# Topsoil removal improves various restoration treatments of a Mediterranean steppe (La Crau, south-eastern France)

Renaud Jaunatre, Elise Buisson, Thierry Dutoit

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Renaud Jaunatre, Elise Buisson, Thierry Dutoit. Topsoil removal improves various restoration treatments of a Mediterranean steppe (La Crau, south-eastern France). Applied Vegetation Science, 2014, 17, pp.236-245. 10.1111/avsc.12063. hal-01329386

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# Topsoil removal improves various restoration treatments of a Mediterranean steppe (La Crau, southeast France)

Renaud Jaunatre, Elise Buisson & Thierry Dutoit

**Keywords**

Community composition; Grassland; Hay transfer; Nurse species seeding; Rehabilitation of former agricultural land; Restoration of grazing; Soil transfer; Species richness; Topsoil removal

**Nomenclature**

Base de Données Nomenclaturales de la Flore de France (Tela-Botanica 2011)

**Jaunatre**, R. (corresponding author, renaud.jaunatre@yahoo.fr),

**Buisson**, E. (elise.buisson@univ-avignon.fr) & **Dutoit**, T. (thierry.dutoit@imbe.fr): Université d'Avignon et des Pays de Vaucluse, Institut Méditerranéen de Biodiversité et d'Ecologie (UMR CNRS/IRD 7263/237), IUT, Site Agroparc BP 61207, 84911, Avignon Cedex 9, France

**Abstract**

**Question:** Can topsoil removal improve restoration efficiency of several ecological engineering techniques (nurse species seeding, hay transfer and soil transfer) for restoring a Mediterranean steppe, species-rich plant community?

**Location:** La Crau area, southeast France (Provence).

**Methods:** After the rehabilitation of a 357-ha herbaceous, sheep-grazed range-land suitable for threatened steppe birds in a formerly abandoned industrial orchard in the last French Mediterranean steppe (La Crau area), four experimental treatments were applied, with or without topsoil removal. Topsoil was removed on a 5000-m² area and the four treatments (control, nurse species seeding, hay transfer and soil transfer) were applied within this area where topsoil was removed and within the area where topsoil was not removed. Three years later, soil chemistry, soil seed bank, vegetation cover and height, plant species richness, composition and diversity were compared.

**Results:** Removing topsoil partly restored soil conditions and significantly reduced the non-target seed bank. Moreover, species richness and similarity to the reference steppe was significantly increased with topsoil removal. Topsoil removal, combined with soil transfer and hay transfer, created a community that most closely resembled the reference during the study period.

**Conclusion:** In order to restore plant community composition, topsoil removal is a relevant method to increase the efficiency of other ecological restoration techniques, especially those that strengthen dispersal processes.

**Introduction**

Restoration ecology is at the interface between theoretical ecology and ecological restoration (Falk et al. 2006). This science, which focuses on how to repair damaged and destroyed ecosystems (Society for Ecological Restoration International Science & Working Policy Group 2004), is increasingly requested since restoration was identified as a tool to slow biodiversity loss (Millennium Ecosystem Assessment 2005; Nellemann et al. 2010; Convention on Biological Diversity 2011). The choice of techniques used to restore ecosystems depends both on ecosystem characteristics and on the causes of their destruction, and this choice is based on knowledge provided by ecological theory (Falk et al. 2006). Intensive cultivation is one of the

modern land uses responsible for a significant part of the 39–50% of transformed land surface (Vitousek et al. 1997).

