The seed bank in soils of disused chalk quarries in the yorkshire wolds, England: Implications for conservation management

Biological Conservation 42 (1987) 287-302

The Seed Bank in Soils of Disused Chalk Quarries in the Yorkshire Wolds, England: Implications for Conservation Management Richard G. Jefferson* & Michael B. Usher Department of Biology, University of York, York YOI 5DD, Great Britain

(Received 28 February 1987; revised version received 9 May 1987; accepted 16 May 1987) ABSTRACT The soil seed bank from three disused chalk quarries in the Yorkshire Wolds, England, is described; these quarries were known to have botanical conservation interest. A number of plant species occurred in the seed bank which were absent .from the existing species-rich vegetation community. Results of this study suggest that rotational scraping of small areas of the open species-rich quarry floor vegetation, removing surface soil and vegetation, may be the most appropriate management technique for maintaining nature conservation interest. I f the succession is allowed to develop too .far, to a tall grassland being invaded with scrub, it may be impossible to return to the open species-rich conditions due to the nature of any secondary succession started as a result of management operations.

INTRODUCTION The soil of most tropical and temperate habitats contains a reservoir of viable seeds of vascular plants: this is termed the seed bank. There is great variation between species in the lifespans of their buried seeds; some species have evolved the capacity to remain viable for many decades (Cook, 1980) whilst other species are seldom found in a buried seed bank. Many studies of the buried viable seeds in soils have been undertaken, especially in temperate agricultural systems. Studies in a variety of other ecosystems include those of upland forest plantations in Britain (Hill & Stevens, 1981), ant hills and acidic grasslands in Wales (King, 1976), tropical * Present address: Nature Conservancy Council, Mariner House, Hull Road, York YOI 3JW, Great Britain. 287 Biol. Conserv. 0006-3207/87/$03"50 © Elsevier Applied Science Publishers Ltd, England, 1987. Printed in Great Britain

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Richard G. Jefferson, Michael B. Usher

rainforest in Australia (Hopkins & Graham, 1983) and freshwater marshes in New Jersey, USA (Leek & Graveline, 1979). Several studies have looked at the buried viable seeds in successional series of known age and vegetational composition (Oosting & Humphreys, 1940; Livingston & Allessio, 1968; Jerling, 1983). The presence of viable seeds of species of former successional stages or plant communities provides insight into the past vegetational history of various habitats, but the focus of attention has not been on the conservation of plant communities. Such buried seeds could have important implications for the management of nature reserves. Moore (1983) posed the question as to how one defines extinction for plant species at a locality given the existence of seed banks. Rowell et al. (1982), for example, reported the germination of the rare Viola persicifolia from soil cores collected in Wicken Fen, where this species of violet had supposedly been extinct since 1916. Subsequently, Rowell (1984) reported further discoveries of populations of V. persicifolia at two sites in Wicken Fen. He considered that these populations originated from viable buried seed following disturbance of the peat by mole activity and past peat digging. Moore (1983) also considered that the reappearance in East Anglia in 1972 of Senecio paludosus, after 70 years of presumed extinction, was the result of germination of buried seeds. Accounts of the nature conservation importance of chalk quarries, both nationally and in the Yorkshire Wolds, can be found in Davis (1979), Jefferson (1984a) and Jefferson & Usher (1986). This present study aimed to provide an assessment of the species composition of the seed bank, and to relate this to a wider investigation into immigration, colonisation and extinction processes operating in disused quarries of plant conservation interest in the Yorkshire Wolds (see Jefferson, 1984b). At such sites, the maintenance of early successional, species-rich plant communities containing assemblages of species characteristic of chalk or limestone grassland is usually the aim of conservation management. The development of tall grassland and transitional grassland-scrub communities is considered undesirable due to both the reduction in floristic diversity (Usher, 1979) and the loss of local or rare species associated with open calcareous grassland communities. Studies of the vegetation dynamics, including the soil seed bank, are thus of relevance in determining the most suitable management strategy.

