# Influence of former cultivation on the unique Mediterranean steppe of France and consequences for conservation management

C. Romermann, Thierry Dutoit, P. Poschlod, Elise Buisson

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C. Romermann, Thierry Dutoit, P. Poschlod, Elise Buisson. Influence of former cultivation on the unique Mediterranean steppe of France and consequences for conservation management. Biological Conservation, 2005, 121, pp.21-33. 10.1016/j.biocon.2004.04.006. hal-02676652

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# Influence of former cultivation on the unique Mediterranean steppe of France and consequences for conservation management ☆

Christine Römermann<sup>a,*</sup>, Thierry Dutoit<sup>b</sup>, Peter Poschlod<sup>a</sup>, Elise Buisson<sup>c</sup>

<sup>a</sup> Institute of Botany, University of Regensburg, Universitätsstrasse 31, D-93051 Regensburg, Germany

<sup>b</sup> UMR INRA-UAPV 406 "Ecologie des Invertébrés", Université d'Avignon, Site Agroparc, Domaine Saint-Paul, 84914 Avignon Cedex, France

<sup>c</sup> UMR/CNRS 6116 IMEP, FST Saint Jérôme, University of Provence, case 461, 13397 Marseille Cedex 20, France

## Abstract

In Europe, the actual landscape has been mainly influenced by human activities. Agricultural intensification led to a considerable habitat loss and fragmentation, especially for dry semi-natural grasslands.

This current study investigates the impact of former melon and cereal cultivation (cultivation period: 1950–1987) on the semi-natural vegetation of the Crau, representing the last xeric Mediterranean steppe in France.

Today, the ex-cultivated melon and cereal fields are characterised by different vegetation compositions, species richness and evenness compared to the undisturbed steppe community. Also the abiotic conditions (N, P, K, pH, soil granule fractions) have been changed by former cultivation practices. The rather transient seed bank of the steppe was depleted during the cultivation periods; ancient weed species and ruderals now determine the seed bank of the ex-cultivated fields.

It is concluded that the conservation of the last parts of undisturbed steppe must have absolute priority. A re-development of the original and unique steppe community on formerly cultivated fields may take decades or centuries, if at all.

Keywords: Crau; Dry grassland; Restoration ecology; Seed bank; Secondary succession

## 1. Introduction

Within the last few decades, agricultural intensification led to a considerable floristic change and decrease of semi-natural ecosystems in Europe (Hodgson and Grime, 1990; Akinola et al., 1998; Poschlod et al., 1998; WallisdeVries et al., 2002). Many formerly continuous types of grassland have become extinct or fragmented, and in most cases only small and isolated patches of ancient remnants remain. In such fragmented landscapes, restoration of ex-arable fields on ancient grasslands plays a major role as restored sites could enlarge and connect these remnants. The restoration depends on the abiotic (e.g. soil fertility) and biotic conditions of the

abandoned arable fields (e.g. modified competitive interactions; Saunders et al., 1991) and on temporal and spatial dispersal abilities of the characteristic species of the target community (Poschlod et al., 1998).

In north-western Europe, the short-term (Austrheim and Olsson, 1999) and long-term (Wells et al., 1976) effects of ploughing on present-day dry calcareous vegetation were clearly identified. Ex-cultivated plots showed a decrease in species-richness and changes in their botanical composition compared with adjacent dry grasslands. Cultivation, however, also influences the direction taken by post-cultural plant succession (Gibson and Brown, 1991). This phenomenon is linked to increased soil fertility of ex-cultivated plots because of ancient fertiliser applications during the cultivation period (Gough and Marrs, 1990) and the presence of ruderal species (*sensu* Grime, 1979), which are known to have persistent seeds in the soil (Graham and Hutchings, 1988a,b; Dutoit and Alard, 1995; Dutoit et al., 2003).
However, also seeds of target species might survive the cultivation period in deeper soil layers, providing the fact that ploughing were not too deep (Bekker et al., 1997). Hence, it is of great importance, to what extent such a “memory” of the ancient vegetation exists (Bakker et al., 1996b), and if it could act as a source of propagules in the restoration of fallow lands (Willems and Bik, 1998). However, most of the characteristic grassland species do not form persistent soil seed banks (Thompson and Grime, 1979; Graham and Hutchings, 1988a,b; Bakker et al., 1996a), and would, therefore, not survive cultivation periods. The restoration value of dry grassland seed banks is, therefore, very small (Dutoit and Alard, 1995; Bakker et al., 1996b). Hence, successful reestablishment of the target species depends on the function of dispersal vectors as for example sheep grazing (Marshall and Hopkinson, 1990; Fischer et al., 1996) since the species own dispersal capacities in space are very low (Poschlod et al., 1998). The introduction of sheep herding would also positively influence vegetation dynamics by limiting competition between stress tolerant target species and ruderal ancient weed species (Gibson and Brown, 1991), especially in those ecosystems where plant communities have been determined by sheep grazing for many centuries.

Though past agricultural land uses have affected the composition and structure of the dry herbaceous ecosystems throughout the landscape around the Mediterranean Sea (Grove and Rackham, 2001), effects of former ploughing on present day vegetation have received little attention in this region compared to north-western Europe. Formerly ploughed sites are often difficult to locate because they are masked by the effects of a high number of wild fires, homogenising the vegetation during secondary succession following abandonment of cultivation (Trabaud and Galtie, 1996).

The Crau represents the last xeric steppe of south-eastern France (Devaux et al., 1983) which has been shaped by sheep grazing for centuries (Rinschede, 1979; Fabre, 1998). The steppe vegetation represents a plant species association which is unique in Europe (Devaux et al., 1983). It provides a habitat for many endangered animal species, e.g. for the endemic grasshopper *Prionotropis hystrix rhodanica* (Foucart and Lecoq, 1998) and the rare bird species *Pterocles alchata* (sandgrouse; Wolff, 1998). However, destruction by agricultural practices started in the 16th century, when irrigation systems were brought up and the northern parts were transformed into intensively used grasslands. At the beginning of the twentieth century, large parts of the dry semi-natural grasslands (called coussous) have been transformed into arable, industrial or military land, leading to the fragmentation of the former 60,000 ha steppe vegetation (Etienne et al., 1998). After abandonment of most arable fields in the 1980s, traditional transhumance has been reintroduced, connecting ex-

cultivated fields with undisturbed steppe vegetation (Dureau and Bonnefon, 1998).

Hence, the aims of the current study were: (i) to describe the vegetation of coussous, ex-cultivated melon and ex-cultivated cereal fields and (ii) to investigate the impact of historical cereal and melon cultivation on the biotic and abiotic conditions of the ex-cultivated fields (e.g. species richness, evenness, soil chemistry, etc.) and (iii) to investigate the impact of historical cultivation on the seed bank of the steppe plant community, and its possible contribution for the restoration of the ex-cultivated fields. The general aim from a conservational point of view is, therefore, to find out if a re-development of the original coussous vegetation is possible on arable fields after abandonment.