Intensive cultivation causes important and lasting damage to established vegetation. One of the first steps of intensive cultivation is usually deep ploughing, which suppresses at least the above-ground vegetation. This disturbance (i.e. a constraint that limits plant biomass by causing its destruction; Grime 1977) releases nutrients in soils and therefore also reduces stress (with stress defined as a growth limitation; Grime 1977). Its effects on vegetation are significant even a very long time after cultivation abandonment (Dupouey et al. 2002; Öster et al. 2009). The pre-disturbance plant community is indeed destroyed by ploughing and the seed bank usually does not contain
many species of this pre-disturbance community (Graham & Hutchings 1988). Moreover, increased nutrient content favours more competitive species (Huenneke et al. 1990). For these reasons, most restoration methods used after cultivation abandonment attempt to strengthen dispersal processes that also compensate for seed limitation (Kiehl et al. 2010; Hölzel et al. 2012) and/or to restore abiotic conditions (Marrs 2002). Among these, one restoration technique is of particular relevance in previously cultivated areas: topsoil removal. This technique indeed decreases the nutrient content and removes the persistent seed bank of arable weeds (Verhagen et al. 2001; Davy 2002). Removing topsoil has already provided promising results in former agricultural areas, favouring low productive plant communities (Aerts et al. 1995; Patzelt et al. 2001; Verhagen et al. 2001; Allison & Ausden 2004; Buisson et al. 2006b, 2008), and especially when combined with enhanced dispersal of target species (Patzelt et al. 2001; Hölzel & Otte 2003; Klimkowska et al. 2010b). Despite the fact that Mediterranean dry grassland plant communities hardly recover after intensive cultivation, to our knowledge topsoil removal has not been assessed as a restoration method. We therefore tested topsoil removal, combined with other restoration techniques, for the first time in a Mediterranean environment. We here present the results after 3 yrs of monitoring.

The objective of this study is to determine if topsoil removal can improve efficiency of other restoration methods – specifically nurse species seeding, hay transfer or soil transfer – in order to restore a Mediterranean steppe plant community. Our experiment took place in the La Crau area: a Mediterranean steppe plant community located in southeast France. Intensive cultivation has been abandoned in this area but, low dispersal and low seed production of target species, as well as higher competition due to increased nutrient content, lead to very low re-establishment of the plant community (Römermann et al. 2005; Buisson et al. 2006a; Coiffait-Gombault et al. 2012). Within this large-scale restoration project, three techniques – nurse species seeding, hay transfer and soil transfer – have been used experimentally either with or without topsoil removal with the objective of restoring the La Crau steppe vegetation (hereafter, steppe).

The design of this experiment made it possible to test five predictions: (1) removing topsoil restores initial soil conditions of the reference steppe ecosystem by removing the layer impacted by intensive cultivation (fertilization, tillage, drip irrigation); (2) removing topsoil decreases the non-target seed bank that originated from cultivation and mainly concentrated in the upper layer; and (3) from predictions (1) and (2), removing topsoil decreases vegetation cover and average height and thus may reduce the potential competitive effect of non-target species. Moreover, as

many target species tend to be poor competitors, topsoil removal may therefore favour their establishment: (4) species richness and similarity to the reference are increased by topsoil removal; and (5) the increase in species richness and similarity to the reference due to strengthening of dispersal processes through hay or soil transfer is improved by topsoil removal.

## Methods

### Study site

La Crau area is a Mediterranean steppe located in south-east France (Bouches-du-Rhône; ca. 43°31′N, 4°52′E), which has been driven by several thousand years of interactions between soil, Mediterranean climatic conditions and traditional sheep grazing (Devaux et al. 1983; Badan et al. 1995; Henry et al. 2010). The 40-cm deep soil is made up of 50% siliceous stones and lies on a calcareous conglomerate that cannot be penetrated by plant roots (Devaux et al. 1983). La Crau area has a Mediterranean climate, with high inter-annual variability, with an average of 540 mm yearly precipitation, mainly in spring and autumn, and 110 d per year with wind of >50 km·h⁻¹ (Devaux et al. 1983). Traditional extensive sheep grazing has taken place in La Crau area for more than 2000 yrs (Devaux et al. 1983; Badan et al. 1995), as in many typical Mediterranean rangelands (Le Houérou 1995). This xeric steppe is a species-rich plant community composed mainly of annuals and dominated by two perennials, *Brachypodium retusum* Pers. and *Thymus vulgaris* L., which represent >50% of overall plant community biomass. Moreover, this ecosystem hosts the largest French steppe bird populations, e.g. *Pterocles alcata* (pin-tailed sandgrouse), *Tetrax tetrax* (little bustard) and *Falco naumanni* (lesser kestrel) (Cheylan et al. 1983). Despite continuous work to protect this ecosystem (Buisson & Dutoit 2006; Dutoit et al. in press), this species-rich plant community, hosting the largest French population of steppe birds, has undergone many destruction events and has lost 80% of its original area mainly due to intensive cultivation or civil engineering (Wolff et al. 2009). Despite the fact that the steppe of La Crau is a unique ecosystem in France, these experiments will provide insights on restoration technique efficiency that are relevant to other relatively dry Mediterranean rangeland ecosystems, such as the Dehesas in Spain, Montados in Portugal and semi-arid ecosystems of north Africa.

