vegetation surveys were conducted each year in May when the majority of plant species can be identified. Nomenclature of vascular plants followed Tison et al. (2014). Diversity parameters were calculated separately for each site. We estimated alpha-diversity (species richness, mean species number) and beta-diversity with the Bray–Curtis distance. The mean Bray–Curtis distance between the ten (site 1) or five quadrats (site 2 and 3) of each treatment was calculated. The mean of this distance varied from 0 to 1, 0 signifying full similarity between treatments.

In addition to vegetation analyses, grazing surveys were conducted in spring 2014. The objectives were to compile the complete history of grazing activities since 1999 and to characterize current grazing practices on the three sites. Survey data was collected from the different farmers and managers of the different herds and the technician in charge of the mechanical cutting on site 1. Socio-economic and technical information on the different grazing or mechanical cutting systems was sought, with interviews conducted in the form of open discussions at the sites, with the herds present. A thematic grid had previously been constructed, listing different essential talking points (periods of grazing, etc.). Any of these points that did not arise spontaneously during the discussion were addressed through specific questioning.

## Soil Analyses

In total, 40 soil samples were analyzed, 10 for site 1 (five replicates in the cattle grazed grassland and five replicates in the MG), and 30 for site 2 consisting of five replicates for each of the six different plant communities identified on this site.

Soil samples were collected in spring 2014 near each vegetation quadrat, in order to evaluate nutrient content, particle size and organic matter. For each treatment on site 1 and 2, five 200-g soil samples were extracted with a soil corer at a depth of 0–10 cm. Soil samples were collected near each pair of quadrats for site 1 and for each quadrat at site 2. No soil samples were collected from site 3, since the introduction of mechanical or grazing disturbance was too recent (2012) to have created any significant impact on soil physico-chemical parameters (Greenwood and Mckenzie 2001). After drying and sieving (2-mm mesh size) the soil, physico-chemical properties (pH water, exchangeable Ca, K

and $P_2O_5$) were measured according to standardized methods: pH in soil:water ratio of 1:2.5 (Thomas 1996); exchangeable cations (CEC) by the Metson method (Metson 1956); and available phosphorous by the Olsen method (Olsen et al. 1954; Sparks et al. 1996; Ciesielski and Sterckeman 1997). Percentages of clay (<2 µm), silt (2–50 µm) and sand (50–2000 µm) were determined according to the Robinson method (Baize 2000) without decarbonation. Organic C (Allison 1965) content and total N (Bremner 1996) content were quantified from bulk soil by dry combustion, using a CHN elemental analyzer (ISO 10694).

## Statistical Analyses

Generalized linear models (GLM) were used to analyze the effects of the different treatments applied on each of the three sites separately on vegetation variables (mean vegetation height, vegetation cover, litter and bare-ground cover, species-richness and Bray–Curtis indices) from the sampling period (2014 and 2015). GLMs were computed with a Poisson distribution error, which is particularly suited to counting data, and with a Gaussian distribution error for continuous data (Bray–Curtis) (Crawley 2009), treatments and years are considered as factors, vegetation properties as variables, (Supplementary Appendix A).

For each site, the characteristics concerning composition and abundance of plant species were ordinated using non-metric multidimensional scaling (NMDS) based on Bray–Curtis similarity (Borcard et al. 2011), to visualize the changes in vegetation composition and distribution and the species most correlated to each treatment.

Soil characteristics of site 1 and 2 were ranked using principal component analysis (PCA) (Borcard et al. 2011). To test the effect of the different treatments on soil physico-chemical parameters, soil particle sizes (fine and coarse sand, fine and coarse silt, clay) and nutrient content (total carbon, total nitrogen, C:N, CEC, Metson, $K_2O$, $P_2O_5$, CaO, $CaCO_3$, pH) were then compared via univariate analyses.