SAMPLE SITES AND METHODS The seed banks were studied at three disused Cretaceous chalk quarries in the Yorkshire Wolds: Burdale Quarry (National Grid Reference SE 872626),

Seed banks mquarrysoi~

289

Wharram Quarry (SE 859653) and Kiplingcotes Chalk Pit (SE 915435). The two latter quarries are nature reserves of the Yorkshire Wildlife Trust Ltd and have been designated as Sites of Special Scientific Interest. Burdale Quarry, although having no statutory protection, was ranked second only to Wharram Quarry when the nature conservation interest of 30 Wolds quarries was evaluated by Jefferson (1984a). Soil core samples were taken in the open, species-rich quarry floor community at each site--this is one of the seven plant communities previously described as occurring in the Wolds quarries (Jefferson, 1984a). Constant species in the species-rich community at all sites include Festuca ovina, Lotus corniculatus, Leontodon hispidis, Linum catharticum and, except at Wharram Quarry, Brachypodium pinnatum (nomenclature follows Clapham et al., 1981). Nine series of samples were taken at intervals of six weeks over the period March 1983 to February 1984, with a final (tenth) set of samples collected approximately four weeks later so as to coincide exactly with the first set of samples collected in March 1983. On each occasion 15 soil cores were taken randomly from the species-rich community within each quarry. The soil cores were 5"2 cm diameter, 9-9 cm deep, having a volume of 210cm 3, though in practice a full core was often not obtained due to the shallow depth of the accumulated soil. The soil cores were broken up, spread out on plastic sheets and allowed to air-dry in a greenhouse. The dry soil was passed through a 1 cm mesh sieve to remove stones, large roots, rhizomes and stolons. Samples were aggregated into units of three to form five c o m p o u n d samples from each site. Each of these was placed over a 2cm layer of sterile coarse sand in a 22.5 × 17 × 5.3 cm plastic seed tray. The 15 trays (five for each of the three quarries) for each sampling date were kept in a greenhouse, with a 16 h day at 20°C and an 8 h night at 15°C, underneath fluorescent tubes (75/85 W), and were watered daily with a fine spray. This method, which assesses both the seeds buried in the soil and those present on the soil surface, was found by T h o m p s o n & Grime (1979) to be suitable for the germination of seeds of a wide range of plant species. No attempt was made to look at the depth distribution of buried seeds as the stony nature of the quarry soil prevented sectioning of the cores. Seedlings were removed and identified as soon as possible to avoid competitive effects. Seedlings which could not be identified initially were transplanted into pots containing potting compost and grown until identification was possible. Chancellor (1966), Muller (1978) and the vegetative key in Hubbard (1968) were used where necessary as identification aids. Soil samples from each sampling date (with the exception of those collected in March 1984) were turned over after approximately 12 weeks; mixing is known to cause more seeds to germinate (Thompson & Grime, 1979).

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Richard G. Jefferson, Michael B. Usher

Additionally, a set of three soil cores was taken in October 1983, and a further set of six cores in March 1984, from the tall grassland community at Wharram Quarry. The methods employed in germinating seeds were exactly the same as described above. Constant species in this tall grassland c o m m u n i t y include Arrhenatherum elatius, Festuca rubra, Dactyl& glomerata, Heracleum sphondylium, Centaurea nigra and Ononis repens. This community was considered by Usher (1979) and Jefferson (1984a) to represent a later stage in the chalk quarry succession which occurs in Northern England.

RESULTS The total numbers of seedlings of species germinating from the soil cores from the three sites are listed in Table 1. Also listed are the percentage frequencies of occurrence of species in quadrat samples taken in the current species-rich quarry floor vegetation (of all of the quadrats recorded in the quarries, the numbers actually assigned to the species-rich community were 24 at Wharram Quarry, 27 at Burdale Quarry and 20 at Kiplingcotes Chalk Pit; see Jefferson, 1984a, for details). Several species, annotated in the table, are completely absent from site species lists compiled in 1982 by Jefferson (1984b). At Wharram Quarry a total of 767 seedlings germinated from the 150 soil cores collected; this represents a seed bank of 0.24 seeds cm-2 (a volume measurement of 0.024 seeds cm - 3 is a minimum estimate due to the fact that a number of soil cores did not achieve a full depth of 9.9 cm). The seedlings were assigned to 42 vascular plant taxa, of which 41 were identifiable to species level. Twenty-six of these 41 species (63%) are currently found in the quadrat samples of the species-rich community. Fifteen species (37 %) in the seed bank were not present in the quadrat samples, although 13 of these species were found elsewhere in the quarry. Two species, Epilobium hirsutum and Matricaria matricarioides, were present in the seed bank but could not be found in a vegetative state in the nature reserve. Ten species currently found in the quadrats in the species-rich community were not found in the seed bank. Counts for Burdale Quarry indicated a similar density of seeds in the seed bank (754 seedlings germinating, 0.24 seeds c m - 2 or a minimum of 0"024 seeds cm-3). A total of 51 taxa were represented in the seed bank; 50 of these were identified to species level. There is an interesting contrast between Burdale and Wharram Quarries since at Burdale only 21 species (42%) were found both in the seed bank and the quadrat samples of the species-rich community, and 29 species (58%) only occurred in the seed bank. Of these 24