## 2. Materials and methods

### 2.1. Study area

The study was carried out in the Crau, located about 50 km northwest of Marseille, France (Fig. 1). The plain is an old, stony river delta which was shaped by several changes of the Durance river bed between 650,000 and 120,000 years ago (Devaux et al., 1983). The dryness of the ecosystem is induced by the Mediterranean climate (mean temperature 14.5 °C, yearly precipitations of 500–600 mm; Cherel, 1988); precipitation does rarely occur within the vegetation period. Soil properties are exceptional: a water impermeable conglomerate in 40–60 cm depth (called locally “taparas”) separates the ground water from the pebbly soil (Devaux et al., 1983).

While the steppe plant community of central Crau is determined by *Brachypodium retusum* (Poaceae), *Thymus vulgaris* (Lamiaceae) is the dominant plant species in the western regions (Loisel et al., 1990).

Between 1950 and 1960 relatively moderate cereal cultivation (without deep ploughing) and fertilisation took place (Devaux et al., 1983). Melon cultivation (1965 until now) was more intensive as it destroyed the “taparas” by deep ploughing, simplifying irrigation afterwards (Borck, 1998). Furthermore, huge amounts of fertilisers and pesticides were applied (Le Gloru, 1956). Before 1975, melon cultivation took place in small plastic tunnels (1.7 m width, 0.7 m height; Borrey, 1965). Afterwards, large plastic tunnels were used (3 m width, 2 m height), leaving always track ways of about 2 m between the tunnels. At most sites, cultivation stopped around 1990 because of melon overproduction, non-profitable cultivation and hard working conditions.

### 2.2. Study sites

As vegetation composition of central and western regions of the Crau is different, four study areas in
<figure><img src="image_2.png" /><figcaption>Fig. 1. Map of the Crau (according to Devaux et al. (1983), modified). Western (Gamadou, Negreiron, Négrès) and central (Peau de Meau, Magnan, Pt. Abondoux, Valigne) study sites are marked.</figcaption></figure>

central Crau (Magnan, Valigne, Peau de Meau, Pt. Abondoux) and three areas in western Crau (Gamadou, Negreiron, Négrès) were chosen (Fig. 1).

The sampling areas provide always an ex-cultivated field with adjacent undisturbed steppe. At Pt. Abondoux, only one ex-cultivated cereal field was sampled. In total six plots of coussous, six ex-cultivated melon fields and three ex-cultivated cereal fields were sampled. At each site, the same flock of sheep was grazing both steppe and ex-cultivated plots.

With the help of documentary evidence and aerial photographs (1947–1998, with gaps), the histories of the sampling sites were reconstructed (Table 1).

## 2.3. Vegetation and soil sampling

Vegetation and soil analyses were carried out in the centre of all study sites. For vegetation analyses, 400 m² (20 m × 20 m) of each site were sampled with 10 randomly placed quadrates of 2 m × 2 m, divided into 25 sub-quadrates of 0.16 m² using the frequency method (Mueller-Dombois and Ellenberg, 1974). Percentage cover of vegetation, stones and bare ground were surveyed.

For soil analyses, five replicate samples (200 g each) were taken per site. After drying and sieving (200 µm), nutrient analyses (C, N, P₂O₅, CaO, MgO, K₂O, O.M., C:N, pH) were carried out by the INRA (Institut National de la Recherche Agronomique) using the methods illustrated in Baize (2000). Percentage content of clay (<0.002 mm), fine silt (0.002–0.02 mm), coarse silt

(0.02–0.05 mm), fine sand (0.05–0.2 mm) and coarse sand (0.2–2 mm) were investigated at the University of Aix-Marseille III (Baize, 2000).

## 2.4. Seed bank sampling and analysis

At the site of Peau de Meau (central Crau), the seed bank of the steppe plant community (PC), the ex-cultivated cereal field (PFc1) and the ex-cultivated melon field (PM) were sampled in April 2001 before the input of fresh seeds and after the natural stratification of dormant seeds in winter (persistent viable seed bank).

For each land-use type, 10 replicate soil samples were taken consisting each of 10 pooled soil cores according to the recommendations in Bakker et al. (1996a). After taking the soil cores (4 cm in diameter, 20 cm depth), they were divided into two depth layers (0–10 and 10–20 cm) in order to account for the cultivation impact on seed distribution in the soil (ploughing regimes, up to 20–40 cm depth!). The soil of the two depth layers was pooled, respectively, to one sample each of 1256 cm³ (1.26 litre) per replicate and depth.

For seed bank cultivation, the “seedling emergence method” (ter Heerdt et al., 1997) was chosen to qualify and quantify germinable seeds in the soil seed bank. Soil samples were concentrated by washing them with water on two different sieves (2 mm and 200 µm) to reduce bulk and clay. The concentrated soil was spread in a thin layer (0.5 cm) in plastic trays filled with 2 cm vermiculite topped with medical compresses (mesh size about 100
<table><caption>Table 1
Cultivation history of the sampled sites of central and western Crau</caption><thead><tr><th></th><th>Sites</th><th>Sampled plots</th><th>Abbreviation</th><th>Cultivation type</th><th>Time period of arable cultivation</th></tr></thead><tbody><tr><td rowspan="8">Central Crau</td><td rowspan="2">Magnan</td><td>Coussous</td><td>MC</td><td>—</td><td>—</td></tr><tr><td>Ex-cultivated melon field</td><td>MM</td><td>Melons (large tunnels)</td><td>1993–98</td></tr><tr><td rowspan="2">Valigne</td><td>Coussous</td><td>VC</td><td>—</td><td>—</td></tr><tr><td>Ex-cultivated melon field</td><td>VM</td><td>Melons (large tunnels)</td><td>1978, 1984</td></tr><tr><td rowspan="6">Peau de Meau</td><td>Coussous</td><td>PC</td><td>—</td><td>—</td></tr><tr><td>Ex-cultivated melon field</td><td>PM</td><td>Melons (small tunnels)</td><td>1967</td></tr><tr><td></td><td></td><td>Melons (large tunnels)</td><td>1984</td></tr><tr><td>Ex-cultivated cereal field</td><td>PFc1</td><td>Cereals</td><td>1948–64</td></tr><tr><td>Ex-cultivated cereal field</td><td>PFc2</td><td>Melons</td><td>1974</td></tr><tr><td>Ex-cultivated cereal field</td><td></td><td>Cereals</td><td>1975</td></tr><tr><td rowspan="2">Western Crau</td><td rowspan="2">Gamadou</td><td>Petit Abondoux</td><td>AFc</td><td>Melons</td><td>1979</td></tr><tr><td>Ex-cultivated cereal field</td><td></td><td>Cereals</td><td>1984</td></tr><tr><td rowspan="2">Negreiron</td><td rowspan="2">Negreiron</td><td>Coussous</td><td>GC</td><td>—</td><td>—</td></tr><tr><td>Ex-cultivated melon field</td><td>GM</td><td>Melons (large tunnels)</td><td>1986</td></tr><tr><td rowspan="2">Négrès (Bergerie Romaine)</td><td rowspan="2">Négrès (Bergerie Romaine)</td><td>Coussous</td><td>NC</td><td>—</td><td>—</td></tr><tr><td>Ex-cultivated melon field</td><td>NM</td><td>Melons (large tunnels)</td><td>1978/79</td></tr><tr><td rowspan="2">Négrès (Bergerie Romaine)</td><td rowspan="2">Négrès (Bergerie Romaine)</td><td>Coussous</td><td>BRC</td><td>—</td><td>—</td></tr><tr><td>Ex-cultivated melon fields</td><td>BRM</td><td>Melons (small tunnels)</td><td>1975, 1982/83</td></tr></tbody></table>