To compare soil particle size, nutrient content, vegetation height, vegetation, litter and bare-ground cover, species richness and Bray–Curtis distances, we used ANOVA tests for multiple comparisons when the data followed a normal law (Shapiro–Wilk or Lilliefors test). This first analysis was followed by post-hoc Tukey tests when a significant difference was detected between treatments. We used *t* tests for two–two comparisons. When data did not follow normal distribution, we used non-parametric Kruskal–Wallis tests for multiple comparisons, followed by pairwise Mann–Whitney–Wilcoxon non-parametric tests, with the value of *p* adjusted according to the method of Benjamini and Hochberg (1995). When a significant difference was detected between treatments, we used the Mann–Whitney–Wilcoxon test for two–two comparisons.
All statistical analyses were performed with R software version 2.15.3. (The R Foundation for Statistical Computing 2012), using its native packages for univariate analyses and additional packages ade4 (Chessel et al. 2004; Dray and Dufour 2007) and vegan (Oksanen et al. 2012) for multivariate analyses (PCA and NMDS).

## Results

In total, 223 species were inventoried on the three sites (Supplementary Appendix B). The most common species were *Dactylis glomerata*, *Clinopodium nepeta*, *Hypericum perforatum*, *Medicago minima* and *Crataegus monogyna*, found in more than 50 % of the 140 quadrats realized during the 2014–2015 period. The rarest species inventoried were the fern *Ophioglossum vulgatum*, with only one occurrence, under a shrub thicket in the horse grazing embankment (site 2), and is listed as a legally protected species in South-eastern France (Region "Provence-Alpes-Côte d'Azur").

## Site 1: Low-Density Cattle Grazing vs. Mowing

The grazing survey revealed that the selected grassland (a park of 16 ha of the 130 ha total) was grazed each year since 1999 between November and July. This corresponds to a stocking rate of 0.50 LU (1 LU = 500 kg ha⁻¹ y⁻¹). Starting from 2012, the MG (4 ha) was cut annually in August, ensuring that the vegetation was cut after seed dispersal and

limiting the potential impact of mechanical cutting on fauna. After cutting, the biomass remained in place on the site.

The NMDS analysis (NMDS stress = 0.21) of the vegetation data (20 quadrats × 148 species) revealed two different herbaceous communities in May 2014 and 2015 (Fig. 2a and b): the grassland grazed by cattle was dominated by short annual plant species such as *A. simplex*, *Veronica arvensis*, *Campanula erinus*, *Petrorhagia prolifera*, *Rostraria cristata*, and the MG was essentially dominated by tall perennial species such as *Brachypodium phoenicoides*, *Dactylis glomerata*, *Reichardia picroides*, with short annuals such as *Euphorbia exigua*, *Lysimachia linum stellatum*, *Valantia muralis* also present.

In May 2014 and 2015, we measured no significant difference between the impact of mowing and cattle grazing for species richness, mean vegetation height, bare ground and litter covers. Nevertheless, vegetation cover was significantly higher in May 2015 in the MG (89.4 % ± 2.15) than in the cattle-grazed grassland (60.5 % ± 4.39) (t = 2.9372, df = 14.366, *p* < 0.05 in 2015). In May 2014 and 2015, the Bray–Curtis dissimilarity indices (Fig. 3) were significantly higher in the cattle-grazed grassland (W = 11, *p* < 0.01 in 2014 and 2015), indicating greater heterogeneity of vegetation in this grassland (0.65 ± 0.01 in 2015) than in the MG (0.51 ± 0.04 in 2015). The significantly higher cover (W = 99, *p* < 0.001) by the tussock perennial grass *B. phoenicoides* in the MG (82 %) than in the cattle-grazed grassland (17 %) in 2015 may explain this difference.

<figure><img src="image_3.png" /><figcaption>**Fig. 2** NMDS ordination (NMDS stress = 0.21) based on 20 vegetation quadrat samples, ten in the MG and ten in the cattle GG (site 1, *n* = 20) in dykes of a power canal of Donzère-Mondragon in the Lower Rhône valley, Southeastern France. The A plot shows only the 2015 samples grouped by treatment with polygons. To clarify, only the</figcaption></figure>

<figure><img src="image_4.png" /><figcaption>most correlated plant species to the two first axes are literally inscribed. The B plot shows the vegetation dynamic for each treatment, according the position of their barycenter during the 2 years of the vegetation survey (2014–2015)</figcaption></figure>
<figure><img src="image_5.png" /><figcaption>**Fig. 3** Means and standard errors of Bray–Curtis index in the cattle grazed (GG and mown grasslands MG in dykes of a power canal of Donzère–Mondragon in the Lower Rhône valley, Southeastern France. (W = 11 for *p* < 0.01 in 2014 and 2015). *Bars* showing common letters do not have any significant differences between treatments the same year according to Tukey honest significant differences post-hoc tests</figcaption></figure>