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TABLE 2 Number of Seeds Germinating from Soil Cores taken from the Tall Grassland Community at Wharram Quarry Species A chillea millefolium A p h a n e s arvensis A r r h e n a t h e r u m elatius C e n t a u r e a nigra Chamerion angustifolium C i r s i u m arvense Dactylis g l o m e r a t a D e s c h a m p s i a cespitosa Festuca rubra Heracleum sphondylium H o l c u s lanatus L e o n t o d o n hispidus Linum catharticum Ononis repens R e s e d a luteola S a g i n a apetala Senecio j a c o b a e a T a r a x a c u m sp. Trisetum flavescens Urtica dioica

October

March

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were found elsewhere in the quarry, but five could not now be found in the quarry (E. hirsutum is again one of these species, but Chaenorhinum minus is particularly notable as this is relatively uncommon in the Wolds). Only six species, found in the quadrats, were absent from the seed bank, the most surprising species in this list being Plantago lanceolata, which was represented in the seed banks of the other two quarries. The seed bank at Kiplingcotes Chalk Pit was more extensive (1743 seedlings germinating, representing a density of 0"55 seeds cm -1 or a minimum of 0"055 seeds cm-3). Forty-six taxa, including 45 identifiable species, germinated, of which 24 species (52%) were present in both the seed bank and the quadrat samples of the present species-rich community. Nine of the 22 species present in the seed bank, but not in the quadrats, could not be located elsewhere in the quarry in 1982. Again both E. hirsutum and C. minus are included in this list, together with another small species of open areas, Sagina apetala. The numbers of seedlings emerging from the soil cores taken from the tall grassland plant community at Wharram Quarry are listed in Table 2. A total of 105 seedlings representing 20 vascular plant species were recorded. This represents a viable seed density of 0.55 cm -z or a minimum estimate of

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TABLE 3 Percentage Frequency of Occurrence of Species in Quadrats from the Scraped Areas, Speciesrich and Tall Grassland Communities at Wharram Quarry (A dash (--) implies that the species was not recorded in the quadrats, and n is the number of quadrats) Species

Achillea millefolium A g r o s t i s stolon(fera A p h a n e s arvensis A renaria serpyllifolia A r r h e n a t h e r u m elatius B a r b a r e a vulgaris Bellis p e r e n n i s Briza m e d i a C a m p a n u l a rotundifolia Carex flacca Carlina vulgaris C e n t a u r e a nigra Chamerion angust(folium C i r s i u m arvense C. e r i o p h o r u m C. vulgate Crataegus monogyna Crepis capillaris Dactylis glomerata Dactylorhiza fuchsii D e s c h a m p s i a cespitosa D e s m a z e r i a rigida Euphrasia n e m o r o s a Festuca ovina F. rubra Galium aparine G. cruciata G. v e r u m Gentianella amarella Heracleum sphondylium H i e r a c i u m pilosella H o l c u s lanatus Koeleria m a c r a n t h a Lathyrus pratensis L e o n t o d o n hispidus Linum catharticum L o t u s corniculatus M e d i c a g o lupulina O n o n i s repens O p h o , s apifera

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R i c h a r d G. Jefferson, M i c h a e l B. Usher

296

TABLE 3 - - c o n t d . Species

P h l e u m p r a t e n s e agg. P l a n t a g o lancoelata P. m e d i a Poa pratensis P o l y g a l a vulgaris P r i m u l a veris Prunella vulgaris R a n u n c u l u s repens Senecio j a c o b a e a T a r a x a c u m spp. Thymus praecox Tr(folium p r a t e n s e T. repens Trisetum flaveseens Tussilago f a r f a r a Veronica c h a m a e d r y s Vicia cracea V. sativa V. s e p i u m

