A quantitative survey of chalk grassland in England
Transcription
A quantitative survey of chalk grassland in England
A Quantitative Survey of Chalk Grassland in England J. W. BLACKWOOD The Nature Conservancy (East Anglia Region), Sandbanks, Saltfleetby, Louth, Lincolnshire, England, & C. R. TUBBS, The Nature Conservancy (South Region), Shrubbs Hill Road, Lyndhurst, Hampshire, England ABSTRACT to give way before an encroaching tide of arable, the trend being accelerated during the scare-price period Precise quantitative information about habitats & important both in considering their conservation and in laying a of the Napoleonic Wars. It is doubtful whether the foundation for more detailed systematic study. This article total acreage of chalk grassland in England in 1815 records a quantitative survey of chalk grassland in England excessively exceeded that of the present time. Naish, in 1966, and demonstrates the practicability of effecting rapid national habitat surveys. The survey revealed that, despite progressive "reclamation', there remained in England at that time a minimum of 107,605 acres (43,546 ha) of chalk grassland, of which almost 70 per cent lay in the county of Wiltshire. Observations indicated that most of these areas of chalk grassland were used as sheep and cattle grazing, and were on escarpments too steep to reclaim. Distribution of C h a l k O u t c r o p s in E n g l a n d a n d Percentage of O u t c r o p O c c u p i e d b y C h a l k Grassland f o r E a c h C o u n t y or T o p o g r a p h i c a l Unit. Yorkshire W o l d s 15 ::::: Lincolnshire W o l d s ~" INTRODUCTION Until the closing decades of the 18th century, unsown grassland, mainly sheep-walk, occupied vast areas of the chalklands of lowland Britain. In the 15th and 16th centuries, the area of sheep-walk would appear to have expanded partly in response to rising market-prices for wool, and partly because considerable sheep flocks were necessary to the maintenance of arable production. The chalkland areas were essentially sheep-and-corn country. The sheep, r u n on the open downs by day, were folded overnight on the arable land. The latter benefitted both from the sheep's dung and from the consolidation of the light soils by trampling. Kerridge (1951) concluded that, in Wiltshire, 'without the sheep-fold the whole agricultural economy would necessarily have collapsed.' In the 18th century, and probably before, the emphasis swung towards arable production. Although the sheep-fold remained indispensable as the chief source of manure, technical innovation (mainly the introduction of the new fodder-crops and legumes, and the widespread irrigating of water-meadows) made it possible to incorporate sheep husbandry into the management of the new enclosed arable farms, arising largely from the increasing numbers of Enclosure Acts. Thus the vast tracts of open downland started ~.,.:, ool Cambridge 01 'v: :: : : : :::~,.. Chiltern H i l l s 04 :: :: : :::::::: : i : / Berkshire 08 i Wiltshire 157 ~ : ::i::: :::: h ~ ; ::::~: i : :::: ::5::: ; " iiiiil;~:.:.:..~aols22 ................ , ................................. :iii: ..............~o;i~ Do~ .... "'~:::::..~. ......... :::ii::!!i::::/ Sout h [)£~vns s ~-~ -- Dorset 3.5 152 Fig. 1. Sketch-map of chalk outcrops in England, with indications of percentage of each area occupied by chalk grassland. (1961), using surveys published in 1791, 1807, 1825 and 1840, demonstrated a rapid reduction and fragmentation of the area of downland sheep-walk in Hampshire until, at the last mentioned date, it occupied very little more than its present area. Subsequently, there has been an ebb and flow of 'reclamation' and abandonment of arable, corresponding to Biological Conservation, Vol 3, N o . 