Concerning the soils of site 1 (Fig. 4), axis 1 of the PCA (51.2%) opposed the MG to the cattle-grazed grassland (CG) based on fine particle content. The univariate analyses confirmed this trend and showed that proportions of clay (179.75 ± 14.98 g/kg), fine silt (122.75 ± 12.02 g/kg) and coarse silt (65 ± 5.04 g/kg) were significantly higher in the MG than in the cattle-grazed grassland (125.40 ± 15.97, 79.80 ± 5.20 and 45.00 ± 5.47 g/kg for *p* < 0.05 respectively). Axis 2 (21.4%) was not correlated with the two different treatments for C, N, P₂O₅ and CEC Metson and probably revealed some local differences in vegetation distribution independently from management types realized in each dyke (cattle grazing or mowing).

## Site 2: Low-Density Horse Grazing vs. No Management

The grazing survey revealed that this embankment (27 ha) was grazed between 2008 and 2012 and again throughout 2015 by 4 Konik-Polski mares, which corresponds to a very low calculated stocking rate of 0.06 LU (1 LU = 500 kg ha⁻¹ y⁻¹).

The NMDS analysis (NMDS stress = 0.15) (Fig. 5a and b) realized on the vegetation (30 quadrats × 146 species) showed that horse grazing in 2014 and 2015 maintained, very OG, and short grasslands (SG) dominated by early successional species (Sagina apetala, Minuartia hybrida, T. suffocatum, Filago germanica, Vulpia myuros etc.) and short grassland species (Medicago minima, Vulpia ciliata, Galium parisiense, Centaurium erythraea, Rostraria crista-tata, Catapodium rigidum, etc.). The plant composition was different between the control (C) and the short grasslands (SG) opened by horse grazing. Nevertheless, there was no

<figure><img src="image_6.png" /><figcaption>**Fig. 4** PCA ordination based on the 10 soil samples and 14 soil variables analyzed in the cattle grazed grassland (GG) and the mown grassland (MG in the soils of dykes along the power canal of Donzère-Mondragon in the Lower Rhône valley, Southeastern France</figcaption></figure>

difference in composition between the control (C) without grazing and, the TG and the TGS found in the grazed area. These three grasslands showed a lot of common species (Schedonorus arundinaceus, B. phoenicoides, Dactylis glomerata, Agrimonia eupatoria, Gallium aparine, etc.). Species-richness was significantly higher in 2014 in the open (28 ± 2.19 sp./4 m²) and short grasslands (25.2 ± 1.46 sp./4 m²) than in the other plots, with the exception of the tall grasslands dotted with shrubs (20.6 ± 3.65 sp./4 m²). In 2015, the Bray–Curtis index was significantly higher (*F* value = 8.99 *p* < 0.001) in the open grasslands (0.72 ± 0.02) and ST (0.77 ± 0.02) compared to all the other plots (Fig. 6).

For the soils of site 2 (Fig. 7), axis 1 of the PCA (37.6%) opposed the open and short grasslands (OG and SG) to the TG and the TGS with a central position for shrub thicket plots (ST). This gradient is based on a fertility gradient (P₂O₅, K, CEC, C, N), whereas axis 2 (24.5%) is a fine particle gradient not correlated with the different vegetation investigated (Fig. 7). Only total organic carbon content showed significant differences between plots (*F* value = 3.21 *p* < 0.05) with higher content in the tall grasslands dotted with shrubs (39.70 ± 2.14 g/kg) and lower content in the short grassland (22.20 ± 1.61/kg).

