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M3 Twyford Down, Hampshire




The scarred landscape © Peter Wakely
Location
County: Hampshire
Nearest town or settlement: Winchester
Contact details
Project description

Introduction:

In February 1990, the decision was taken that the final section of the M3 motorway would follow an easterly route through Twyford Down. Although the route chosen would run through land that was largely agricultural, it would affect the East Hampshire AONB, parts of the St Catherine's Hill and the Itchen Valley SSSI, and would isolate the most easterly parts of the St Catherine's Hill SSSI, the "Dongas" with its colony of Chalkhill Blue butterflies.

Due to concern about losses of habitat, it was proposed to recreate downland on top of Twyford Down, on an area of arable land surrounding the Arethusa Clump, and on the restored route of the existing Winchester bypass (A33). This would create areas of downland considerably greater than those to be lost, and it would provide a link between the water meadows and St Catherine's Hill.
Since 1990, staff from the Institute of Terrestrial Ecology have been acting as advisors in order to ensure that the construction of the road had the minimum effect on wildlife and to suggest methods by which a range of habitats could be created on land made available by the motorway construction.

Following the decisions of Public Enquiries, it was expected that there would be scope for habitat creation which would considerably increase the area of chalk grassland, a habitat which is in decline nationally. Although the line of the motorway was fixed, there was considerable scope for modifying construction procedures to take account of wildlife.
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Preliminary Surveys:
As a first step, the animal and plant communities were surveyed along the route itself, and also in the adjacent areas, particularly the SSSIs. The purpose of this was: *to identify at an early stage any factors which might need special provision in the construction of the motorway, such as major deer or badger crossings/paths,*to identify any habitat or species of particular conservation interest which lay directly on the line of the new road and which could be of use in the downland restoration, *to ensure that the reconstructed downland would contain appropriate plant and animal communities which would fit into the mosaic of adjacent downland.
The surveys showed that there was no particular problem with either deer or badgers on the chosen route, and therefore no special provision would be needed. However, two areas of grassland of interest were identified.

The first was a small area of flood meadow which would be lost. The area was found to contain no particularly rare species, but was a good example of a declining habitat type and therefore merited translocation for use elsewhere, if possible.

The second area was located at the northern end of Twyford Down in as limb of the St Catherine's Hill SSSI, known locally as "the Dongas". These are thought to be ancient trackways connecting Winchester with the Down. Although much of the Dongas is species-rich downland, the motorway route had been carefully chosen to cross where scrub had encroached and most of the conservation interest had already been lost. It was decided that all the remaining turf in this area would be moved and used in the first stage of the downland restoration.
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Restoration Methods:
The aim of the restoration was to create new areas of herb-rich downland with a characteristic, rich invertebrate fauna on the restored A33 and around the Arethusa Clump on Twyford Down. It was intended that, within a relatively short time, these areas would become integrated into the St Catherine's Hill reserve of the Hampshire Wildlife Trust.
The process of habitat restoration can be divided into three distinct phases: *The first phase is the preparation of the site. This can involve denutrification by various methods, adjustment of soil depths, and in some cases landscaping to alter aspect. *The second phase is the introduction of the relevant plants in suitable proportions. This can be accomplished by variations of turf translocation, seedling and planting of pre-grown plants. The invertebrate fauna would not have to be introduced on Twyford Down as all the restoration sites were adjacent to established chalk grassland. *The third phase is the manipulation of management techniques to drive the development of the plant and animal communities towards the desired grassland types. The most commonly used methods on chalk grassland are mowing or grazing, with grazing generally considered to give the most desirable grassland.
This programme of downland restoration involved the use of three main methods: turf translocation, seeding and plug plants. Approximately 0.5 ha could be turfed and the remaining area would receive seed and plug plants which would, where possible, be of local provenance.

Turf Translocation:
Two turf translocation methods were used. Before the main works began, a small area of herb-rich turf was identified above the Hockley Junction which was rich in Horseshoe Vetch, the food plant of the Chalkhill Blue. As this was of value to the restoration, all 500 square metres were translocated into the first restoration area using traditional hand methods.The main translocation of turf employed large-scale turfing equipment, using a technique known as "macroturfing", moving large, thick turvse. This method has many advantages over traditional turfing, virtually eliminating problems of frost and drought damage, and because the turves are thick most burrowing invertebrates and deep-rooted plants survive.