### Experimental design

The experiment was implemented within a larger rehabilitation project: a 357-ha intensively cultivated orchard located approximately in the centre of the steppe area was abandoned in 2006 and rehabilitated in 2009 (Jaunatre
et al. 2011; Dutoit et al. 2013). The former orchard was mainly dominated by grasses after its abandonment, such as *Avena barbata* Link and *Bromus madritensis* L. The rehabilitation project within which the restoration experiment was implemented began in 2009, with the aim of re-creating a herbaceous steppe-like habitat for steppe birds. The rehabilitation consisted in (1) removing trees from the abandoned orchard; (2) leveling soils; and (3) reintroducing traditional sheep grazing in spring 2010. The experimental design was influenced by the needs within the rehabilitation project and allowed only pseudo-replication, but on relatively large areas. The topsoil was removed on a 5000-m² area, while the area without topsoil removal was >350 ha (Fig. 1). Four restoration treatments were tested, both where the topsoil was removed and where it was not removed: (1) control: i.e. no particular restoration technique; (2) nurse species seeding to rapidly cover bare ground and occupy vacant niches, and then to provide safe sites for recolonizing target species once sheep grazing is reintroduced; (3) hay transfer in order to provide seeds of local species; and (4) soil transfer in order to provide propagules of local species with associated microorganisms. Within the topsoil removal area, these four treatments were each applied on three replicate, randomly located 10 m × 10-m plots (Fig. 1). Within the area without topsoil removal, the four restoration treatments were applied on different areas described in Jaunatre et al. (2012): the control on ca. 270 ha, within which nurse species seeding was applied on 60 ha (2 × 30 ha), hay transfer on 20 ha (2 × 10 ha) and soil transfer on 3 ha (3 × 1 ha).

Topsoil removal consisted in removing the 20-cm upper soil layer. Three nurse species (*Lolium perenne* L., *Festuca arundinacea* Schreb. and *Onobrychis sativa* Lam) were cho-

sen for their palatability, purchase availability, ability to rapidly cover bare ground, but also for their low competitive ability in the long term under Mediterranean environmental conditions and their presence in artificial meadows of La Crau area (Devaux et al. 1983). The hay was gathered using a leaf vacuum truck in summer in a steppe patch located <5 km from the restoration experiment and was then spread at a 1:3 ratio (gathered on 1 ha, to be spread on 3 ha). The transferred soil is the 20-cm upper soil layer of a steppe patch of which the destruction was previously planned for construction work, located <2 km from the experiment. The bulk soil was gathered and spread at a 1:3 ratio (gathered on 1 ha, to be spread on 3 ha) within 1 d just before the first significant autumn rains (i.e. no stockpiling).

## Data collection

Soil seed bank samples were collected immediately after treatment implementation (i.e. in autumn 2009), whereas soil and vegetation sampling were performed 3 yrs after restoration.

Ten seed bank samples were spatially randomly collected in the steppe, in the control (i.e. after rehabilitation) and in the topsoil removal area, with a total of 30 samples. The seed bank was estimated according to the concentrated seedling emergence method (Ter Heerdt et al. 1996), with ten 2-L soil samples spread on a 1:4 (compost: vermiculite) substrate. Samples were randomly disposed in a greenhouse and germination counted, identified and removed each week for 3 mo. These data were used to estimate total number of germinated seed as well as number of germinated seeds from target and non-target species;

<figure><img src="image_2.png" /><figcaption>**Fig. 1.** Experimental design of areas without topsoil removal (a) and with topsoil removal (b); (b) is a detail of (a). Restoration techniques are nurse species seeding (N), hay transfer (H), soil transfer (S) and control (Ctrl).</figcaption></figure>
target species being species found in the reference steppe
community (Molinier & Tallon 1950).