Scraped (n = 20)

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Tall g r a s s l a n d (n = 18) --

10

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60

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0"055 cm - 3. The majority of the species characteristic of this community are represented in the seed bank, but the list in Table 2 contains some species that would be unable to grow in the tall grassland (e.g.S. apetala, Linum catharticum, Reseda luteola and Aphanes arvensis. As part of the investigation into the most appropriate form of management for disused quarries of nature conservation interest, 20 randomly located, 40 x 40 cm, quadrats were recorded in July 1983 in two areas of Wharrum Quarry where surface soil and vegetation had been scraped away from the underlying chalk in 1976. The aim of this TABLE 4 C o m p a r i s o n o f Species L i s t s f r o m the S c r a p e d A r e a s , T a l l G r a s s l a n d and Species-rich Communities at Wharram Quarry using S~renson's

Coefficient Community

Scraped areas Species-rich Tall grassland

Community Scraped

Species-rich

Tall g r a s s l a n d

--

0.74 --

0"53 0"52

Seed banks in quarry soils

297

management had been to restart the quarry succession in areas which had developed into a tall grassland community. Table 3 lists the percentage frequency of occurrence of species in the scraped areas and in the speciesrich and tall grassland communities. Pairwise comparisons of these species lists were made using Sorenson's coefficient of similarity C s = 2j/(a + b) wherej is the number of species common to the two samples and a and b are the total number of species in each sample respectively. This is a simple measure of the extent to which the two samples have species in common; Table 4 gives the matrix of coefficient values.

DISCUSSION

Comparison of the seed bank with the current vegetation The majority of the more frequent species (44 of the 47 species with a frequency of 25 % or more) occurring in the vegetation at the three sites was represented as seedlings (Table 1). However, species such as H. pilosella, L. corniculatus and F. ovina at all three sites, and Deschampsia cespitosa at Wharram Quarry, which are of high frequency in the vegetation, were absent or only sporadically recorded as seedlings. A study of the seed banks of a number of habitats (Thompson & Grime, 1979) also found that very few seedlings of L. corniculatus germinated from soil cores taken beneath plant communities where this species was abundant in the above ground vegetation. Jones & Turkington (1986), however, reported both that L. corniculatus forms large seed banks in the soil and that a large proportion of recently shed seed would only germinate after scarification. Lack of scarification could account for the small number of L. corniculatus seedlings germinating both in Thompson & Grime's (1979) and in this study. The seeds of H. pilosella have a pappus for wind dispersal. Grime (1979) suggested that this species is adapted for rapid colonisation of bare habitats; the viability of the seeds declines rapidly and they are not recruited into the persistent seed bank. Once established, H. pilosella spreads by vegetative reproduction from daughter rosettes produced at the ends of stolons. The absence of D. cespitosa from the seed bank may be related to the low levels of flowering of this species when growing in the shallow, nutrient-poor quarry soil since it occurred in the seed bank of the tall grassland soil (Table 2). Thompson (1986) recovered no seedlings of F. ovina from soil cores taken from acid grassland on Dartmoor even although the species was an abundant component of the above-ground vegetation. The scarcity of F.