1, O c t o b e r 1 9 7 0 - - O Elsevier Publishing C o m p a n y Ltd, E n g l a n d - - P r i n t e d in Great Britain Biological Conservation the level of investment in arable production. The fragmentation of the 'old down' or chalk grassland, however, has been progressive, and at the present time it seems clear that reclamation for arable farming, etc., has reached its highest ever level. Chalk grassland characteristically carries a rich flora (Wells, 1969) and invertebrate fauna (Morris, 1969). The formerly extensive tracts also attracted a distinctive avifauna. The attention of biologists, however, has generally been focussed on a limited number of wellknown sites, and few quantitative data relating to the total extent, management, and fragment-size, of chalk grassland are available. The objects of the survey recorded here were twofold: first to demonstrate the practicability of carrying out rapid and extensive quantitative habitat surveys, and secondly to obtain quantitative data for chalk grassland as a basis for further systematic study. The field-work in this case occupied 82 man-days and was carried out mainly by the authors and N. E. King (who surveyed Wiltshire). The actual mapping was completed between April and September 1966 at a scale of 1:25,000. (Fig. 1). The maps are held by the Nature Conservancy and are available for reference on request. DEFINITION OF HABITAT AND METHOD OF SURVEY The survey was necessarily extensive rather than intensive. In the time available it was not possible to investigate in detail the soils and vegetation of each individual site. For the purpose of the survey, therefore, the habitat was defined as unsown grassland on soils of which the parent material was chalk. This was considered more appropriate than the definition of specific plant communities on chalk soils, and was in fact the primary definition of chalk grassland adopted by Tansley & Adamson (1926). The definition does, however, introduce some element of subjective judgement in an extensive survey such as that described here. For example, sites which had clearly received treatment with herbicides, but which retained a proportion of chalk grassland herbs and grasses, were excluded. On the other hand areas of abandoned arable (few in number and mostly small in area) were included where it was known that for peculiar reasons (e.g. National Nature Reserve status) no further cultivation would take place on them. Small areas of grassland arising on superficial deposits overlying the chalk and isolated within extensive tracts of chalk grassland were also included. Sites carrying more than a 50 per cent cover of scrub were excluded from the survey; areas of chalk grassland of less than five acres (two hectares) were not recorded. Areas were estimated by using a transparent grid superimposed on the survey maps. Within each grid square, a block of dots was printed for the estimation of parts of squares. The areas estimated in this way do not include a correction for steep slopes where the true surface area is greater than that shown on the map. RESULTS The data obtained from the survey are summarized in Table I, which refers to 107,605 acres (43,546 ha) of chalk grassland grouped, as appropriate, according to the administrative county or topographical unit involved. This total area was distributed in 1,225 separate fragments, of which 931 or more than threequarters, were of less than 50 acres (20 ha) but more than 5 acres (2 ha) in extent. Only 67 sites with an area in excess of 200 acres (81 ha) each were recorded. As sites of less than five acres (2 ha), together with roadside verges, railway embankments, and small artefacts such as chalk quarries and earthworks, were not included in the survey, the recorded acreage falls short of the true total area of the habitat. It should also be stressed that the figures in Table I represent the mapped and not the projected area, and that the greater part of the grassland is on steep slopes. Both the distribution and survival of the chalk grasslands recorded in the survey depend on two main factors: the occurrence of escarpments too steep to cultivate, and the use of certain areas of the chalk for military training. In addition, a few sites receive protection from 'reclamation' because they are Nature Reserves, Public Open Spaces, National Trust property, or common lands, or are protected by a private owner. In most cases these are on escarpments too steep to reclaim. Table I shows that only in Wiltshire, Dorset, Hampshire, the South Downs, the Yorkshire Wolds, and the Isle of Wight, does the area of chalk grassland exceed one per cent of the area of the chalk outcrop, and that fragments of more than 200 acres (81 ha) are almost exclusively confined to these areas. Perhaps the most striking feature of the Table is the large area of chalk grassland--73,085 acres (29,576 ha)--and the large number of fragments of