µm). The trays were covered with fine gauze material to prevent contamination. All trays were watered frequently from below. Emerged seedlings were identified (Muller, 1978; Marmarot, 1997), counted and removed weekly. Unknown seedlings were grown for identification. The seed bank samples were cultivated for in total six months including a six week chilling period after the third month to break secondary dormancy of the seeds.

## 2.5. Data analyses

### 2.5.1. Vegetation and soil analyses

A detrended component analysis (DCA) was carried out on the raw vegetation data (PC-ORD 4.0, McCune and Mefford, 1999). Plots and species frequencies were ordinated and, to present a bi-plot, species were correlated with it.

Comparisons of vegetation and soil parameters were carried out on different levels:

* for western and central sites (coussous and ex-cultivated melon fields with 30 quadrates each),

* for the three land-use types (coussous, ex-cultivated cereal fields and ex-cultivated melon fields with 30 quadrates each),

* for all different plots (for each site and each vegetation type with 10 quadrates each).

The parameters species richness and evenness were calculated (formulas see Mühlenberg, 1993). These two parameters and vegetation, stone and bare ground cover, soil nutrients and granule sizes were compared on the different levels using univariate tests. Normal distributed data with homogeneous variances were analysed by single-factor ANOVAs with subsequent post-hoc LSD tests (least significant difference). The remaining data were compared by Kruskal-Wallis *H* tests with following Mann-Whitney *U* tests. For the comparison of only two data sets, pair-wise *t* tests were applied. For all statistical tests, SPSS Inc. (Release 10.0) for Windows was used.

### 2.5.2. Combination of vegetation data with abiotic parameters

To identify the factors which have influenced the species compositions of the various plots, a principal component analysis was carried out. Vegetation data was the main matrix; the standardised second matrix contained the parameters soil chemistry, soil granule fractions, vegetation, stones and bare ground cover, time of abandonment, time period of land use, cultivation types (cereals, melons, none) and melon cultivation regime (small vs. large tunnels).

### 2.5.3. Seed bank analyses

Total number of species and seeds were counted and recorded for each plot. Mean number of species per sample and mean number of seeds per species were calculated and compared. Mean number of seeds per sample of 126 cm² were calculated and statistically compared. For the different depths levels (0–20, 0–10 and 10–20 cm) seed numbers per sample were extrapolated to 1 m² and compared. For all statistical comparisons, the same methods of analyses were applied as described above using SPSS 10.0. Evenness was calculated for each plot.
### 2.5.4. Combination of seed bank data with vegetation data

To compare seed bank composition with vegetation composition, a DCA was carried out on a power-transformed matrix containing the combined data of seed bank and established vegetation.

## 3. Results

### 3.1. Vegetation attributes

In the bi-plot (Fig. 2) the 30 quadrates (4 m²) of western and central coussous are clumped, respectively, confirming different species compositions of western and central undisturbed steppe vegetation. Differently, the quadrates (4 m²) of central and western ex-cultivated melon plots are mixed. The ex-cultivated cereal plots PFc2 and AFc have intermediate positions between coussous and ex-cultivated melon plots.

Species being strongly correlated with the described plot ($r^2$ = 0.55) are presented as vectors pointing to the corresponding direction. *Neatostema apulum*, *Trifolium scabrum*, *Sherardia arvensis* and *Thymus vulgaris* are characteristic for western coussous. Representatives for the central coussous are *B. retusum* and *Linum strictum*.

None of the typical species of the ex-cultivated plots were strongly correlated with the axes. However, *Bromus madritiensis*, *B. rubens*, *Rostraria cristata*, *Lobularia maritima* and *Calamintha nepeta* were very abundant in the ex-cultivated plots; they were dominant species in ex-cultivated melon fields. Characteristic and dominant species in ex-cultivated cereal plots were *Aegilops ovata*, *Trifolium subterraneum*, *Hedypnois cretica* and *Geranium molle*.

### 3.2. Species richness and evenness

Concerning species richness ($S$) and evenness ($E$) the western coussous had significantly higher values than central coussous ($S_{west} = 45.8$ vs. $S_{centre} = 36.1$; $U_S = 199.5, p < 0.001$; $E_{west} = 0.91$ vs. $E_{centre} = 0.88$; $U_E = 183, p < 0.001$). Significant differences between western and central ex-cultivated melon plots were not detected ($S_{west} = 29.97$ vs. $S_{centre} = 32.53$; $E_{west} = 0.86$ vs. $E_{centre} = 0.87$). Significant differences were found when comparing all studied plots ($\chi_S^2 = 123.09, p < 0.001$; $F_E = 20.451, p < 0.001$; see Table 2). Generally, undisturbed steppe vegetation had higher values in both species richness and evenness (Table 2).

### 3.3. Abiotic factors influencing vegetation dynamics

#### 3.3.1. Vegetation, stone and bare ground cover

Vegetation and stone cover were higher in central compared to western coussous (vegetation: 55% vs. 49%, $T = 2.04, p < 0.05$; stones: 50% vs. 15%, $T = 19.2, p < 0.001$). Bare ground cover was higher in western coussous (39% vs. 11%, $T = -0.918, p < 0.001$). Western and central ex-cultivated melon plots showed equal values for stone and bare ground cover (stones: 44.87% vs. 44.88%; bare ground: 19.65% vs. 22.96%). In vegetation cover, western ex-cultivated melon plots had even higher values than central ones (43.4% vs. 39.5%; $T = -2.1, p < 0.05$).

As shown in Fig. 3, melon and cereal cultivation resulted in a modification of vegetation ($F = 19.11, p < 0.001$), stone ($F = 47.0, p < 0.001$) and bare ground cover ($F = 25.24, p < 0.001$).

