# Permanent seed banks in chalk grassland under various management regimes: their role in the restoration of species-rich plant communities

Thierry Dutoit, Didier Alard

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Thierry Dutoit, Didier Alard. Permanent seed banks in chalk grassland under various management regimes: their role in the restoration of species-rich plant communities. Biodiversity and Conservation, 1995, 4, pp.939-950. hal-05470878

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# Permanent seed banks in chalk grassland under various management regimes: their role in the restoration of species-rich plant communities

THIERRY DUTOIT and DIDIER ALARD*

Université de Rouen, Laboratoire d'écologie, U.F.R. Sciences, 76821 Mont Saint Aignan Cedex, France

A survey was carried out on four stands of a chalk grassland (open, intensively grazed, tall and scrub) in order to monitor the permanent soil seed bank under various management regimes responsible for the decline of the ecological interest of this ecosystem. There was no real similarity between the seed banks and the associated vegetation, except in the intensively grazed stand. When vegetation changes, seed banks appear to be quite stable and are mostly dominated by three species (*Centaurium erythraea*, *Origanum vulgare*, *Hypericum perforatum*). The seed bank seems to be quite useless in order to restore a species-rich community after both grazing intensification or abandonment. Because recolonization processes are the critical factor, the management of species-rich grassland has to be designed with a view to preserving a given plant community as well as to improving dispersion processes from a source to target areas.

**Keywords:** chalk grassland; soil seed bank; vegetation succession; plant-richness; conservation management; France

## Introduction

The accumulation and persistence of seeds in the soil of arable field and pastures of most temperate habitats is a well-known phenomenon. The existence of this reservoir of buried viable seeds has been recognized for a long time and is described as the soil seed bank (Grime, 1979; Thompson and Grime, 1979).

A striking feature of the seed bank is the general discordance between the species contained in the soil and the associated vegetation. Chippindale and Milton (1934) showed that the presence of viable seeds of species of former successional stages or plant communities can reflect an earlier stage in vegetation succession history. The loss of the vegetative stage in a plant's life history, even if absent for several decades, does not necessarily show that the species is not present as dormant seed (Moore, 1983). This phenomenon can be applied in nature conservation to recreate former or endangered plant communities (Bakker, 1989).

The maintenance of species-rich plant communities containing assemblages of species characteristic of mature chalk grassland is usually the aim of conservation management (Duffey *et al.*, 1974; Arlot and Hesse, 1981). The development of tall grassland and scrub communities, as a consequence of grazing abandonment, is considered undesirable due to both the reduction in floristic diversity and the loss of local or rare species associated with open calcareous grassland communities (Smith, 1980; Bobbink, 1987). Several studies have
looked at the buried viable seeds in other community types in successional series of known age and vegetational composition (Oosting and Humphreys, 1940; Livingstone and Allessio, 1968). However, studies have shown that the potential of seed banks for recolonizing large areas of tall grassland or scrub, which have been respectively mown or cleared (Grubb and Key, 1975), was quite weak. In France, there have been only a few studies on the seed bank in grassland focusing on the conservation of plant communities (Guyot and Massenot, 1950; Guyot, 1965; Delpech, 1969).

The aim of this study was to investigate the size and the composition of the permanent seed bank beneath different stands of a chalk grassland under various management regimes. In a context of former abandonment, the results are used to assess the feasibility of restoring and managing chalk grassland vegetation from seeds already present in the seed bank.

# Materials and methods

## Study site

The study was conducted in Saint-Adrien nature reserve (Fig. 1), 15 km south of Rouen (1°5'3"E, 49°19'22"N), an area of 32 ha of old chalk grassland. The vegetation consists of a mosaic of different age stands of chalk grassland succession on Cretaceous chalk.

Four stands were chosen for further investigations of the vegetation and the seed bank (open [A], grazed [B], tall [C] grasslands and scrub [D]). These four stands were on the same slope and have had the same land-use history prior to 1950. They were used as a vineyard before 1914 and sheep-grazed from then to approximately 1950 (Liger, 1952). These former agricultural practices on previously arable areas still have some effects on the composition of the chalk grassland (Wells et al., 1976), with the presence of typical weeds from old vineyards (e.g. *Allium sphaerocephalum*, *Muscari racemosum*, *Thlaspi perfoliatum*). Aerial photographs (1947, 1955, 1963, 1973, 1991) show that management since 1950 is the main difference between the stands.