Seeding and Plug Plants:
The 260 kg of seed needed for the 6.5 ha to be sown could not be collected locally without causing unacceptable damage to both flora and fauna of the local source sites, most of which were SSSIs. Therefore the majority of seed mixes comprised commercial seed of known origin, virtually all species coming from south of England sources, and many from areas close to Hampshire. Where species were unavailable commercially of were in very short supply, seed was collected by hand from local downland and incorporated into the mixes.
The seeding was supplemented by 100 000 plug plants of eight species which were important butterfly food plants or were unlikely to grow well from seed. These were grown from seed or cuttings which were collected locally, many from St Catherine's Hill. (Back to Top|#top)

Downland Restoration Areas:
The timing of the main construction contract dictated when the areas destined for downland restoration would become available. Therefore it was necessary to divide the restoration work into three phases:

October 1992:
- Arethusa A - the southern section of the Arethusa Clump, some 1.5 ha of arable land

Winter 1993:
- Arethusa B - the rest of the Arethusa Clump, another 2 ha

Late 1994:
- the old Winchester Bypass (A33) would be infilled, landscaped and prepared for restoration, 3.5 ha of which would be to chalk grassland.
Both hand cutting and macroturfing methods were used to remove turf to the site, with the most species-rich turf being placed in the optimum positions and the less rich in less favourable areas. The areas not turfed were harrowed to form a seedbed and in March sown in three sections with three distinct seed mixes. A short turf mix for the area surrounding the species-rich turf, a mix for longer turf, and a mix of seed collected from local sites using suction equipment. After sowing the area was lightly rolled. Over the next 18 months some 31 000 pot plants of seven species of downland plants - Kidney Vetch, Horseshoe Vetch, Rockrose, Thyme, Clustered Bellflower, Cowslip and Hairy Violet - all except the last having been grown from local seed or cuttings, were planted into this area.
Some 200 Juniper plants were also planted in five rabbit- and stock-proof exclosures in order to add further diversity. Juniper is a typical downland plant which is declining in southern England.

Arethusa B:
The topsoil was stripped and removed as in the previous site. No turf was available, so the entire area was sown with a single mixture of seed. The bulk of this mix, 59 species in all, was seed of commercial origin, mixed with hand-collected seed of species otherwise unobtainable. To supplement to seeding 22 000 plug plants of six species of downland plants - Devil's-bit Scabious, Horseshoe Vetch, Cowslip, Hairy Violet, Clustered Bellflower and Rockrose - were planted. These were all grown from local source material.