## Site 3: Clearing vs. Low-Density Goat Grazing vs. No Management

The grazing survey confirmed that this embankment had been grazed (20 ha) since 2013 from May to November, by
<figure><img src="image_7.png" /><figcaption>**Fig. 5** NMDS ordination (NMDS stress = 0.15) based on vegetation quadrat samples in different vegetation successional stages localized in the horse grazed site (site 2, n = 30) in a tailrace embankment in Donzère-Mondragon in the Lower Rhône valley, Southeastern France. The A plot shows only the 2015 samples grouped by treatment with polygons: open grasslands with the presence of bare soils (OG), short grasslands (SG), tall grasslands (TG), tall grasslands dotted by shrubs (TGS), shrubs (ST) and not grazed controls (C). To clarify, only the most correlated plant species to the two first axes are literally inscribed. The B plot shows the vegetation dynamic for each treatment, according the position of their barycenter during the 2 years of the vegetation survey (2014–2015)</figcaption></figure>

<figure><img src="image_8.png" /><figcaption>**Fig. 6** Means and standard errors of Bray–Curtis index in 2014 and 2015 of a horse grazed site in a tailrace embankment in the site of Donzère-Mondragon in the Lower Rhône valley, Southeastern France: open grasslands with the presence of bare soils (OG), short grasslands (SG), tall grasslands (TG), tall grasslands dotted by shrubs(TGS), shrub thickets (ST) and not grazed controls (C). (F = 7.163 for *p* < 0.001 in 2014 and F = 8.996 for *p* < 0.001 in 2015). Bars showing common letters do not have any significant differences between treatments the same year according to Tukey honest significant differences post-hoc tests</figcaption></figure>

a flock of 300 goats in 2013 and only 100 goats in 2014 and 2015, a stocking rate of 1.06 LU (1 LU = 500 kg ha⁻¹ y⁻¹). Mechanical clearing was performed in 2012 to thin the forest vegetation of this embankment for better mobility of the goat flock, with some 20 % of shrub-forest vegetation cleared.

The NMDS analysis (NMDS stress = 0.14) realized on the 40 quadrats and 76 species (Fig. 8a and b) showed that

the treatments (grazing, clearing and grazing × clearing) impacted plant composition. In 2015, plots that had been cleared and grazed (CG) were dominated by annual grasses (Anisantha sterilis, Anisantha madritensis, Hordeum murinum, etc.), unpalatable tall forbs (Carduus tenuiflorus, A. minus, etc.) and species known to have permanent seed-banks (Lysimachia arvensis, Hypericum perforatum, Blackstonia perfoliata, etc.). The plots only cleared in 2012
and fenced in 2013 were characterized by perennial species (Brachypodium sylvaticum, Poa trivialis, Torilis arvensis) and ruderal species (Bryonia cretica). The plots only grazed by goats since 2013 without clearing in 2012 (G) were characterized by ruderal species (Galium aparine, Sonchus oleraceus, etc.) as well as by a decrease in the most palatable shrub and tree species (Hedera helix, Cornus sanguinea, Fraxinus angustifolia), which still characterized the non-cleared and non-grazed control plots (C). In 2015, a significant decrease in vegetation cover was obviously measured in the cleared and goat-grazed plots (56% ± 9.27) compared to all the other plots (F = 11.31 for *p* < 0.001). We found no difference in species richness in 2014, but the cleared and fenced treatment, CF, (16 ± 2.90 sp./25 m²) had significantly higher species richness in 2015 (F = 4.802 for *p* < 0.05) than the control (6.8 ± 0.73 sp./25 m²) (Fig. 9). Bray-Curtis indices were also significantly higher in 2014 and 2015 in the CG and CF treatments than in the control (Fig. 10).

<figure><img src="image_9.png" /><figcaption>**Fig. 7** PCA ordination based on the 30 soil samples and 14 soil variables analyzed in the horse grazed site: open grasslands with the presence of bare soils(OG), short grasslands (SG), tall grasslands (TG), tall grasslands dotted by shrubs(TGS), shrub thickets (ST) and not grazed controls(C) in the soils of tailrace embankments in the site of Donzère-Mondragon in the Lower Rhône valley, Southeastern France</figcaption></figure>

Discussion

Vegetation Dynamics on the Three Sites

The different grazing systems examined here were implemented under different conditions, for different objectives and at different times. Nevertheless, converging trends are clearly observed from the three experiments. The low-density grazing applied appears more relevant for the objectives of plant species diversity conservation and/or restoration, than mowing at a rate of one cut/year or to the