The open grassland is a typical species-rich short-herb vegetation naturally grazed by rabbits. The most frequently recorded species are *Sesleria albicans*, *Brachypodium pinnatum*, *Carex flacca*, *Festuca cf. lemanii*, *Teucrium chamaedrys* and *Hieracium pilosella*. Flints are locally abundant, and the ground surface is colonized by a dense cover of bryophytes and lichens, probably by selected grazing by the rabbits. Although this grassland is not managed, it can be considered as a good example of a natural extensively grazed species-rich area.

The second stand, a fenced area of 0.7 ha, has been continuously grazed since 1984 with a mean stocking rate of 4 Texel sheep ha⁻¹yr⁻¹. Changes in the structure and botanical composition of the grassland are the increase of *F. cf. lemanii* and the decrease of *B. pinnatum*. Due to the intensive grazing, *S. albicans* was almost eliminated while species of mesotrophic calcicolous grassland appeared, e.g. *Lotus corniculatus*, *Medicago lupulina* and *Trisetum flavescens* (de Foucault, 1988).

The vegetation of the third stand corresponds to a tall grassland which has been abandoned since 1950 and is dominated by tussock grasses, e.g. *B. pinnatum* and *S. albicans*, with encroachment by some scrub species. Grass litter is abundant, with seedlings of annuals and biennials e.g. *Campanula rotundifolia*, *Galium pumilum* and *Digitalis lutea* occurring on the ground surface.

The fourth stand corresponds to the third stand except that the vegetation is now
<figure><img src="image_2.png" /></figure>

**Figure 1.** Map showing the study area in the Saint-Adrien nature reserve. Letters indicate the stands referred to in the text, [A] open grassland, [B] grazed grassland, [C] tall grassland, [D] scrub.

dominated by woody phanerophytes e.g. *Cornus sanguinea*, *Crataegus monogyna* and *Viburnum lantana*. The stand can be dated by historical events which changed the patterns of land use (Boulet, 1989). Aerial photographs show that scrub communities appeared after the cessation of grazing in 1950 and are now forty years old.

More details of the vegetation of the site are given by Liger (1952) and de Foucault and Frileux (1988).

## Methods

The soil seed bank was sampled in January 1993 because the highest number of species germinating in chalk grassland is in the winter (Graham and Hutchings, 1988a). The soil cores were 7 cm diameter, 8.5 cm deep, having a volume of 327 cm³, though in practice a full core was sometimes not obtained due to the presence of stones and the shallow rendzina soil. Twenty sampled cores were taken randomly from each stand. This sampling intensity seems sufficient when compared with the recommended minimum soil volume of 1200 cm³ for recovery of most of the species in grassland seed banks (Roberts, 1981).