more than 200 acres, situated in Wiltshire. This circumstance is accounted for in part by the extensive military training areas on Salisbury Plain, and in part by the varied topography of the county. Those counties or topographical units in which the area of chalk grassland was less than one per cent, exhibit only a limited distribution of escarpment (e.g. Norfolk) or are extensively wooded (e.g. the Chilterns). Blackwood& Tubbs: A Quantitative Survey of Chalk Grasslandin England TABLE I Chalk Grassland Survey 1966: Summary of Data Obtained No. of fragments by size categories (acres*) County or % ~ % topographical unit ~ ~ ~~ ~ ~ ~ ~ ~ ~ o Dorset ~ ~? ~ ¢',1 ¢m + 236,880 3-5 145 101 28 10 2 1 3 73,085 464,400 15'7 529 407 51 34 16 5 16 Hampshire 5,224 239,760 2.2 119 95 13 7 1 -- 3 Berkshire 1,573 185,400 0'8 59 50 7 2 -- -- -- Isle of Wight 2,128 14,040 15.2 24 13 6 2 -- 2 1 South Downs (Sussex) 8,592 223,200 3"8 117 83 13 11 2 1 7 North Downs (Kent and Surrey) 2,226 437,760 0.5 94 83 10 1 -- -- -- Chiltern Hills (Beds., Berks., Bucks., Herts., and Oxon.) 2,028 550,080 0"4 59 47 7 4 -- -- 170 156,600 0.1 3 2 1 44 332,640 0.01 3 3 . 225 143,280 0.2 12 12 . 3,939 262,440 1'5 61 35 15 5 4 2 -- 107,605 3,246,480 3.3 1,225 931 151 76 26 11 30 Wiltshire Cambridgeshire Norfolk Lincolnshire Wolds Yorkshire Wolds TOTAL 8,371 .-. 1 . . . . . . . . . . . . . * 1 acre = 0"405 ha. A brief appraisal of the distribution and management of chalk grassland in each county (or topographical unit) follows, using Table I together with notes made in the field and other information gathered during the survey. Wiltshire Wiltshire has almost 70 per cent of the total area of chalk grassland in England, including 37 sites of more than 200 acres (81 ha). The Ministry of Defence training areas on Salisbury Plain account for slightly less than half the county's chalk grassland acreage, or 7.4 per cent of the county's chalk outcrop. Intensive grazing is here confined to the periphery of the training areas, but great tracts of the Imber Ranges are extensively grazed by large herds of cattle which are 'drifted' across the Ranges when they are not in use. Previous to their acquisition early in this century, many of the training areas were in arable cultivation, which is perhaps reflected in the widespread abundance of Tall Oat-grass (Arrhenatherum elatius) and Upright Brome-grass (Bromus erectus). The ranges as yet exhibit little scrub development--probably because much of their extent is comparatively remote from hedgerow Hawthorn (Crataegus monogyna) which might provide seed sources. The remaining chalk grasslands in Wiltshire are almost all on scarp slopes--the scarp of the Marlborough Downs overlooking the Vale of Pewsey, the edges of Salisbury Plain, and the numerous chalk ridges of southern Wiltshire. Practically all are retained as sheep or cattle grazings which form integral parts of farm units. Grazing management has resulted in the retention of characteristically 'clean' slopes, few sites exhibiting scrub colonization. Dorset The 8371 acres (3388 ha) of chalk grassland recorded in Dorset are distributed mainly along the Purbeck Biological Conservation chalk ridge, along the northerly-facing scarp overlooking the Blackmoor Vale, and on the sides of dipslope valleys. As in Wiltshire, practically the whole of the total area (apart from the military training area on Purbeck ridge) is grazed by either sheep or cattle; however, because of the more limited distribution of steep slopes as compared with Wiltshire, the present tide of cultivation has left fewer and relatively less extensive areas of unsown grassland in Dorset. The botanical composition of many steep slopes in northern Dorset has also been partially modified by reseeding and spraying. Hawthorn encroachment is generally more widespread in Dorset than in Wiltshire. Hampshire Apart from three sites in excess of 400 acres (162 ha), one of which is an eastern extension of the Salisbury Plain Ministry of Defence Ranges, another is a Public Open Space owned by Hampshire County Council, and the third is an actively managed Common, the Hampshire chalk grassland is restricted to a scatter of sites of less than 200 acres on steep and uncultivable slopes. Most of those are either ungrazed or grazed only occasionally, and hawthorn colonization is a recurrent