<figure><img src="image_10.png" /><figcaption>**Fig. 8** NMDS ordination (NMDS stress = 0.14) based on vegetation quadrat samples in a goat grazed site (site 3, n = 20) of a tailrace embankment in Donzère-Mondragon in the Lower Rhône valley, Southeastern France. The A plot shows only the 2015 samples grouped by treatment with polygons: cleared areas grazed by goats (CG), cleared and fenced areas (CF), not cleared areas only grazed by goats (G) not cleared and not grazed areas (C). To clarify, only the most correlated plant species to the two first axes are literally inscribed. The B plot shows the vegetation dynamic for each treatment, according the position of their barycenter during the 2 years of the vegetation survey (2014–2015)</figcaption></figure>

<figure><img src="image_11.png" /></figure>
<figure><img src="image_12.png" /></figure>

**Fig. 9** Means and standard errors of plant species-richness in 25 m² quadrats in 2014 and 2015 of a goat grazed site of a tailrace embankments in the site of Donzère–Mondragon in the Lower Rhône valley, Southeastern France: cleared areas grazed by goats (CG), cleared and fenced areas (CF), not cleared areas grazed by goats (G) not cleared and not grazed areas (C) (W = 0.9307 for *p* < 0.05 in 2014 and F = 4.802 for *p* < 0.05 in 2015). *Bars* showing common letters do not have any significant differences between treatments the same year according to Tukey honest significant differences post-hoc tests

<figure><img src="image_13.png" /></figure>

**Fig. 10** Means and standard errors of Bray–Curtis index in 2014 and 2015 of a goat grazed site along a tailrace embankments in Donzère–Mondragon in the Lower Rhône valley, Southeastern France: cleared areas grazed by goats (CG), cleared and fenced areas (CF), not cleared areas grazed by goats (G) not cleared and not grazed areas (C) (F = 8.437 for *p* < 0.01 in 2014 and F = 4.802 for *p* < 0.05 in 2015). *Bars* showing common letters do not have any significant differences between treatments the same year according to Tukey honest significant differences post-hoc tests

absence of management, for the artificial dykes and embankments studied.

On site 1, the power canal dykes grazed by rustic Galloway cattle since 1999 and a dyke where annual mowing system is performed since 2012 showed differences in plant communities. Cattle selected short annual species, whereas mowing favored tall perennial tussock grasses, especially *B. phoenicoides*, which covered around 75% of the soil in the MGs over the 2014–2015 period. In the grazing system, cattle are expected to feed mainly on palatable grassland species, thus lowering the competitive ability of perennial grasses such as *B. phoenicoides* and allowing the establishment of low-productive species such as short annuals.

As a consequence, plant species can be expected to co-exist and their number to increase (Osem et al. 2002 and Lezama et al. 2014). Both types of management failed to discriminate the two grasslands by their number of species or their number of species rare in the Vaucluse. Indeed, the species inventoried in this study and listed as rare or very rare species of the Vaucluse county (Girerd 1991) were *Alyssum simplex*, *Anisantha lanceolatus*, *Centaurium tenuiflorum*, *Limonium echioides*, *Ranunculus parviflorus*, *Trifolium suffocatum*. They were all inventoried in the grasslands grazed by cattle or horses at sites 1 and 2, except *A. simplex* and *A. lanceolatus*, which were also inventoried in the quadrats of the MG at site 1.

However, cattle grazing led to increased heterogeneity of plant communities in comparison with the mown area, at the beta-diversity scale. This greater vegetation heterogeneity in the grazed grassland may be linked to the impacts of these heavy herbivores on soils. Galloway cows may indeed have caused surface erosion of the fine particles by trampling, which could explain the significantly lower proportions of clays, fine and coarse silts in the grazed area as revealed by the greater heterogeneity in the distribution of physico-chemical soil parameters analyzed in the grazed grassland. Nevertheless, it must be emphasized that it was not possible to measure the variation of defoliation following times in the grazed grassland and that uncollected residue in autumn and winter which probably covering the soil, reduced also the runoff losses.