<figure><img src="image_3.png" /><figcaption>Fig. 2. DCA ordination of the established vegetation in central and western Krau. Matrix: 160 species, 150 quadrates of 15 sites (see Table 1 for abbreviations of the study sites; first letter: site; second/third letter: land use type C=Coussous, M=ex-cultivated melon field; Fc=ex-cultivated cereal field); $r^2$-cut-off-value = 0.55. Species correlated with axis 1: *Trifolium scabrum* ($r^2$ = 0.698), *Sherardia arvensis* ($r^2$ = 0.574), *Neatostema apulum* ($r^2$ = 0.573) and *Thymus vulgaris* ($r^2$ = 0.55). Species correlated with axis 2: *Brachypodium retusum* ($r^2$ = 0.748) and *Linum strictum* ($r^2$ = 0.664).</figcaption></figure>
<caption>Table 2</caption>

Mean species richness (S) and mean species evenness (E) of the different sites. Maximums (>45 species) are bold; minimums (<20 species) are bold and italic

<table><thead><tr><th></th><th>Site</th><th>S</th><th>E</th></tr></thead><tbody><tr><td rowspan="3">Western coussous</td><td>GC</td><td><strong>48.2 ± 0.76</strong></td><td><strong>0.92 ± 0.00</strong></td></tr><tr><td>NC</td><td><strong>46.7 ± 1.02</strong></td><td><strong>0.90 ± 0.01</strong></td></tr><tr><td>BRC</td><td>42.5 ± 0.95</td><td>0.91 ± 0.00</td></tr><tr><td rowspan="3">Central coussous</td><td>MC</td><td><strong>47.5 ± 0.93</strong></td><td><strong>0.90 ± 0.00</strong></td></tr><tr><td>VC</td><td>37.2 ± 0.89</td><td>0.90 ± 0.01</td></tr><tr><td>PC</td><td>23.5 ± 1.44</td><td>0.84 ± 0.01</td></tr><tr><td rowspan="3">Western ex-cultivated melon plots</td><td>GM</td><td>30.9 ± 1.53</td><td>0.85 ± 0.01</td></tr><tr><td>NM</td><td>40.7 ± 1.81</td><td>0.88 ± 0.00</td></tr><tr><td>BRM</td><td><strong>18.3 ± 1.83</strong></td><td><strong>0.84 ± 0.01</strong></td></tr><tr><td rowspan="3">Central ex-cultivated melon plots</td><td>MM</td><td><strong>45.2 ± 2.17</strong></td><td><strong>0.88 ± 0.00</strong></td></tr><tr><td>VM</td><td>38.8 ± 1.9</td><td>0.88 ± 0.01</td></tr><tr><td>PM</td><td><strong>13.6 ± 0.43</strong></td><td><strong>0.83 ± 0.01</strong></td></tr><tr><td rowspan="3">Central ex-cultivated cereal plots</td><td>PFc1</td><td>40.9 ± 0.72</td><td>0.89 ± 0.00</td></tr><tr><td>PFc2</td><td>32.9 ± 1.02</td><td>0.87 ± 0.01</td></tr><tr><td>AFc</td><td>36.4 ± 1.35</td><td>0.89 ± 0.00</td></tr></tbody></table>

For abbreviations of the sites see Table 1.

<figure><img src="image_4.png" /><figcaption>Fig. 3. Percentage cover of vegetation, stones and bare ground each compared between coussous, ex-cultivated cereal and ex-cultivated melon plots of central Crau. Different letters indicate significant differences within each block.</figcaption></figure>

### 3.3.2. Soil chemistry and soil physics

As shown in Table 3, western coussous had higher nitrogen, phosphorous and potassium contents compared to central coussous. The central coussous were richer in magnesium and C:N content. Regarding the physical soil conditions, the western coussous had higher contents of fine sand and the central coussous was superior in the percentage of clay and coarse sand content. These differences between central and western sites have been disappeared after melon cultivation; only nitrogen and carbon content and percentage coarse sand were different for central and western ex-cultivated melon fields (Table 3).

Coussous, ex-cultivated cereal and melon fields have significantly different phosphorous and potassium contents, C:N ratios and pH-values (Table 4). Compared to

the coussous, the soil of both ex-cultivated cereal and melon plots had increased values in pH and potassium content. The ex-cultivated melon fields had elevated phosphorous contents. C:N ratio was lowest in ex-cultivated fields; the ex-cultivated melon fields had higher values than the ex-cultivated cereal fields.

Regarding the physical soil conditions, the percentage of clay was highest in the coussous compared to ex-cultivated lands. Ex-cultivated melon fields had significantly the highest content of fine silt (Table 4).

## 3.4. Abiotic parameters being correlated with vegetation dynamics

As pointed out by the PCA (Fig. 4), enhanced bare ground cover and low stone cover of western coussous are correlated to the different species compositions of western and central coussous. Phosphorous content, land use period and time since abandonment are the factors correlated to different species compositions of the ex-cultivated melon and cereal fields.

### 3.5. Soil seed bank results

#### 3.5.1. Seed densities and species diversity

The investigation of the seed bank gave for thirty soil samples of the three land-use types and both depths in total only 463 germinations of 36 species or 17 families. Differences were detected in species number when comparing the seed bank of the three land-use types (F = 2.593, *p* < 0.05). The seed bank of the ex-cultivated cereal plot had significant higher species numbers than the ex-cultivated melon fields (LSD, *p* < 0.05).

The seed bank of the coussous and of the ex-cultivated cereal field seemed to have fewer seedlings compared to the seed bank of the ex-cultivated melon field (Table 5), though it could not be proved statistically.

As indicated by relatively small means, high standard errors and low evenness values, species distribution was extremely heterogeneous in the seed bank of the ex-cultivated melon field (Fm). It was determined by *L. maritima*, making up more than 80% of all emerged seedlings. Frequent species in the coussous seed bank were *Dactylis glomerata* and *Verbena officinalis*. The ex-cultivated cereal plot seed bank was dominated by *L. maritima* and *C. nepeta* (Table 6).

Comparing number of emerged seedlings of the two different sampling depths within each land-use type (Table 5), only the ex-cultivated cereal field seed bank showed significant differences (*U* = 24, *p* < 0.05).