Soil analyses were carried out on six samples taken in
2012 in the steppe, in the control (i.e. after rehabilitation)
and in the topsoil removal area, with a total of 18 samples
that were spatially randomly gathered. For each sample,
three 70-g subsamples of soil were randomly gathered
before being pooled and sieved through a 2-mm mesh
sieve for analyses at the INRA (Institut National de la
Recherche Agronomique). Nutrient analysis (organic C
(Anne's method), total N, C:N, Olsen-P (Olsen et al.
1954), Ca and K) and water pH were conducted following
standard methods (Baize 2000).

In each combination of treatments (i.e. topsoil removed
x restoration treatment), nine vegetation 2 m × 2 m
relevés were carried out in May 2012, 3 yrs after set-up,
totaling 126 samples. Where topsoil was not removed and
in the steppe, 18 vegetation 2 m × 2 m relevés were car-
ried out on the same date. Presence and abundance using
the abundance coefficient of all plant species were
recorded (0%: 0; <1%: 0.5; <5%: 1; <25%: 2; <50%: 3;
<75%: 4 and >75%: 5), as well as estimation of total vege-
tation cover and average height.

Data analysis

Similarity between each of the samples and the steppe was
assessed after calculating the mean similarity with each
steppe sample using the Bray-Curtis similarity index (Bray
& Curtis 1957). To compare soil chemistry in the steppe,
topsoil removal area and control area, as parametric condi-
tions were met, we performed ANOVA followed by Tukey
HSD *post-hoc* tests. To compare seed bank characteristics in
the steppe, topsoil removal area and control area, as para-
metric conditions were not met, we performed Kruskal-
Wallis tests, tested against a χ² distribution, followed by
Mann-Whitney *post-hoc* tests with Bonferroni p adjust-
ment. For vegetation characteristics (average height, vege-
tation cover, species richness and Bray-Curtis similarity),
parametric conditions were met, we therefore performed
three analyses: (1) an ANOVA to compare the steppe with
the effect of each treatment without removing topsoil, fol-
lowed by Tukey HSD test; (2) a second ANOVA to compare
the steppe with the effect of each treatment after removing
topsoil, followed by Tukey HSD test; (3) four *t*-tests to com-
pare treatments with or without topsoil removal in each of
the four restoration treatments (control, nurse species
seeding, hay transfer and soil transfer), the *P*-values were
recalculated with Bonferroni adjustment. Non-metric mul-
tidimensional scaling (NMDS) ordination was performed
on species occurring in at least three relevés to obtain an
overview of vegetation composition according to each
treatment, as this is the most robust unconstrained ordina-

tion method in community ecology (Minchin 1987). This
ordination used 20 starts from random configuration to
avoid local minima. This NMDS ordination was performed
with species occurring in at least three relevés and was fol-
lowed by multiple response permutation procedure analy-
sis (MRPP) to detect significance of differences between
with and without topsoil removal. All calculations and
analyses were performed with packages 'stats' and 'vegan'
in R 2.13.0 (R Foundation for Statistical Computing,
Vienna, AT).

Results

Effect of topsoil removal on soil chemical characteristics

Soil chemical characteristics were significantly affected by
topsoil removal (Fig. 2). Nitrogen and carbon content
were significantly lower when topsoil was removed, com-
pared either to the control or to the steppe (respectively,
F<sub>2,12</sub> = 23.79, *P* < 0.001; Fig. 2a; F<sub>2,12</sub> = 16.84, *P* < 0.001;
Fig. 2b). Phosphorus and potassium were significantly
higher in the control but there was no significant differ-
ence between the steppe and after topsoil removal
(respectively, F<sub>2,12</sub> = 14.43, *P* < 0.001; Fig. 2c;
F<sub>2,12</sub> = 18.02, *P* < 0.001; Fig. 2d). The pH increased from
the steppe (6.79 ± 0.06) to the control (7.37 ± 0.08) to
the topsoil removal (8.09 ± 0.12) treatment
(F<sub>2,12</sub> = 52.77, *P* < 0.001; Fig. 2e).