Thus, the increase in short annual species could also reflect reduced nutrient availability and the absence of competition with tall perennial grasses, consumed by herbivores (Milchunas et al. 1988; Noy-Meir et al. 1989; Peco et al. 2005; Golodets et al. 2010). This may have increased the number of ecological niches, resulting in the coexistence of more species (Harpole and Tilman 2007). Conversely, not gathering the hay may have led to increased fertility and organic matter content in the soils of the mown area. The expected increase in species richness under moderate grazing is in accordance with results from other studies, which also reported an increased number of species after re-establishment of low-density grazing in abandoned grasslands with a homogeneous vegetation structure, due to the dominance of perennial tussock grasses (Öckinger et al. 2006).

On site 2, the introduction in 2005 of low-intensity horse grazing on a tailrace embankment led to different stages of plant secondary succession on a poor mineral soil. Even at very low intensity, Konik–Polski horse grazing created early successional plant communities (e.g. very open and short grasslands) which were not present in the neighboring ungrazed area. These two types of grassland were characterized by an increase in short annual plants, some of which are particularly rare in the Vaucluse (*C. tenuiflorum*,
L. echioides, R. parviflorus, T. suffocatum). Here again, the decrease in the perennial tussock grass B. phoenicoides and the increase in short annual species measured are in accordance with results from other studies (Marriott et al. 2009). The tall-growing, competitive species, present during an interim phase characterized by an absence of disturbance, are able to suppress smaller light-demanding species (Pykälä et al. 2005). Controlling their dominance by grazing provides open space for germination and establishment of less competitive species, which is supposed to be favorable to species richness (Rosenthal et al. 2012). However, in our case, the very low horse grazing density (0.06 LU) could explain why the significant differences measured for species richness in 2014 were not confirmed in 2015, long-term monitoring will then confirm or not this hypothesis The increasing dominance of tall perennial tussock grasses (B. phoenicoides, S. arundinaceus, D. glomerata, etc.) and the resulting decrease in species richness due to their competition can be attributed to heterogeneity in grazing pressure between the different vegetation patches even at low grazing intensity (Duncan 1992). As a result, the species richness of tall grasslands dotted with shrubs tends to increase again due to the presence of pioneer shrub species (e.g. C. sanguinea Rubus ulmifolius, C. monogyna, etc.). While ST are obviously particularly poor in herbaceous plant species due to the higher ability of shrub species to compete for light, it should be noted that the only species protected at regional scale, O. vulgatum, was found solely under shrub canopies. This result therefore confirms the importance of maintaining a heterogeneous mixture of open grasslands and ST to maximize plant diversity on this low-intensity horse-grazed embankment (Beever et al. 2008).

On site 3, forest clearing in 2012 and the introduction in 2013 of goat grazing by the rustic “Chèvres du Rove” on a tailrace embankment allowed the creation and maintenance of cleared areas in the spontaneous forest vegetation; but these areas were still dominated by R. pseudoacacia planted in 1954. Cleared and grazed areas are characterized in 2015 by the presence of annual grass species and other annual species with permanent seed banks (e.g. L. arvensis, Hypericum perforatum, Blackstonia perfoliata, etc.) (Dutoit and Alard 1995). The presence of Cynoglossum creticum may also be attributable to the presence of goats, as its seeds are known to be transported in the fleece of animals (Selvi et al. 2011). After clearing and without goat grazing, the fenced areas were rapidly colonized by perennial grasses. Areas only grazed by goats since 2013 showed a decrease solely in the shrub species most palatable to goats. As observed for sites 1 and 2, forest clearing with or without grazing clearly changed the plant composition. If this kind of goat grazing (pressure of more than 1 LU) is maintained, increased species richness and heterogeneity can be then expected in the future.

## Implications for the Restoration and Management of the Vegetation of Dykes and Embankments

The challenge of the introduction of sustainable management systems using rustic herbivores at artificial sites such as dykes and embankments lies in determining whether low-density grazing indeed generates heterogeneous vegetation with distinct successional stages. At the scale of our three experimental areas as at the scale of the whole natural reserve and of all the dykes and embankments along the Lower Rhône river (13,000 ha), managements for conservation purposes will not be implemented unless they proved to be consistent with the safety objectives.