feature. forms a chain of chalk grassland broken only by the valleys of the Adur, Ouse, and Cuckmere, and by woodlands between Washington and Steyning. On the dip-slope, chalk grassland is confined to coombes and trenches. Almost all the chalk grassland on the South Downs is grazed and so relatively few sites have been invaded by scrub. North Downs The scarp slope ranging from Guildford, Surrey to Ashford, Kent contains scattered small fragments of chalk grassland that are largely unmanaged and often bear much scrub development. The dip-slope is mainly wooded, being on a clay-with-flints capping over chalk so that little chalk grassland was located. Around Folkstone and Dover, steep-sided chalk valleys and cliff edges produce a number of grassland fragments most of which are grazed. The Chiltern Hills The 2028 acres (821 ha) of chalk grassland recorded in the Chiltern Hills is confined almost entirely to the north-west-facing scarp of the Upper Chalk in Oxfordshire and Buckinghamshire. Colonization of the scarp by scrub is common, and in several places woodland is developing. Most areas of grassland are Berkshire clearly too small (59 fragments of which 47 were less The ease with which the gently undulating plateau than 50 acres (20 ha) in extent) to be managed as of the Berkshire Downs can be brought into cultivation economic parts of farm units. The dip-slope of the in reflected in the small percentage (0.8 per cent) of Chilterns is largely capped with drift and extensively the chalk outcrop remaining as unsown grassland. The wooded, and chalk grassland is confined to the sides total area of chalk grassland, 1573 acres (637 ha), is of a few valleys. distributed in 59 fragments, of which 50 are between five acres (2 ha) and 50 acres (20 ha) in area and none of the remaining nine is more than 200 acres. Most of Cambridgeshire and Norfolk the larger sites, however, are grazed and exhibit little In the virtual absence of steep slopes, chalk grassscrub colonization. land in these counties is confined to artefacts (i.e. earthworks and quarries) and a single small glacial scar at Ringstead Downs, Norfolk. Isle of Wight More than 2000 acres (809 ha) of chalk grassland, representing about 15 per cent of the total chalk Lincolnshire Wolds outcrop, are found in the Isle of Wight. This high Chalk grassland is here confined to artefacts such as proportion of chalk grassland compared with arable deserted medieval village sites and to a few steep land arises partly from the incidence of steep slopes slopes. These sites have been but little colonized by and partly from the activities of the National Trust, scrub and are mostly grazed by cattle or sheep. whose interests in the island include extensive tracts of chalk grassland, most of which is grazed by cattle. Yorkshire Wolds To the north and west the Wolds are segmented by South Downs West of the River Arun the South Downs of Sussex are complex systems of U-shaped glaciated valleys, the extensively wooded, chalk grassland being confined to steep sides of which carry 3939 acres (1594 ha) of a three-mile (4.8 km) stretch of the north-facing scarp chalk grassland according to our survey. Most such and to a few smaller fragments on the dip-slope. From areas are grazed by sheep but a few have been colonized the Arun to Beachy Head the steep north-facing scarp by scrub. Blackwood & Tubbs : A Quantitative Survey of Chalk Grassland in England ACKNOWLEDGEMENTS W e wish to a c k n o w l e d g e the help o f o u r colleagues M. J. W o o d m a n , G. M. A. Barker, R. D. Jennings, N. E. King, a n d J. M. Schofield, in c a r r y i n g o u t this survey. W e also wish to t h a n k T. C. E. Wells for c o m m e n t i n g on a d r a f t o f this p a p e r , a n d R. A. F e n t o n for p r e p a r i n g the m a p a c c o m p a n y i n g it. References KERRIDGE, E. W. J. (1951). The Agrarian History of Wiltshire. Ph.D. thesis, University of London. A Code of Conduct for the Conservation of Flowering Plants and Ferns In the interests of the Conservation of Flowering Plants and Ferns it is suggested that members of the BSBI* consider the following recommendations as a guide to collecting, introducing, or visiting, rare and local species. The Society hopes that all those interested in the conservation of the Countryside will also follow these recommendations. Collecting 1. Members will not pick or collect any material of nationally rare species as defined in a listt published by the Society. 