<figure><img src="image_3.png" /></figure>

**Fig. 2.** Effect of topsoil removal on nitrogen (g·kg⁻¹) (a), carbon (g·kg⁻¹) (b), phosphorus (g·kg⁻¹) (c) potassium (g·kg⁻¹) (d) and pH (e). Average ±SE of the Steppe (Ste.) compared to control (Ctrl), and topsoil removal (TSR). Bars with common letters do not differ significantly according to Tukey HSD *post-hoc* tests (*P* > 0.05). Asterisks indicate significant differences according to ANOVA (****P < 0.001; **P < 0.01; *P* < 0.05 and NS: *P* > 0.05).
## Effect of topsoil removal on the seed bank

The total number of germinated seeds was not significantly different between the steppe, the control and after topsoil removal ($\chi^2 = 5.66$, *df* = 2, *P* = 0.059; Fig. 3a). Nevertheless, the number of non-target species germinated seeds was significantly higher in the control and was reduced to values similar to the steppe after topsoil removal ($\chi^2 = 6.97$, *df* = 2, *P* = 0.031; Fig. 3b). In contrast, there were no germinated seeds of target species in the control and after topsoil removal, while around 9.63 ± 4.17 seeds germinated in the steppe ($\chi^2 = 21.51$, *df* = 2, *P* < 0.001; Fig. 3c).

## Effect of topsoil removal on the established vegetation

With or without topsoil removal, average plant height was not affected by restoration treatments, the only difference (ANOVA, respectively, $F_{4,85} = 6.93$, *P* < 0.001 and $F_{4,49} = 3.59$, *P* = 0.012) was in the steppe, which had the lowest average height (6.22 ± 0.68 cm; Fig. 4a). The results were similar for vegetation cover: the steppe had lower vegetation cover (58.61 ± 2.77%) compared to hay

transfer or soil transfer only without topsoil removal ($F_{4,85} = 4.84$, *P* = 0.002; Fig. 4b), whereas there was no significant differences between the steppe and treatments after topsoil removal ($F_{4,49} = 2.06$, *P* = 0.101). Within each restoration treatment, removing topsoil did not significantly reduced average plant height, while it significantly reduced vegetation cover on hay transfer (*df* = 11, *t* = 5.13, *P* = 0.002) and on soil transfer (*df* = 10, *t* = 2.83, *P* = 0.036).

Without topsoil removal, soil transfer had no significant effect on species richness from the steppe (respectively, 28.72 ± 1.80 and 32.94 ± 1.33 spp. 4 m⁻²; Fig. 4c), while other restoration treatments had lower species richness but no significant differences among them ($F_{4,85} = 60.08$, *P* < 0.001). After removing topsoil, soil transfer and hay transfer produced similar species richness to the steppe (>28 spp. 4 m⁻²). Species richness for control and nurse species seeding after topsoil removal remained significantly lower than on the steppe ($F_{4,49} = 12.42$, *P* < 0.001). Over all restoration treatments, topsoil removal significantly increased species richness. The NMDS ordination of vegetation (Fig. 5) and MRPP showed significant discrimination among restoration treatments with or without topsoil removal (observed delta: 0.45; expected delta: 0.59, *P* = 0.001). Without topsoil removal, grasses dominated the vegetation, e.g. *Bromus diandrus* Roth or *B. madritensis* L., representing an average of 50% cover, while after topsoil removal forbs dominated the vegetation, e.g. *Veronica arvensis* L., *Crepis sancta* (L.) Bornm. or *Trifolium scabrum* L. Seeding with nurse species was characterized by the sown species *Lolium perenne* L. and *Festuca arundinacea* Schreb., both with and without topsoil removal. Soil transfer produced the closest composition to that of the steppe, characterized by e.g. *Logfia gallica* (L.) Coss. & Germ. *Brachypodium distachyon* (L.) P. Beauv. or *Poa bulbosa* L. According to this ordination, each restoration treatment seemed closer to the steppe with topsoil removal (Fig. 5). This trend was confirmed as Bray–Curtis similarities to the steppe (Fig. 4d). Topsoil removal indeed significantly increased Bray–Curtis similarity to the steppe within each restoration treatment (Fig. 4d). In addition, Bray–Curtis similarity in hay transfer was significantly higher than in the control after topsoil removal, while it was not significantly different without topsoil removal (Fig. 4d).