The soil cores were crumbled, spread out on plastic sheets and air-dried in the laboratory. When dry, the soil was sieved through a 1 cm mesh sieve to remove the coarsest plant fragments and stones. Samples were aggregated into units of five soil cores to form four compound samples from each grassland. Each of these was placed over a 2 cm layer of sterile coarse sand (sterilized at 120°C for several periods of 2 h) in a 45 × 30 × 8 cm plastic seed tray. The 16 trays (4 vegetation types × 4 replicates) were kept in a greenhouse, with a diurnal cycle of 8 h/16 h darkness/light and a temperature regime of 15/20°C. Light was provided by fluorescent tubes (75/85 W). The soil was kept moist. Emerging seedlings were
<table><caption>**Table 1.** Number of germinated seeds of each taxon recorded per sample of soil cores taken of the four stands. (The percentage of the total number of each stand seedlings and the percent occurrence of all species in quadrats are also given.) Species absent from quadrats but observed in each stand of the succession are denoted by (+). Species are arranged alphabetically: nomenclature follows Clapham *et al.* (1962)</caption><thead><tr><th rowspan="2">Species</th><th colspan="3">Open grassland</th><th colspan="3">Grazed grassland</th><th colspan="3">Tall grassland</th><th colspan="3">Scrub communities</th></tr><tr><th>No. of Seeds</th><th>% of total</th><th>Rel. abun. in quadrats</th><th>No. of Seeds</th><th>% of total</th><th>Rel. abun. in quadrats</th><th>No. of Seeds</th><th>% of total</th><th>Rel. abun. in quadrats</th><th>No. of Seeds</th><th>% of total</th><th>Rel. abun. in quadrats</th></tr></thead><tbody><tr><td>Cladonia sp.</td><td>—</td><td>—</td><td>0.6</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td></tr><tr><td>Moss</td><td>—</td><td>—</td><td>5.5</td><td>—</td><td>—</td><td>2.4</td><td>—</td><td>—</td><td>—</td><td>9.1</td><td>—</td><td>—</td></tr><tr><td>Achillea millefolium</td><td>—</td><td>—</td><td>(+)</td><td>1</td><td>0.5</td><td>16.0</td><td>1</td><td>0.2</td><td>6.7</td><td>1</td><td>0.3</td><td>—</td></tr><tr><td>Anagallis arvensis</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>2</td><td>0.5</td><td>—</td></tr><tr><td>Anthericum ramosum</td><td>—</td><td>—</td><td>(+)</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td></tr><tr><td>Anthyllis vulneraria</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>0.3</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td></tr><tr><td>Asperula cynanchica</td><td>—</td><td>—</td><td>(+)</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td></tr><tr><td>Avenula pubescens</td><td>—</td><td>—</td><td>(+)</td><td>—</td><td>—</td><td>0.7</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td></tr><tr><td>Brachypodium pinnatum</td><td>—</td><td>—</td><td>12.0</td><td>—</td><td>—</td><td>15.7</td><td>—</td><td>—</td><td>—</td><td>28.5</td><td>—</td><td>(+)</td></tr><tr><td>Briza media</td><td>—</td><td>—</td><td>(+)</td><td>—</td><td>—</td><td>0.6</td><td>—</td><td>—</td><td>(+)</td><td>—</td><td>—</td><td>—</td></tr><tr><td>Bromus erectus</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>0.3</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td></tr><tr><td>Campanula rotundifolia</td><td>—</td><td>—</td><td>0.9</td><td>3</td><td>1.4</td><td>1.2</td><td>1</td><td>0.2</td><td>5.0</td><td>1</td><td>0.3</td><td>(+)</td></tr><tr><td>Carex flacca</td><td>—</td><td>—</td><td>11.1</td><td>—</td><td>—</td><td>4.4</td><td>1</td><td>0.2</td><td>—</td><td>—</td><td>—</td><td>—</td></tr><tr><td>Carlina vulgaris</td><td>—</td><td>—</td><td>(+)</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td></tr><tr><td>Centauria scabiosa</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td></tr><tr><td>Centaurium erythraea</td><td>79</td><td>56.8</td><td>—</td><td>111</td><td>50.5</td><td>—</td><td>182</td><td>45.0</td><td>—</td><td>157</td><td>40.5</td><td>(+)</td></tr><tr><td>Cerastium arvense</td><td>1</td><td>0.7</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td></tr><tr><td>Cirsium acaule</td><td>—</td><td>—</td><td>1.2</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td></tr><tr><td>Cirsium arvense</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>(+)</td></tr><tr><td>Clematis vitalba</td><td>3</td><td>2.2</td><td>—</td><td>—</td><td>—</td><td>—</td><td>1</td><td>0.2</td><td>(+)</td><td>—</td><td>—</td><td>4.2</td></tr><tr><td>Clinopodium vulgare</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>9</td><td>2.3</td><td>4.9</td></tr><tr><td>Coryza canadensis</td><td>—</td><td>—</td><td>(+)</td><td>—</td><td>—</td><td>—</td><td>1</td><td>0.2</td><td>—</td><td>—</td><td>—</td><td>31.5</td></tr><tr><td>Cornus sanguinea</td><td>—</td><td>—</td><td>(+)</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>(+)</td><td>—</td><td>—</td><td>21.0</td></tr><tr><td>Crataegus monogyna</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td></tr><tr><td>Daucus carota</td><td>—</td><td>—</td><td>1</td><td>0.5</td><td>(+)</td><td>—</td><td>—</td><td>—</td><td>(+)</td><td>—</td><td>—</td><td>—</td></tr><tr><td>Digitalis lutea</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td></tr><tr><td>Echium vulgare</td><td>—</td><td>—</td><td>(+)</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>5.6</td></tr><tr><td>Epipactis atrorubens</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td></tr><tr><td>Eryngium campestre</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>(+)</td><td>—</td><td>—</td><td>—</td></tr><tr><td>Euphrasia stricta</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>(+)</td><td>—</td><td>—</td><td>—</td></tr><tr><td>Festuca cf. lemarii</td><td>1</td><td>0.7</td><td>8.6</td><td>4</td><td>1.8</td><td>20.4</td><td>—</td><td>—</td><td>2.0</td><td>1</td><td>0.3</td><td>(+)</td></tr><tr><td>Galium mollugo</td><td>2</td><td>1.4</td><td>0.6</td><td>—</td><td>—</td><td>0.3</td><td>—</td><td>—</td><td>(+)</td><td>—</td><td>—</td><td>—</td></tr><tr><td>Galium pumilum</td><td>—</td><td>—</td><td>(+)</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>2.0</td><td>1</td><td>0.3</td><td>—</td></tr><tr><td>Genista tinctoria</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>2.4</td><td>—</td><td>—</td><td>—</td></tr></tbody></table>
<table><caption>**Table 2.** Total number of germinated seeds and taxa recorded for each stand in the seed bank and in the above ground vegetation</caption><thead><tr><th>Stand</th><th>Seeds</th><th>Seeds per m<sup>2</sup></th><th>Taxa</th><th>Taxa present only in seed bank</th><th>Taxa present only in quadrats</th><th>Taxa present in quadrats and seed bank</th></tr></thead><tbody><tr><td>Open grassland</td><td>139</td><td>1800</td><td>15</td><td>7</td><td>12</td><td>8</td></tr><tr><td>Grazed grassland</td><td>220</td><td>2860</td><td>18</td><td>3</td><td>13</td><td>15</td></tr><tr><td>Tall grassland</td><td>404</td><td>5252</td><td>24</td><td>11</td><td>10</td><td>13</td></tr><tr><td>Scrub community</td><td>388</td><td>5044</td><td>19</td><td>10</td><td>6</td><td>9</td></tr></tbody></table>