### 3.6. Combination of seed bank data with vegetation data

The bi-plot of Fig. 5 indicates that the vegetation composition of coussous, ex-cultivated cereal and ex-cultivated melon field were very different to each other:
<table><caption>Table 3
Results of chemical and physical soil analyses of central and western coussous and central and western ex-cultivated melon plots of central Crau</caption><thead><tr><th rowspan="2"></th><th colspan="3">Coussous</th><th colspan="3">Ex-cultivated melon plots</th></tr><tr><th>Centre</th><th>West</th><th>Statistics</th><th>Centre</th><th>West</th><th>Statistics</th></tr></thead><tbody><tr><td>Nitrogen (g/kg)</td><td>1.29 ± 0.05</td><td>1.54 ± 0.06</td><td>T = 3.21***</td><td>1.31 ± 0.04</td><td>1.52 ± 0.06</td><td>T = 3.05**</td></tr><tr><td>Phosphorous (g/kg)</td><td>0.005+0.002</td><td>0.01 ± 0.00</td><td>U = 40.5**</td><td>0.03 ± 0.01</td><td>0.05 ± 0.01</td><td>U = 94.0 n.s.</td></tr><tr><td>Potassium (g/kg)</td><td>0.09 ± 0.01</td><td>0.15 ± 0.01</td><td>U = 15.5***</td><td>0.15 ± 0.01</td><td>0.15 ± 0.02</td><td>U = 96.5 n.s.</td></tr><tr><td>Magnesium (g/kg)</td><td>0.19 ± 0.01</td><td>0.16 ± 0.01</td><td>T = -2.98**</td><td>0.17 ± 0.01</td><td>0.18 ± 0.02</td><td>U = 0.63 n.s.</td></tr><tr><td>C:N</td><td>9.48 ± 0.08</td><td>8.54 ± 0.23</td><td>U = 21.0***</td><td>8.77 ± 0.09</td><td>8.78 ± 0.15</td><td>U = 106.5 n.s.</td></tr><tr><td>Carbon (g/kg)</td><td>12.28 ± 0.62</td><td>13.10 ± 12.28</td><td>T = 1.03 n.s.</td><td>11.54 ± 0.43</td><td>13.30 ± 0.47</td><td>T = 2.79*</td></tr><tr><td>Calcium (g/kg)</td><td>2.03 ± 0.06</td><td>1.87 ± 0.07</td><td>T = -1.69 n.s.</td><td>2.1 ± 0.09</td><td>3.32 ± 0.68</td><td>U = 71.5 n.s.</td></tr><tr><td>pH</td><td>6.46 ± 0.05</td><td>6.53 ± 0.08</td><td>T = 0.83 n.s.</td><td>6.73 ± 0.12</td><td>6.91 ± 0.23</td><td>T = 0.68 n.s.</td></tr><tr><td>Organic matter<br>content (g/kg)</td><td>21.11 ± 0.86</td><td>22.53 ± 1.06</td><td>T = 1.04 n.s.</td><td>19.85 ± 0.73</td><td>22.87 ± 0.81</td><td>T = 2.77 n.s.</td></tr><tr><td>Coarse sand (%)</td><td>20.4 ± 0.89</td><td>17.47 ± 0.81</td><td>T = -2.43*</td><td>17.93 ± 0.65</td><td>15.31 ± 0.57</td><td>T = -3.03***</td></tr><tr><td>Fine sand (%)</td><td>21.96 ± 0.56</td><td>24.94 ± 0.71</td><td>T = 3.28**</td><td>23.52 ± 0.44</td><td>23.18 ± 0.50</td><td>T = -0.52 n.s.</td></tr><tr><td>Coarse silt (%)</td><td>15.74 ± 0.42</td><td>21.70 ± 2.63</td><td>U = 94.5 n.s.</td><td>16.88 ± 1.19</td><td>16.81 ± 0.92</td><td>T = 0.05 n.s.</td></tr><tr><td>Fine silt (%)</td><td>19.33 ± 0.30</td><td>18.83 ± 0.76</td><td>U = 102.0 n.s.</td><td>27.96 ± 2.09</td><td>29.61 ± 2.28</td><td>T = 0.54 n.s.</td></tr><tr><td>Clay (%)</td><td>22.57 ± 0.56</td><td>17.11 ± 1.89</td><td>U = 61.0*</td><td>13.72 ± 1.70</td><td>15.09 ± 2.11</td><td>U = 110.0 n.s.</td></tr></tbody></table>

Means and standard errors and the test values of *t* tests (*T*) or Mann-Whitney *U* tests (*U*) and the *p*-values (*p* < 0.05, **p* < 0.01, ***p* < 0.001, n.s. – not significant) are given.

<table><caption>Table 4
Comparison of the different land-use types: chemical and physical soil parameters showing significant differences between coussous, ex-cultivated cereal and melon fields</caption><thead><tr><th></th><th>Coussous</th><th>Ex-cultivated cereal plots</th><th>Ex-cultivated melon plots</th><th>Statistical results</th></tr></thead><tbody><tr><td>pH</td><td>6.46 ± 0.05<sup>a</sup></td><td>6.71 ± 0.05<sup>b</sup></td><td>6.73 ± 0.1<sup>b</sup></td><td>F = 3.82, p < 0.05</td></tr><tr><td>K (g/kg)</td><td>0.09 ± 0.01<sup>a</sup></td><td>0.13 ± 0.01<sup>b</sup></td><td>0.15 ± 0.01<sup>b</sup></td><td>F = 8.22, p < 0.01</td></tr><tr><td>P (g/kg)</td><td>0.005 ± 0.001<sup>a</sup></td><td>0.013 ± 0.002<sup>a</sup></td><td>0.031 ± 0.006<sup>b</sup></td><td>F = 13.1, p < 0.001</td></tr><tr><td>C:N ratio</td><td>9.48 ± 0.08<sup>a</sup></td><td>9.21 ± 0.09<sup>b</sup></td><td>8.78 ± 0.09<sup>c</sup></td><td>χ<sup>2</sup> = 19.91, p < 0.001</td></tr><tr><td>Clay (%)</td><td>22.57 ± 0.56<sup>a</sup></td><td>14.84 ± 2.00<sup>b</sup></td><td>13.72 ± 1.70<sup>b</sup></td><td>F = 9.60, p < 0.001</td></tr><tr><td>Fine silt (%)</td><td>19.33 ± 0.30<sup>a</sup></td><td>21.84 ± 2.08<sup>a</sup></td><td>27.96 ± 2.09<sup>b</sup></td><td>F = 6.75, p < 0.01</td></tr><tr><td>Coarse sand (%)</td><td>20.41 ± 0.89<sup>a</sup></td><td>22.41 ± 0.52<sup>b</sup></td><td>17.93 ± 0.65<sup>c</sup></td><td>χ<sup>2</sup> = 16.26, p < 0.001</td></tr><tr><td>N (g/kg)</td><td>1.29 ± 0.047<sup>a</sup></td><td>1.29 ± 0.040<sup>a</sup></td><td>1.31 ± 0.039<sup>a</sup></td><td>χ<sup>2</sup> = 0.099, n.s.</td></tr><tr><td>Mg (g/kg)</td><td>0.19 ± 0.006<sup>a</sup></td><td>0.17 ± 0.007<sup>a</sup></td><td>0.17 ± 0.010<sup>a</sup></td><td>χ<sup>2</sup> = 3.763, n.s.</td></tr><tr><td>C (g/kg)</td><td>12.28 ± 0.499<sup>a</sup></td><td>11.88 ± 0.389<sup>a</sup></td><td>11.54 ± 0.428<sup>a</sup></td><td>χ<sup>2</sup> = 1.363, n.s.</td></tr><tr><td>Ca (g/kg)</td><td>2.03 ± 0.061<sup>a</sup></td><td>2.07 ± 0.065<sup>a</sup></td><td>2.10 ± 0.090<sup>a</sup></td><td>χ<sup>2</sup> = 0.204, n.s.</td></tr><tr><td>O.M. (g/kg)</td><td>21.11 ± 0.857<sup>a</sup></td><td>20.43 ± 0.673<sup>a</sup></td><td>19.85 ± 0.732<sup>a</sup></td><td>χ<sup>2</sup> = 1.48, n.s.</td></tr><tr><td>Fine sand (%)</td><td>21.96 ± 0.562<sup>a</sup></td><td>23.20 ± 1.386<sup>a</sup></td><td>23.52 ± 0.441<sup>a</sup></td><td>χ<sup>2</sup> = 3.947, n.s.</td></tr><tr><td>Coarse silt (%)</td><td>15.74 ± 0.421<sup>a</sup></td><td>17.71 ± 1.684<sup>a</sup></td><td>16.88 ± 1.194<sup>a</sup></td><td>χ<sup>2</sup> = 0.367, n.s.</td></tr></tbody></table>