(a) Number of total germinated seeds

<figure><img src="image_4.png" /></figure>

(b) Number of non-target germinated seeds

<figure><img src="image_5.png" /></figure>

(c) Number of target germinated seeds

**Fig. 3.** Effect of topsoil removal on total number of germinated seeds (a), number of non-target species germinated seeds (b), and number of target species germinated seeds in the seed bank (c). Average ± SE of the Steppe (Ste.) compared to control (Ctrl), and topsoil removal (TSR) communities. Bars with common letters do not show any significant differences according to Mann–Whitney *post-hoc* tests with Bonferroni *P* adjustment (*P* > 0.05). Asterisks indicate significant differences according to Kruskal–Wallis tests (***$P < 0.001$; **$P < 0.01$; *$P < 0.05$, NS $P > 0.05$).

## Discussion

As reported in other studies in more temperate region (Aerts et al. 1995; Patzelt et al. 2001; Verhagen et al. 2001; Allison & Ausden 2004), the current experiments in the La Crau area show that topsoil removal has a significant effect on soil properties, seed bank and vegetation characteristics and hence on restoration success.
<figure><img src="image_6.png" /><figcaption>**Fig. 4.** Effect of topsoil removal on average plant height (cm) (a), vegetation cover (%) (b), species richness (4 m²) (c) and Bray-Curtis similarity (d). Average ±SE of the Steppe (Ste.) compared to control (Ctrl), nurse species seeding (Nurse), hay transfer (Hay t.) and soil transfer (Soil t.), either without topsoil removal (no) or with topsoil removal (TSR). Bars with common letters do not differ significantly according to Tukey HSD post-hoc tests (*P* > 0.05), without topsoil removal (lowercase letters) and with topsoil removal (uppercase letters). Within a restoration technique, asterisks indicate significant differences according to t-tests between with or without topsoil removal (***P < 0.001; **P < 0.01; *P* < 0.05; NS *P* > 0.05).</figcaption></figure>

## Effect of topsoil removal on soil properties and seed bank

Primary conditions can strongly affect the course of vegetation dynamics (Ross & Harper 1972; Kardol et al. 2013), which is why the seed bank, i.e. the internal species pool, is one of the key parameters necessary to understand secondary succession. Another key parameter is abiotic conditions (soil, climate) that have a large influence on relative competition. Six years after intensive orchard cultivation abandonment and 3 yrs after soil leveling, the soil content of phosphorus and potassium was still higher than in the steppe, while nitrogen and carbon content were

similar. Such an increase in nutrient content has already been identified as having a very long-term effect on vegetation (Dupouey et al. 2002; Jacquemyn et al. 2003; Semelová et al. 2008; Smits et al. 2008; Královec et al. 2009). While topsoil removal allowed recovery of steppe phosphorus and potassium content, it also significantly decreased nitrogen and carbon content below steppe levels and increased the pH above steppe levels. Our first prediction of the restoration of soil chemical properties after topsoil removal is therefore partly rejected. Nevertheless, a reduced nutrient content may reduce growth and prevent arriving non-target species from forming a dense cover (Marrs 2002). The number of germinated seeds in
<figure><img src="image_7.png" /><figcaption>**Fig. 5.** Non-metric multidimensional scaling ordination based on 126 2 × 2 m vegetation relevés and the 120 species present in at least three samples (with 15.46 stress). Samples are grouped by treatment: steppe in white, control, nurse species seeding, soil transfer and hay transfer either without topsoil removal (light grey) or with topsoil removal (dark grey). For clarity, only the 38 most correlated species are shown.</figcaption></figure>

the seed bank was not significantly different in the con-
trol, in the steppe or after removing topsoil. Nevertheless,
we obtained contrasting results when separating target
and non-target species: the number of non-target species
was significantly reduced to steppe levels after topsoil
removal, while the number of target species remained
zero with or without topsoil removal, which confirms that
the seed banks of previously cultivated areas are rarely
made up of target species; the latter not remaining viable
in the soil during several years of intensive cultivation
(Thompson & Grime 1979; Graham & Hutchings 1988;
Hutchings & Booth 1996; Stroh et al. 2002; Buisson et al.
2006a). Our second prediction was a decrease in the non-
target seed bank after topsoil removal, which is therefore
not rejected.