identified and removed or replanted for later identification. The descriptions of Muller (1978) were used when necessary as an identification aid. After two months, each sampling was sprayed with gibberellic acid (1 g l⁻¹), following the method of Barralis and Chadoeuf (1980, 1987). The soil was thoroughly stirred before the start of the second germination period which lasted two months, because mixing is known to cause more seeds to germinate (Roberts, 1981).

A point quadrat method was used to investigate the vegetation in the four stands (Stampfli, 1991). For each stand, two permanent 1 m² quadrats were marked for observation in homogeneous areas. For each of the eight plots, point frequency records of 100 regularly spaced points (2 cm), consisting of 2 rows at regular distances of 33 cm were made with a needle which was long enough to contact even the tallest plants possible. These quadrats were used to assess the relative contribution of dominant plant species. The flowering plants which occurred outside the quadrats were also noted as potential seed sources in order to record the whole plant community. This study was made in June 1993 with complementary visits in order to monitor the plants which occurred later in the year.

## Results

The species identified in the seed bank and the number of recorded seedlings plus the associated vegetation in the quadrats are listed in Table 1.

Viable seeds of 38 species were found in the soil samples. This number is quite small compared with the 69 taxa of above-ground species. Of the total seedlings recorded in all the samples, 84% derive from three species: *Centaurium erythraea* (46%), *Hypericum perforatum* (21%) and *Origanum vulgare* (17%). The first germination period (two months) provided a total of 204 seedlings identified to 23 different species. The second germination period revealed an additional 947 seedlings of 27 species of which 15 were not recorded in the previous germination period.