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ovina was thought to be due to its habit of possessing a transient seed bank whereby none of the seed output persists in a viable condition for more than a year (Thompson & Grime, 1979). The few seedlings recorded in Table 1 from all three quarries may therefore be related more to the method of sampling, including both surface and buried seeds, than to the presence of viable buried seeds of this species. From Table 1 it can be seen that a few species are absent both from quadrat samples and from the quarry species lists, but present in the seed bank. Some of these species are abundant components of the seed flora in the quarry soils (for example, Aphanes arvensis and Reseda luteola at Wharram and Burdale Quarries and Arenaria serpyllifolia at Kiplingcotes Chalk Pit). Other species occurred only occasionally as seedlings (Senecio vulgaris at Burdale Quarry, Cardamine hirsuta at Kiplingcotes Chalk Pit) or only once (Polygonum convolvulus at Burdale Quarry, Sagina apetala at Kiplingcotes Chalk Pit). Many of these species are typically associated with disturbed ground and their presence in the seed bank is not surprising given that these quarries are at least partly surrounded by land under intensive arable cultivation. Such species are capable of maintaining a reservoir of viable seeds in the soil long after their 'extinction' as individuals in the vegetation community. Regeneration will only be possible if future disturbance creates environmental conditions amenable to germination and establishment. Epilobium hirsutum is of particular interest in that its presence in the seed bank of all three quarries is probably the result of immigration of seeds from nearby populations; it is an unlikely pioneer colonist of exposed chalk. The seeds of this species are light (0-05 mg; Grime et al., 1981) and highly vagile. Jefferson (1984b) trapped its seeds on sticky traps at both Wharram and Burdale Quarries, providing independent evidence for an immigration hypothesis. The presence of early successional species in the seed bank, and differences between the seed flora and the flora represented in the current above-ground vegetation, have been noted frequently (see, for example, Chippindale & Milton, 1934; Oosting & Humphreys, 1940; Jerling, 1983). In disused quarries in the Yorkshire Wolds, pioneer species such as R. luteola do not persist in the species-rich communities which have developed over a period of ten or more years. However, disturbances, such as rabbit scratching at Wharram Quarry, often result in the transient appearance of the seedlings of such species. Implications of seed bank studies for nature conservation

Reference to Table 1 indicates that possibly only one species present in the seed bank and absent from the current vegetation of the quadrat samples

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could be considered to be of nature conservation significance in terms of its rarity. This is Arabis hirsuta, which occurs in the seed bank at Kiplingcotes Chalk Pit. This calcicolous species was classified by Jefferson (1984a) as being of restricted distribution since it occurs in less than one quarter of the 10 km squares of the National Grid in the British Isles and in less than half of the l0 km squares covering the Northern Chalk (Yorkshire and Lincolnshire Wolds). In the Wolds, A. hirsuta is confined to open situations in disused quarries (it was found in six of the 30 pits and quarries examined by Jefferson, 1984b), disused railways and in open microsites in chalk grassland. Its absence from the existing vegetation at Kiplingcotes Chalk Pit may reflect the temporal limitation of observations; at Wharram Quarry A. hirsuta appears in some years on areas that have been scraped by rabbits, though it is itself very susceptible to rabbit grazing. Jefferson & Usher (1986) stressed the problems involved in defining extinction (as opposed to temporary absence) of species of flora and fauna with reference to such successional environments. Successional vegetational changes in disused quarries pose problems for the management of such sites for nature conservation where the aim is to maintain species-rich grassland communities akin to the semi-natural calcareous grassland of the region. Most quarry nature reserves and SSSIs have received little or no deliberate management (Davis, 1979). The two management techniques which could be used to arrest successional change in quarry sites are grazing and mowing. The former method is, in reality, not always a practical solution due to the expense of fencing, the availability of stock or the problem of providing drinking water. Mowing is similarly not often a practical solution due to the uneven nature of the terrain and the unavailability of suitable equipment. As an alternative, management at Wharram Quarry has involved the mechanical scraping and removal of both the vegetation and the surface soil from the underlying chalk with a view to restarting the succession. Two areas, each over 200 m 2, which had developed into tall grassland with scattered shrubs, were scraped in 1976. Comparison of the species occurring in the scraped areas with the speciesrich community reveals differences in species composition and abundance, though in interpreting such differences the different ages of the respective plant communities, as well as the greater openness of the scraped areas, must be considered (Table 3). Comparison of the scraped area with both the species-rich and tall grassland communities using Sorenson's coefficient of similarity (Table 4) indicates that the scraped areas, at present, most closely resemble the species-rich community but also have affinities, in terms of shared species (e.g. Centaurea nigra, Heracleum sphondylium and Ononis repens, cf. Tables 2 and 3), with the tall grassland plant community. As evidenced by observations immediately after the scrapes were made (e.g. the