Effect of topsoil removal on vegetation cover and average plant height

Although soil chemical properties changed after topsoil
removal, they did not reach steppe values. The changes
induced more stressful abiotic conditions (i.e. nutrient-
poor sites), which can be expected to limit the develop-
ment of dense vegetation. The third prediction is only
partly rejected after 3 yrs: topsoil removal indeed
decreased average vegetation height and cover but was sig-
nificant only within some restoration treatments (hay
transfer and soil transfer). Topsoil removal has already
been shown to decrease above-ground vegetation cover in
other restoration experiments (Allison & Ausden 2004;
Buisson et al. 2006b). The lack of significant changes in
vegetation cover and average height in our case study may

be explained by relatively severe climatic conditions in the
third year, which almost homogenized cover and average
height to low values. Nevertheless the slightly decreased
vegetation density may have reduced competition toward
less competitive, light-demanding target species and thus
favoured their recolonization (Henry et al. 2004; Hautier
et al. 2009).

Effect of topsoil removal on the plant community

Species richness was increased within all restoration
treatments, allowing hay transfer and soil transfer plots
to become as species-rich as the steppe. Moreover, our
results showed that composition changed after topsoil
removal, with a lower cover of grass species and more
occurrences of target species. Our fourth prediction (that
species richness and similarity to the reference commu-
nity increased after topsoil removal), is therefore not
rejected with our results. This is in accordance with pre-
vious studies in more temperate ecosystems, which have
shown that topsoil removal is an efficient technique to
restore species richness and composition of species-rich,
semi-natural grasslands (Verhagen et al. 2001; Kiehl &
Pfadenhauer 2006). Our fifth prediction is also not
rejected: species richness and similarity to the reference
community due to reduced seed limitation after hay or
soil transfer was improved with topsoil removal. Hay
transfer indeed gave very limited positive effects when
abiotic conditions were not specifically restored with
topsoil removal, the establishment of target species being
conditioned by the presence of available microsites
(Standish et al. 2007; Dybzinski & Tilman 2012). Topsoil
removal allowed improvement of the effect of other restoration techniques, and confirmed that a combination of hay transfer with topsoil removal is an effective way to restore species-rich plant communities even within a few years, as shown in more temperate grasslands (Patzelt et al. 2001; Hölzel & Otte 2003; Klimkowska et al. 2010b).

## Perspectives

Our results highlight the importance of both abiotic and dispersal filters, which both influence competition and hence influence the biotic filter. Simple models of thresholds (Whisenant 1999) or filters (Keddy 1992; Zobel 1997; Fattorinni & Halle 2004; Lortie et al. 2004) are useful conceptual models, but should not underestimate the complexity of ecosystem functioning (Hobbs & Norton 2004). Applied to ecological restoration, our results confirm that neither restoring abiotic conditions or strengthening dispersal of target species alone guarantee successful restoration of a Mediterranean steppe, but should rather be implemented together (Donath et al. 2003; Öster et al. 2009). Once the processes that drive successful plant community restoration are identified, techniques should be optimized in order to increase efficiency and decrease cost. The removal of topsoil has high environmental and financial costs (Klimkowska et al. 2010a), and further research should be carried out to mimic its positive effects and avoid its negative effects. Soil compaction or turning the soil profile through ploughing could be used (Bakker & Van Diggelen 2006): at lower cost, the first could decrease soil nutrient availability and space available for roots, and thus decrease the density of potentially competitive species; the second would replace the upper soil layer with a soil having a lower nutrient content and would bury the seed bank. To optimize and strengthen dispersal techniques, the methods and application ratio should be adapted to the target community, as shown in Kiehl et al. (2010): the more the nutrient content is reduced, the lower the ratio to be used.

## Acknowledgements

This study was supported by CDC Biodiversité, CEEP Ecomusée de Crau, the Réserve Naturelle des Coussouls de Crau, the Conseil Régional de Provence Alpes Côtes d'Azur and CNRS RTP Ingecotech funding. We are also grateful to Laurence Affre for greenhouse space and to students and field assistants for help with fieldwork and seed bank studies: Jean-François Alignan, Noëllie Fonvieille, Alexandre Fuentes, Laurie Gariglio and Daniel Pavon. We are also grateful to two anonymous reviewers for their thoughtful comments on the manuscript.

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