In our case, the same trends regarding restoration success were observed over the 2014–2015 period. Low grazing pressure always led to increased heterogeneity of short grasslands, characterized by increased beta-diversity, in comparison with no grazing or a year of mechanical control in site 1. The proportion of rare short annual species increased after the implementation of low-density grazing, regardless of the rustic breed (cattle, horse, goat). Nevertheless, grazing as it was applied here had no significant impact on alpha-diversity. On grazed grasslands, we still observed species-poor communities dominated by tall species (B. phoenicoides and B. sylvaticum), whose capacity for competition will prevent a significant increase in species richness.

The experiments conducted remain insufficient to determine whether grazing can really increase plant species richness, because the flocks and herds had to be managed in a context where safety objectives (the need to detect potential water infiltrations immediately) took priority over nature conservation objectives. This is particularly true for site 1, along the power canal, where low-density cattle grazing, initiated in 1999 and considered unsuitable for safety reasons, was replaced by mowing at the rate of one cut/year in 2012. Yet it is impossible to conclude from this experiment that domestic livestock grazing does not meet safety requirements. The impact and consequences of grazing management can vary according to the intensity, the period and the stocking cycle (Ralphs et al. 1990; Hart and Ashby 1998; Fynn and O’Connor 2000; Dumont et al. 2009, 2012). Our results indicate that replacing grazing by mowing at this frequency and at this period (late in the season) was a bad choice in terms of nature conservation, revealed by the significant decrease in beta-diversity in 2014–2015 correlated with the dominance of B. phoenicoides.

On site 2, the low livestock density applied resulted in areas with initially species-rich short grasslands being neglected, leading to the development of species-poor tall grassland vegetation also dominated by B. phoenicoides. In this case, one main focus of restoration management by
grazing should be a better adjustment of grazing to forage
availability so as to increase the area of tall species-poor
grassland opened, thereby ensuring high light availability
within stands (Kotowski and Van Diggelen 2004). A higher
stocking rate than that applied in this embankment seems to
be required if this goal is to be achieved. On site 3, goat
grazing changed the botanical composition and seems able
to sustainably maintain open vegetation after forest clearing
without the domination of perennial grasses such as
*B. sylvaticum*.

These first experiments suggest how extensive grazing
may favorably impact such habitats. They should, however,
be considered only as a first step; far more extensive
experimentation needs to be performed, at a larger scale
(Bailey et al. 1996). In management by grazing, the choice
of animal is obviously crucial, but stocking rate and
stocking period are also determining factors (Ash and
Stafford Smith 1996; Metzger et al. 2005). Different kinds
of mixed grazing, such as horses and goats to prevent the
colonization of grassland by shrubs, or goats and sheep to
control the dominance of perennial grassland species where
shrubs have previously been eradicated by goats, could thus
be tested. However, different levels of instantaneous pres-
sure also need to be tested, seeking a better compromise
between conservation and safety objectives.

The grazing surveys revealed that the different breeds are
owned by different organizations (a farmer, the land con-
servationist and a private company), which may make it
difficult to set up an ideal grazing system. Experimenting
with new mixed grazing systems should not prevent man-
agers from continuing to monitor the impacts of current
grazing systems on plant species composition, richness and
diversity in order to characterize the long-term successional
processes and the consequences for the plants of high
conservation value that may still colonize these recent
ecosystems. Beyond purely environmental or agronomic
issues, decisions on adopting extensive grazing and espe-
cially on its duration will also be governed by socio-
economic and technical constraints (Poschlod and Wallis
De Vries 2002). Thus, it will be vital to specify the costs of
these new grazing systems, to ensure their sustainability.

**Acknowledgments** We would like to thank the *Compagnie Nationale du Rhône* (CNR), the owner of the study sites and the *Office National de la Chasse et de la Faune Sauvage* (ONCFS), the manager of the study sites, for their financial support. Many thanks also to SFR Tersys for the grant awarded to this project in 2014. Our thanks to Romain Brusson (CNR) and Denis Roux (ONCFS) for permission to access the different sites. Thanks to Daniel Pavon, Hervé Ramone, Arthur Rocher, Bénédicte Beylier, Laurent Garde, Bruna Romanini, Yannick Scali and Jean-Noël Laffitte for their technical help with the botanical inventories and grazing surveys. We thank Marjorie Sweetko for English language revision. Finally, we thank the two anonymous reviewers for their useful comments.

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