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almost immediate germination of Reseda luteola), the scraping initiates a secondary succession in that buried seeds and possibly also plant fragments from the original community are already present for in situ recolonisation following the disturbance. This suggests that the course of secondary vegetation succession in the quarry will differ from that of the original primary succession, at least in terms of the species composition. The species composition of the seed pool of potential colonising species in the area surrounding the quarry will also have changed between the time when the quarry was originally abandoned and the time when the scrapes were made; this could also contribute to the differences in species composition. Species characteristic of the tall grassland community (C. nigra, H. sphondylium and Plantago lanceolata) are currently present in the scraped areas but would not have been present in the early stages of the primary succession following the cessation of quarrying. Reference to Tables 3 and 4 shows that although there are similarities between the scraped areas and the species-rich community, a number of species characteristic of the latter are absent from or occur at reduced frequencies in the scrapes. Species such as Carlina vulgaris and Ophrys apifera fall into the former category, whereas

Festuca ovina, Gentianella amarella, Hieracium pilosella, Lotus corniculatus and Thymus praecox all occur in greater abundance in the species-rich community. Conversely, a number of pioneer species still occurred in the scraped areas seven years after their creation (Table 3). These include Aphanes arvensis, Arenaria serpyllifolia and Desmazeria rigida. Comparison of Table 2 with column 2 of Table 3 indicates that few of the species in the seed bank beneath the tall grassland community are those present in the current species-rich community. This has important implications from a conservation standpoint if the small number of samples are an accurate assessment of the composition of the seed bank beneath the tall grassland community. Thompson (1986), for example, argued that many studies of seed banks have taken too few soil samples to assess accurately the relative abundance of species in the seed bank. In contrast, in this study estimation of the abundance of species in the seed bank was not intended and the data in Table 1 are used to reflect species richness. The data suggest that seeds of species such as T. praecox, G. amarella and Prunella vulgaris are absent from the tall grassland community seed bank; such species clearly do not remain viable in the soil for long periods, and it is such species which are valued when they occur in the species-rich quarry floor community. Given the apparent absence of certain earlier successional species from the seed bank beneath the tall grassland community (Table 2) and the differences in species composition and structure between the scraped areas and the species-rich community (Tables 3 and 4), a better method of management at Wharram Quarry may involve the rotational scraping of small areas (perhaps 10-20 m 2) of the species-rich community rather than

Seed banks mquarrysoils

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scraping larger areas (c. 200 m 2) of the tall grassland community. Inspection of Table 1 shows that many of the species occurring in the species-rich community are also present in the seed bank beneath it. Jefferson (1984b) also found that seedlings of species such as T. praecox, G. amarella and Leotodon hispidus germinated in small plots (25 x 25 cm) scraped clear of soil and vegetation in the species-rich community at Wharram Quarry in 1983. For the species not represented in the seed bank, colonisation of a mosaic of small scrapes by seed shed from plants in the adjacent species-rich community, together with regeneration from root fragments, may enable the re-establishment of a similar species-rich plant community; however, further long-term experimental work would be needed to prove such a hypothesis. One potential disadvantage of scraping as a conservation management tool lies in its initial apparent destructive impact of the existing vegetation and its unacceptability to uninformed members of the general public interested in natural history. This is potentially a serious problem, as demonstrated at Wharram Quarry, where a large population of the visually attractive Ophrys apifera occurs in the open, species-rich community. This orchid attracts a large number of visitors to the reserve in the summer months. Scraping appears at first sight to be destroying both the habitat and the species; hence it is important to inform visitors to the reserve of the management techniques in operation. What is certain is that the management of an early successional community is not straightforward; considerable management effort will be required to ensure the retention of the complete guild of colonising species. A C K N O W L E D G E M ENTS We thank the Yorkshire Wildlife Trust Ltd and Birdsall Estates for permission to work on their land. This research was funded by a Natural Environment Research Council studentship to R.G.J., for which we are grateful. Finally, we thank Mrs M. Wetherill for typing and editing the manuscript. REFERENCES Chancellor, R. J. (1966). The identification of weed seedlings of farm and garden. Oxford, Blackwell Scientific Publications. Chippindale, H. G. & Milton, W. E. J. (1934). On the viable seeds present in the soil beneath pastures. J. Ecol., 22, 508-31. Clapham, A. R., Tutin, T. G. & Warburg, E. F. (1981). Excursion flora of the British Isles, 3rd edn. Cambridge, Cambridge University Press. Cook, R. (1980). The biology of seeds in the soil. In Demography and evolution in plant populations, ed. by O. T. Solbrig, 107-29, Botanical Monographs, 15. Oxford, Blackwell Scientific Publications.

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