Different letters indicate significant differences within one parameter. The results of the ANOVAs or Kruskal-Wallis *H* tests are given.

*B. retusum*, *L. strictum*, *Euphorbia cyparissias* and *D. glomerata* are characteristic for coussous and, though weakly, for ex-cultivated cereal field vegetation. *Lobularia maritima* is associated with ex-cultivated melon field vegetation and weakly with ex-cultivated cereal field vegetation. However, there are no species being characteristic for seed bank species composition; always the same main species built up the related seed banks!

## 4. Discussion

The results of the current study show clearly that former arable cultivation has considerably changed the original steppe community. Past cultivation practices led to significant changes in physical and chemical soil parameters resulting not only in a changed vegetation composition of the ex-cultivated plots, but also in lower species richness and evenness which is today, decades after abandonment and reintroduction of grazing, still visible. Similar observations were made by Dutoit (1996) and Wells et al. (1976) in calcareous grasslands. Also, the species composition of the actual seed bank has been influenced by historical cultivation, results being consistent with those of Graham and Hutchings (1988a,b).

### 4.1. Vegetation attributes

#### 4.1.1. Differences between western and central undis- turbed steppe vegetation

Though the land-use histories of central and western Crau were supposed to be similar, different species
<figure><img src="image_5.png" /><figcaption>Fig. 4. PCA ordination of vegetation data (main matrix: 15 sites, 159 species) and data of 17 parameters: soil data, vegetation, stone and bare ground cover, time period of land use, time of abandonment, cultivation type, cultivation practice. Explained percentage of variance by the first three axes: 60.66% (axis 1: 23.88%, eigenvalue = 12.103, axis 2: 22.95%, eigenvalue = 11.634; axis 3: 13.83%, eigenvalue = 7.009). r²-value = 0.4.</figcaption></figure>

<table><caption>Table 5
Density of germinated seeds and species in the soil seed bank of coussous (C), ex-cultivated cereal field (Fc) and ex-cultivated melon field (Fm)</caption><thead><tr><th></th><th>C</th><th>Fc</th><th>Fm</th></tr></thead><tbody><tr><td><strong>Vegetation</strong></td><td></td><td></td><td></td></tr><tr><td>Total number of species in the vegetation</td><td>50</td><td>68</td><td>28</td></tr><tr><td><strong>Soil seed bank</strong></td><td></td><td></td><td></td></tr><tr><td>Species of established vegetation in seed bank</td><td>22%</td><td>22%</td><td>18%</td></tr><tr><td>Total number of species</td><td>20</td><td>24</td><td>14</td></tr><tr><td>Mean number of species per sample (0–20 cm)</td><td>5.7 ± 1.07<sup>ab</sup></td><td>8.0 ± 1.16<sup>a</sup></td><td>4.7 ± 0.91<sup>b</sup></td></tr><tr><td>Mean number of seeds per species</td><td>4 ± 0.9<sup>a</sup></td><td>5 ± 1.3<sup>a</sup></td><td>17 ± 13.7<sup>a</sup></td></tr><tr><td>Total number of seeds</td><td>81</td><td>142</td><td>240</td></tr><tr><td>Mean number of seeds per sample (126 cm<sup>2</sup>)</td><td>8 ± 1.7<sup>a</sup></td><td>14 ± 2.2<sup>a</sup></td><td>24 ± 17.1<sup>a</sup></td></tr><tr><td>Mean number of seeds on 1 m<sup>2</sup> (0–20 cm)</td><td>644 ± 137<sup>a</sup></td><td>1130 ± 178<sup>a</sup></td><td>1909 ± 1360<sup>a</sup></td></tr><tr><td>Mean number of seeds on 1 m<sup>2</sup> (0–10 cm)</td><td>398 ± 104<sup>a</sup></td><td>780 ± 148<sup>a</sup></td><td>1161 ± 764<sup>a</sup></td></tr><tr><td>Mean number of seeds on 1 m<sup>2</sup> (10–20 cm)</td><td>247 ± 46.6<sup>a</sup></td><td>350 ± 61.9<sup>a</sup></td><td>748 ± 599.4<sup>a</sup></td></tr><tr><td>Comparison of number of seeds in 0–10 and 10–20 cm depth per plot</td><td>n.s.</td><td>*</td><td>n.s.</td></tr><tr><td>Evenness (seed bank)</td><td>82.2%</td><td>81.8%</td><td>33.3%</td></tr></tbody></table>

Means and standard errors are given. Similar letters (a) indicate no significant differences on the *p* < 0.05 level between the sites. As a measure of diversity, the evenness index is given.

dominate undisturbed steppe vegetation of western and central sites: *B. retusum* is the main species of central coussous. Western coussous are dominated by *Thymus vulgaris* and have higher species richness and evenness. Different abiotic conditions have strongly affected vegetation patterns in the Crau: central coussous had higher stone covers and lower fine sand contents, western coussous higher bare ground covers. These divergences between central and western sites might be due to former differences in the dynamics of the Durance (Devaux et al., 1983), which have influenced border and central