Of the 38 identified taxa in the seed bank, 18% of the taxa identified emerged from plots where the species had not been recorded, however, no examples were found in which large numbers of germinated seeds coincided with the absence of the species from the vegetation. The greater part of these species consisted of short-lived weeds (*Anagallis arvensis*, *Cerastium arvense*, *Conyza canadensis*, *Sonchus asper*). The majority of seedlings (82%) were restricted to plots where the species grew in the sampling season. The data about the seed banks from the four stands are summarized in Table 2.
<figure><img src="image_3.png" /><figcaption>**Figure 2.** Seedlings and the frequency of the three predominant species for each stand of the study area.</figcaption></figure>

The open grassland had the smallest number of seeds recorded from any of the four stands but the dominance of the three species (*C. erythraea*, *H. perforatum*, *O. vulgare*) was less important in this sample (65%) although *Centaurium* had its maximum contribution here. In the grazed grassland, except for *C. erythraea*, *H. perforatum* and *Reseda lutea*, all the species present in the seed bank were observed in the quadrats. The seed bank of the tall grassland was the most important in the number of seedlings and species richness. The majority of the characteristic species of this community are represented in the seed bank; *Linum catharticum* and *Muscari* sp. could be considered of nature conservation and ecological significance, whilst *L. corniculatus* and *M. lupulina* could be considered of agronomic interest. In the scrub communities, all the species (except *A. arvensis*) were present in the seed bank but absent from the quadrats. No seedlings of scrub species were found in the seed bank.

The total numbers of seedlings of the three dominant species were quite different in the four stands. Moreover, there was also some disparity in the contribution of each of these species (Fig. 2). Whilst *H. perforatum* was decreased in the scrub stand, *C. erythraea* and *O. vulgare* had their highest value in the tall grassland. The proportional contribution of the three dominant species was similar, with a maximum for the tall grassland (90%) and a minimum for the open grassland (65%).

Sorensen's similarity indices (Cs) have been calculated to compare the composition of seed banks within and between seed banks and quadrats of each vegetation stand (Sorensen, 1948 quoted by van der Valk and Davis, 1976; Jefferson and Usher, 1987), where $Cs = 2j/(a + b)$, j is the number of species common to the two samples, a and b are the
<table><caption>**Table 3.** Sorensen's indices comparing the floristic composition of seed banks and plant communities of each stand of chalk grassland ([A] open grassland, [B] grazed grassland, [C] tall grassland, [D] scrub)</caption><thead><tr><th>Source of species list comparison</th><th>A</th><th>B</th><th>C</th><th>D</th></tr></thead><tbody><tr><td>Vegetation/vegetation</td><td></td><td></td><td></td><td></td></tr><tr><td>A</td><td>—</td><td>0.56</td><td>0.54</td><td>0.24</td></tr><tr><td>B</td><td>—</td><td>—</td><td>0.5</td><td>0.19</td></tr><tr><td>C</td><td>—</td><td>—</td><td>—</td><td>0.38</td></tr><tr><td>Seed bank with each plant community</td><td>0.3</td><td>0.61</td><td>0.42</td><td>0.45</td></tr><tr><td>Seed bank/seed bank</td><td></td><td></td><td></td><td></td></tr><tr><td>A</td><td>—</td><td>0.54</td><td>0.46</td><td>0.47</td></tr><tr><td>B</td><td>—</td><td>—</td><td>0.74</td><td>0.54</td></tr><tr><td>C</td><td>—</td><td>—</td><td>—</td><td>0.46</td></tr></tbody></table>

total number of species in each sample. The only pattern revealed by Sorensen's index values (Table 3) is that seed banks from different stands are generally more similar than are seed banks and the above-ground vegetation. However, the comparison between the seed bank of the grazed grassland and the vegetation of the sample quadrats and seed bank of the tall grassland show the highest values (respectively 0.61 and 0.74). This is due to the presence in both of some species which persist in the seed bank of the tall grassland but disappear in the above-ground vegetation (e.g. *Achillea millefolium*, *Campanula rotundifolia*, *Leucanthemum vulgare* and *Medicago lupulina*) after grazing abandonment.

## Discussion

Many studies of seed banks have shown the disparity between the species composition of the seed bank in soil with that of the above-ground vegetation (Chippindale and Milton, 1934; Champness and Morris, 1948; Major and Pyott, 1966; Russi *et al.*, 1992). In this study, there was also an absence of agricultural weeds which often contributes to the lack of correspondence between vegetation and seed bank (Graham and Hutchings, 1988a; Booth and Hutchings, 1990). This could be due to the long period of continuous grazing (Milberg, 1992; Poschlod and Jackel, 1993) since the end of wine cultivation in 1914 (Allorge, 1922).