The Restored A33:
The road was broken up and the old, overgrown cutting faces and backs were cleared of scrub. Chalk from the excavation of the northbound carriageway of the new cutting was used to fill and landscape the site. Care was taken to give a natural effect and recreate as closely as possible the original shape of the hill. The landscaping of the site was carried out from the bottom upward in order to preserve areas of potentially useful turf which lay at the top of one of the old cuttings. Topsoil was spread over the chalk, thinly in areas which were to become downland, but much more thickly where shrubs and trees were to be planted. All shrub and tree planting was protected by stock-proof exclosures. The slopes were seeded with a downland mix of 51 species prepared as before. However, an area at the foot of St Catherine's Hill and Plague Pit Valley which, due to its south-facing aspect, was potentially of special interest was sown with a richer, shorter turf mix of 37 species. In early May, some 44 500 plug plants were planted in appropriate areas, the same species used as on Arethusa A with the addition of Devil's-bit Scabious.
All the downland restoration areas were surrounded by rabbit- and stock-proof fencing. This was to protect not only the developing turf and invertebrate populations in the early stages of colonisation, but also grazing stock from the public and their dogs. As soon as the turf develops sufficiently, rabbits are allowed controlled access as they are seen as an important element in the management of downlands. The fences also give close control of the grazing animals, the most successful of which was a small flock of Shetland sheep. As soon as the turf on the restoration areas is considered to be robust enough, the dividing fences will be removed and the areas managed as an integral part of Hampshire Wildlife Trust's St Catherine's Hill reserve.
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Habitat Management:
Nearly all herb-rich calcareous grassland requires management if it is not to be taken over by coarser vegetation, scrub, and eventually by woodland. The nature and speed of this process - and hence the intensity and frequency of management required to counter it - vary greatly with the depth and fertility of the soil, and with topography and local climate.
Of the downland created following the construction of the M3, only the steep embankments of the new motorway are unlikely to need frequent management if they are to develop a wildlife interest. All the other grasslands will require annual management. To begin with, the restoration areas are considered individually while they are in a very early stage of development and require special treatment. However, as soon as is practical, the management of the reconstructed downlands will become an integral part of the management regime of the St Catherine's Hill SSSI.
Grazing is the traditional form of management on most chalk grassland and is the preferred method for St Catherine's Hill. Grazed swards usually support a greater diversity and abundance of invertebrates than mown hay meadows, and a different, though not necessarily richer, flora.
Mowing, however, is sometimes the only practical way of managing small isolated sites, or those where livestock are unsuitable, and it is far better to mow grasslands than to abandon them. On chalk, an annual cut in late summer is usually adequate to maintain the flora in the short term, but it is important that the cuttings are removed. In the longer term, rotational mowing will produce a more diverse grassland, supporting a wider range of invertebrates. Mowing is also the most satisfactory way of managing new grassland created from seed, for the first year or two after it is sown, as grazing by heavy livestock can damage the seedbed.
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Monitoring:
It has been a major criticism of most previous attempts at habitat translocation and restoration that there has been little or no follow up, to ascertain the "success" of the work. This means that little has been learnt about the processes involved or what can be done to improve methods for use in the future. However, at Twyford Down the Highways Agency funded botanical and invertebrate monitoring for a period of ten years. The progress of all the reconstructed downland areas and the translocated flood meadows has enabled assessment of the success of the work and to give feedback to "fine-tune" the management works.
Preliminary results suggest that the downlnand turf translocations have been very successful and that the development of the turf from seed has worked well. The development of the invertebrate community is also following expected trends.
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Progress as recorded in the late 1990's:
The three areas of downland restoration, though in different stages of colonisation, are progressing well. On Arethusa A, the first area to be completed, the turf translocations have been very successful so far. In the machine-translocated turf, the average number of downland species per plot dropped very slightly but, as fluctuations are expected, this is not regarded as significant. Orchids are still present and flowering. Even in the hand-translocated turf virtually all the downland species have been maintained, with only a small turnover of gains and losses. In all cases most of the species lost are weeds which one would expect to die out fairly quickly. The three seed mixes are showing a steady improvement and are reaching a stage when nearly all the sown species are represented, providing a fine show of flowers in summer. As expected, weed species were numerous in the first years but are now dying out and the downland species are becoming dominant.
The sown grassland on Arethusa B is maturing more slowly than A with fewer coarse grasses. Although the colonisation is slower it is forming an open-ground, short, slow-growing grassland with an excellent representation of downland species. Much the same picture can be seen on the A33 restoration, although this is a further year behind in growth. The sward is developing well despite the newly germinated plants having to survive very long, hot summer of 1995.
The invertebrate populations are also developing as expected. The initial large numbers of early invasive species are decreasing, to be replaced by the true downland species. Several deep-burrowing species such as the purse web spider successfully moved with the translocated turf, as did many ants nests.
At the Public Inquiry, fears were expressed about the future of the Chalkhill Blue butterfly on the Dongas following the construction of the road. In fact there is no evidence that the population of this species on the Dongas is declining; actually in 1995 it increased, following the national trend. To this can be added a flourishing new population on the Arethusa A area and both the Arethusa B and A33 restoration sites are being colonised. All the colonial species of butterfly that inhabited St Catherine's Hill and the Dongas SSSIs now have discrete additional populations established on the Arethusa Clump restoration area, many at a higher density than on the rest of the reserve.

Sources: Ecology and Twyford Down published by the Institute of Terrestial Ecology R.G.Snazell(1997)
The Butterfly Handbook - English nature 2005
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Page last modified on Monday 10 of December, 2012 13:25:09 GMT