sites dissimilar by different degrees of sedimentation (Bourrelly, 1984). Furthermore, divergences in botanical composition between western and central Crau could be due to differences in edaphic xericity resulting from contrasting soil depths (smaller soil depth at western Crau; Bourrelly et al., 1983) and different climatic conditions (western Crau generally 1 °C warmer than central Crau; Devaux et al., 1983). Additionally, different soil nutrient levels, as observed for central and western steppe, are associated with different vegetation compositions (Willems et al., 1993; Dutoit and Alard, 1996).
<table><caption>Table 6
Dominant species of the soil seed bank of coussous (C), ex-cultivated cereal (Fc) and melon (Fm) plot (seed numbers of all 10 samples per cultivation type) and of the established vegetation</caption><thead><tr><th rowspan="2"></th><th rowspan="2">Species</th><th colspan="3">Soil seed bank (seed numbers)</th><th colspan="3">Established vegetation (%)</th></tr><tr><th>C</th><th>Fc</th><th>Fm</th><th>C</th><th>Fc</th><th>Fm</th></tr></thead><tbody><tr><td rowspan="10">Dominant species of the seed bank</td><td>Bellis sylvestris</td><td>3</td><td>7</td><td></td><td>9.6</td><td>78.0</td><td>0.4</td></tr><tr><td>Calamintha nepeta</td><td></td><td>14</td><td>3</td><td></td><td>59.2</td><td>78.8</td></tr><tr><td>Chenopodium album</td><td>1</td><td></td><td>4</td><td></td><td></td><td></td></tr><tr><td>Dactylis glomerata</td><td>14</td><td>2</td><td></td><td>94.8</td><td>24.4</td><td>2.8</td></tr><tr><td>Gastridium ventricosum</td><td>7</td><td>1</td><td></td><td>53</td><td></td><td></td></tr><tr><td>Geranium molle</td><td>1</td><td>11</td><td></td><td></td><td>50.4</td><td></td></tr><tr><td>Linum strictum</td><td>7</td><td></td><td></td><td>92.4</td><td>0.4</td><td></td></tr><tr><td>Lobularia maritima</td><td>6</td><td>19</td><td>194</td><td></td><td>62.0</td><td>85.2</td></tr><tr><td>Portulaca oleracea</td><td>2</td><td>6</td><td>11</td><td></td><td></td><td></td></tr><tr><td>Sagina apetala</td><td>12</td><td>6</td><td>1</td><td></td><td></td><td></td></tr><tr><td rowspan="3">Dominant species of the established vegetation</td><td>Senecio vulgaris</td><td>1</td><td>13</td><td>11</td><td>5.6</td><td>6.0</td><td>3.2</td></tr><tr><td>Verbena officinalis</td><td>14</td><td>23</td><td>4</td><td></td><td></td><td></td></tr><tr><td>Aegilops ovata</td><td></td><td></td><td></td><td>16.4</td><td>100</td><td>1.2</td></tr><tr><td></td><td>Brachypodium retusum</td><td></td><td></td><td></td><td>100</td><td></td><td></td></tr><tr><td></td><td>Bromus rubens</td><td></td><td></td><td></td><td>0.8</td><td>10.8</td><td>98.0</td></tr></tbody></table>

<figure><img src="image_6.png" /><figcaption>Fig. 5. DCA ordination of vegetation and seed bank data. Vegetation data (%) of coussous, ex-cultivated melon and ex-cultivated cereal fields and original seed bank data (0–10 and 10–20 cm) of coussous, ex-cultivated melon and ex-cultivated cereal field was analysed. r²-cut-off-value = 0.48. Species correlated with axis 1: *Dactylis glomerata* (r² = 0.788); *Sideritis romana* (r² = 0.685), *Linum strictum* (r² = 0.633), *Euphorbia exigua* (r² = 0.631), *Gastridium ventricosum* (r² = 0.535), *Galium parisiense* (r² = 0.517), *Aira cupaniana* (r² = 0.501). Species correlated with axis 2: *Lobularia maritima* (r² = 0.488).</figcaption></figure>

### 4.1.2. Effects of former melon cultivation in central and western Cau

At all study sites, the observed differences of central and western steppe vegetation have disappeared after melon cultivation. Ploughing and fertilising had a homogenising effect on the physical and chemical soil structure (Table 3) and, consequently, also on vegetation composition. The most important factor in determining the development of ex-cultivated plots seems to be the type of melon cultivation. Ex-cultivated melon fields that were once cultivated under large plastic tunnels and more recently abandoned (west: GM, NM; centre: VM, MM) were more similar to the undisturbed steppe-vegetation than ex-cultivated melon fields where small plastic tunnels were used (centre: PM, west: BRM). Cultivation under small plastic tunnels included
the whole field (Borrey, 1965). Differently, untilled track ways between large tunnels functioned as remnants for the survival of target species which are important in determining community composition and species richness (Zobel et al., 1998).

Hence, the factor time since abandonment is masked, because recently ex-cultivated field having safe sites could re-develop faster in the direction of the coussous vegetation than old ex-cultivated melon fields without track ways.

### 4.1.3. Effects of historical cereal cultivation in central Crau

The effects of former cereal cultivation on the current vegetation structure are likely to be less severe as cultivation practices were reasonably gentle in terms of fertiliser and pesticide applications, ploughing and irrigation (Khalifa, 1987; Borck, 1998). Furthermore, time since abandonment was generally quite high, leaving more time for succession and for the arable weed community to change towards the target community. However, the bi-plot of the DCA (Fig. 2) reveals that species compositions of ex-cultivated cereal fields are still very different to undisturbed steppe. These dissimilarities can be strongly related to the still enhanced soil nutrients, changed granule fractions and surface covers (vegetation, stone and bare ground), as these parameters strongly influence vegetation composition (Wells et al., 1976; Graham and Hutchings, 1988a).

### 4.1.4. Comparison of ex-cultivated cereal and melon plots

As pointed out by the PCA (Fig. 4) and the soil results (Table 4), differences in species composition of ex-cultivated cereal and melon fields are mainly determined by time since abandonment, duration of land use and soil fertility (mainly phosphorous). Associated with ancient fertiliser application, soils under previous cultivated land displayed elevated phosphorous and potassium contents. In percentage stone cover and proportion of soil particle fractions, ex-cultivated melon and ex-cultivated cereal fields had different values due to different cultivation practices. Changes in soil structure and increased soil fertility determine not only present-day species composition (Wells et al., 1976; Willems et al., 1993; Janssens et al., 1998), but also strongly influence the succession process by preventing successful establishment of target species by modified competition patterns (Grime, 1979; Austrheim and Olsson, 1999). Whilst the steppe plant community is mainly determined by stress tolerators (sensu Grime, 1979), representing a sub-climax in comparison to potentially forested stages of Mediterranean plant successions (Devaux et al., 1983), ex-cultivated melon and cereal plots are controlled by ruderals (*B. madritiensis*, *B. rubens* and *C. nepeta*,

etc.). Vegetation succession of the ex-cultivated fields passes through a state dominated by competitive ruderals. Under grazing regimes, this can take at least 55 years in temperate regions (Gibson and Brown, 1992), before stress-tolerators may manage to re-establish.