In the long-term, species richness, species diversity, and density of seed banks generally decreases with successional age (Donelan and Thompson, 1980). Seed banks usually contain a high proportion of early successional (r-selected) species which rely on long-term viability as part of their opportunistic strategy. These species represent the predominant part of the persistent seed banks in our results (*C. erythraea*, *H. perforatum*, *O. vulgare*). Late successional (k-selected) species gain no advantage from such persistence of seeds but must in consequence re-invade sites by new propagules following disturbance (Moore, 1980).
In another way, plant richness in the seed bank and in the vegetation are not well correlated especially for ecologically significant species. This may suggest the importance of vegetative processes in the maintenance and/or the consolidation of these species (Grime, 1979). Similar conclusions were reached by Thompson and Grime (1979) and Graham and Hutchings (1988a).

In conservation terms, it is necessary to focus on the different roles of species featuring a transient seed bank which can play a role in the maintenance of settled populations and species featuring a persistent seed bank which can play a role in the regeneration of populations after unsuitable conditions (i.e. grazing intensification or abandonment) have caused species extinctions in the plant communities (Bakker *et al.*, 1991). For instance, this investigation suggests that the re-establishment of species-rich open grassland from abandoned tall grassland may not occur only with the restoration of extensive management (Wells, 1980) or even a simple removal of vegetation cover (Hillier, 1990) if ecological significant species are not able to colonize (Graham and Hutchings, 1988b). Moreover, the results of the scrub stand show that there is a risk of increasing the germination of undesirable species after disturbance (e.g. *Anagallis arvensis*, *Cirsium arvense*, *Turritis glabra*) (Grubb and Key, 1975).

The possibility to utilize the soil seed bank, originating from earlier vegetation stands, to restore species-rich grassland seems then to be quite small (Willems, 1988; Berendse *et al.*, 1992) and for successful restoration of a diversified flora, the seed rain must be more important than the soil seed bank (Jefferson and Usher, 1989). Since the remaining species-rich grasslands are few and isolated islands in an otherwise intensively farmed or semi-urban landscape (Green, 1990) and because many grassland plants have low seed dispersability (Verkaar *et al.*, 1983; Dessaint *et al.*, 1991) the critical factor for successful restoration is seed dispersal and recolonization processes. Moreover, it is known that sheep can transport large amounts of seeds, especially in their fleece (Hillegers, 1983), and that they influence the recolonization process (Gibson *et al.*, 1987; Mitchley, 1988). Grazing systems have then to pay special attention to the restoration of an 'ecological network': for example, by means of an itinerant flock moving between several areas. In this sense, chalk grassland management appears to be a multiscale problem, involving both ecological and agronomic approaches (Alard and Frileux, 1992).

It must be emphasized that species-rich grasslands must be considered not only for their ecological significance, as regards the occurrence of some plant populations, but also, in a broader view, as a possible source for emigration and recolonization processes in the landscape (Alard *et al.*, 1993). In fact, the maintenance of a species-rich plant community is related to an actual management compatible with the extensive requirement of this community, while the restoration of this kind of community is strongly related to internal or external regeneration potentialities. This leads to a double statute of natural reserves: on the one hand, species-rich and supplying reserves; on the other hand, potentially species-rich and target reserves.

Due to both the absence of an appropriate seed bank and the fertility of scrub soils (Marrs, 1985), restoration of the formerly species-rich grasslands will probably be a long-lasting process. In this case, it may be possible, with appropriate management, to establish a moderately rich grassland sward which in the long term may recruit the ecologically significant species, given the presence of a nearby source pool and the long-term effects of the seed rain (Jefferson and Usher, 1989). On the other hand, the solution could be the deliberate reintroduction of desirable species by sowing a seed
mixture containing the appropriate species in grasslands isolated in an 'unsuitable landscape' (Wells, 1990, 1991).

## Acknowledgements

We thank Jérôme Chaïb (Regional Conservatory of Natural sites) and AXA-insurance company for permission to work in the nature reserve of Saint-Adrien. This study was partly supported by the Ministry of Environment (comité EGPN), the Limagrain Foundation and the France Fund. Dr J. Foster and Dr S. Warrington (University of Hertfordshire, UK) provided useful comments on an earlier version of the paper, as did two anonymous referees.

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