2. Members will not collect specimens from any Nature Reserve, Nature Trail, or National Trust property, without first obtaining permission from the appropriate authorities. 3. Members will not collect specimens of any species in a locality in which it is scarce. 4. When leading excursions, members will ensure where possible that material is demonstrated without removing, or severely damaging, the plant. 5. Members who find it necessary to collect voucher material to support new County or other important records should do so with discretion. It is rarely necessary to take a whole plant to ensure accurate identification. A photograph may be adequate in some cases. 6. Members who wish to collect plants for the preparation of duplicate sets of herbarium material should only do so from very large populations. When material in the field is insufficient, it should be cultivated for this purpose. 7. When living material is required for experimental work, members should raise it from seed or cuttings wherever possible. With modern methods of propagation, uprooting of plants should rarely be necessary. Note: If material of species in Clauses 1 or 2 is needed for scientific purposes, reference should first be made to the Secretary of the Conservation Committee of the Botanical * Whereas this Code of Conduct was prepared for the members of the Botanical Society of the British Isles, it is published by us internationally with the happy approval of the Council of that body, as it is felt that it will be of far wider interest and could with advantage be followed in other heavily-populated countries.--Ed. -~ This will be published in an early number of the journal Watsonia and may be subject to amendment from time to time, MORRIS, M. G. (1969). Populations of invertebrate animals and the management of chalk grassland in Britain. Biol. Conserv., 1(3), 225-31. NAXSH, M. C. (1961). The Historical Geography of the Hampshire Chalklands. M.A. thesis, University of London. TANSLEY, A. G. & ADAMSON, R. S. (1926). Studies of the vegetation of the English Chalk. IV: A preliminary survey of the chalk grasslands of the Sussex Downs. J. Ecol., 14, 1-32, illustr. WELLS, T. C. E. (1969). Botanical aspects of conservation management of chalk grasslands. Biol. Conserv., 2(1), 36-44, illustr. Society of the British Isles, who will have access to the latest information about the status of the species, and will attempt to build up an index of sources of seed or living material already available. Visiting 1. Members must ensure that, when land is private, they have adequate permission for access to the site. 2. Members should take care not to damage the site of rare species by 'gardening' before taking photographs. 3. Photographs should not be exhibited or published which disclose the locality of a rare species, nor should details accompanying a photograph do this. 4. Members visiting the sites of rare species should be aware that too many visitors can damage the habitat of a plant by, for example, preventing seedling establishment. It is suggested that parties numbering more than three are often undesirable; larger parties may draw attention to the site of rare species, and are more likely to cause damage to the habitat. Members should always respect requests from Conservation organizations not to visit sites at certain times. 5. Members visiting rare species should always avoid any activity, such as trampling around the plant, or creating an obvious track, which would expose the plant to the attention of unwelcome visitors. 6. Organizers of excursions should take these considerations into account when planning routes. Introductions Members must not introduce plants into the countryside, and especially into Nature Reserves, without the knowledge and agreement of the appropriate conservation organization. F o r further guidance on this subject, reference should be made to ' A Policy on Introductions', SPNR Technical Publication No. 2, 1970, obtainable from the Secretariat, SPNR, The Manor House, Alford, Lincolnshire, England, for 15p. E. MILNE-REDHEAD, President, Botanical Society of the British Isles, The Herbarium, Royal Botanic Gardens, Kew, Richmond, Surrey, England