Weak dispersal (Fischer et al., 1996) and colonisation (Poschlod et al., 1998) abilities of the target species might also prevent the restoration of the original steppe community. Sheep grazing was reintroduced immediately after abandonment of the arable fields. It represents a successful management tool in enhancing seed dispersal from dry grassland to ex-arable fields (Gibson and Brown, 1992). However, most coussous species are low-growing species; adhesive dispersal is therefore reduced (Fischer et al., 1996). Additionally, the dissemination period of most species is in summer (Bourrelly et al., 1983), when the flock has left to the "Haute-Provence" (transhumant migrations; Rinschede, 1979; Fabre, 1998). Sheep dispersal might be less effective compared to other sites where the flock stays on the grassland during the entire vegetation period.

However, even if dispersal is successful for some species, the time has to be suitable for establishment (Primack and Miao, 1992). Gaps and small scale disturbances, as created by sheep, attribute to better germination and establishment conditions (Bullock, 2000) especially for stress-tolerant ruderals (Bullock et al., 2001). But again, compared to temperate regions, in the Mediterranean region, the vegetation succession is generally decelerated as the colonisation and establishment rate of plants are reduced under these dry habitat conditions (Jackel, 1999).

## 4.2. Attributes of the soil seed bank

### 4.2.1. Seed bank characteristics of the steppe community

In the small spring seed bank (644 seeds/m², 0-20 cm depth; 20 species), only 22% of the species of the established vegetation are represented. Species composition was dominated by forbs (e.g. *V. officinalis*); only a few grass species were detected, though with higher seed densities (e.g. *D. glomerata* and *Gastridium ventricosum*, Table 6). Though it was not proved statistically when including all species in the analyses (Table 5), for many species more seeds emerged from the upper soil layer, indicating the transient character of most identified seed bank species as depth distribution is related to seed persistence (Poschlod and Jackel, 1993).

It can be concluded, that the seed bank of the steppe plant community is very small and rather transient, as has been observed for many other dry grassland seed banks (Dutoit and Alard, 1995; Bakker et al., 1996b; Jackel, 1999). Hence, it seems unlikely, that it could act
as a source of propagules in the restoration of ex-cultivated fields.

### 4.2.2. Effects of historical cereal and melon cultivation on the seed bank composition

As also revealed in other studies, e.g. Graham and Hutchings (1988a,b), historical cultivation has had an impact on seed bank patterns in the Crau. In the seed bank of the ex-cultivated melon field we find today, 17 years since abandonment, still ruderal and ancient arable weed species (*Chenopodium album*, *Portulaca oleracea*). Seed numbers were comparatively high (1909 seeds/m², 0–20 cm depth), species numbers very low (14 species). High seed numbers are related to the high proportion of ruderal species (e.g. *L. maritima*) producing many persistent seeds (Grime, 1979). Low species numbers in turn may be a consequence of still enhanced nutrient conditions (Milberg, 1992). Seeds of most coussous species have been lost due to the effects of deep ploughing regimes. The mainly transient target seed bank became depleted and only very few ubiquitous target species have been persisted. At the same time as the seed densities of the target species declined in response to disturbance, pesticide application and increased fertility of arable weed species (ruderals) increased (Akinola et al., 1998).

The seed bank of the ex-cultivated cereal field had enhanced species numbers (24 species) compared to the ex-cultivated melon fields. These differences might be due to different cultivation practices and the longer time since abandonment. Target species could have managed to re-invade and re-establish providing an opportunity to refresh the seed bank. Probably they could have also survived the cultivation period as ploughing was not too deep and fertiliser application was very low (Masip, 1991).

### 4.3. Relationship of established vegetation and soil seed bank

Though differences in the seed bank composition between coussous and ex-cultivated fields exist, they were not as clear as observed for the corresponding vegetation (Fig. 5). Other authors stressed the disparity between vegetation and seed bank species composition as well (Lavorel et al., 1991; Dutoit and Alard, 1995). Nevertheless, many species were, if not absent, only scarce in the seed bank. Species with very low frequencies in the established vegetation did not emerge at all from the seed bank, results being consistent with those of Milberg and Hansson (1994). Though seeds of many arable weeds still occur in the seed bank of the ex-cultivated fields, they do not manage to establish in the aboveground vegetation. Decreased fertility and cessation of irrigation since abandonment are important factors preventing

nitrophilous arable weeds to re-establish (e.g. *P. oleracea*).

### 4.4. Conservation management and restoration ecology

Vegetation and seed bank results suggest clearly, that a re-development of the ex-cultivated fields into the direction of the steppe plant community seems unlikely. The ex-cultivated fields still show modified abiotic conditions, even decades since abandonment.

Therefore, the protection of undisturbed coussous remnants must have absolute priority. Otherwise, the typical and irreplaceable vegetation composition of the steppe with *B. retusum*, *Thymus vulgaris*, *L. strictum*, etc. would disappear. Most target species do not survive any kind of steppe alteration because they do not build up long lasting seed banks. The colonisation potential appears to be very weak since a successful reinvasion and reestablishment occurred only in very few cases. The climatic conditions are not very favourable for seedling/species establishment. Very high temperatures and low humidity at dissemination time may prevent germination and successful establishment (Jackel, 1999), especially at the ex-cultivated fields with changed soil conditions. It can be suggested, that traditional sheep grazing has to be maintained as it links up ex-cultivated fields and steppe vegetation. Plant dispersal is favoured for some coussous species and the succession of the ex-cultivated fields enhanced. As observed in other studies in the Crau (Borck, 1998), vegetation structure would alter significantly under the exclusion of grazing sheep by an increase of competitive species (e.g. *D. glomerata*). However, it is not clear, if, as proposed by Graham and Hutchings (1988a), an avoidance of any other kind of disturbance is crucial in the Crau. Though soil disturbances could bring arable weed seeds to the soil surface, water and nutrient contents of the soil are not favourable for their re-establishment. Only in exceptional wet years, they might manage to re-establish again. Then in turn, the competition structure would change again, and a setback of the succession process may not be a very desirable result.

Therefore, time seems to be the most important factor for the restoration of the ex-cultivated fields. From our results, it must be concluded that the re-invasion of the target species into the ex-cultivated fields from direct adjacent coussous is a process that may take decades if not centuries! The seed reservoirs of the arable weeds would get depleted with the time, though it might take several decades or centuries until their long-term persistent seeds lose their viability (maximal longevity: *P. oleracea* 40 years; *C. album* >660 years according to Thompson et al. (1997)).

In the year 2001, a nature reserve was established including the main area of central Crau (Boutin, 2002). But still, large parts of the steppe are at present still
cultivated. Orchards, mainly peach plantations, are grown throughout the Crau. In order to protect the sensitive and long evolved ecosystem of the Crau, any steppe alteration must be omitted from now on and any cultivated field must be abandoned. The last parts of undisturbed steppe must be conserved!

## Acknowledgements

We acknowledge the C.E.E.P. Écomusée de la Crau for a good cooperation, Markus Bernhardt for field work support and Dr. Christiane Rolando for laboratory helps.

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