Case study: Reversion of arable land to lowland chalk grassland

Author: Lynne Roberts:  The RSPB’s Manor Farm, Newton Tony, Wiltshire

Aims and setting

Chalk grassland is one of the richest landscapes for wildlife in the UK and is a habitat which has suffered significant losses in past decades. This case study focuses on Manor Farm in Wiltshire, now the operational base of RSPB’s Winterbourne Downs nature reserve, which was acquired by the RSPB in 2005 with the overall objective of creating a habitat 'stepping-stone' for wildlife between two internationally important chalk grassland areas: Salisbury Plain and Porton Down.

To enable wildlife species to thrive on the farm and expand their range from the neighbouring chalk grassland areas, much of the arable farmland was reverted to lowland calcareous grassland, a UK priority Biodiversity Action Plan (BAP) habitat. An initial reversion of about 40ha in 2006 has been followed by further grassland creation projects at the rate of around 25ha per year, and now almost 200ha of the total farm area of 296ha has been reverted to semi-natural chalk grassland.

Agri-environmental scheme grants from Natural England and capital grants from the Heritage Lottery Fund were major contributors to the reversion works.
This case study describes in detail the reversion of a 24ha area of arable land, started in 2008.

Site preparation and sowing

In order to select the most conducive sites for reversion, soil samples from across the farm were analysed. It is recognised that land with low nutrient levels, particularly available phosphorous, is the most suitable for creating and restoring species-rich grassland - the best sites will have a phosphate index of between 0 and 1. The results of this showed much of the farm to have high soil phosphate levels and two fields totalling 24ha with a phosphate index of around 2 were selected.

Machine-sowing of harvested seed was chosen as the most suitable method of starting off the reversion process. Natural re-colonisation was thought to be unlikely to produce the right levels of target species, since this requires plentiful supplies of nearby seed - something which could not be guaranteed, especially over such a large area.

A mixture of brush-harvested wildflower and grass seed was sourced from four local downland sites, including the Salisbury Plain Training Area, where there are several thousand hectares of good quality calcareous grassland. Using locally sourced seed increases the chances that the plants will thrive, as they will be better adapted to the local conditions.

After harvesting the last arable crop from the selected fields, a sterile seed bed was created by spraying with glyphosate and cultivating with a spring-tine cultivator to provide a good tilth. A few weeks later, in mid-September, the wildflower/grass seed mix was sown using a pneumatic fertiliser spreader, which blows out the seed from tubes spaced 1m apart along an 18m boom width.

Seed Sowing

The seed was mixed with kiln-dried sand to improve the flow of seed through the spreader and sown at a rate of 20kg per hectare. Four complete passes of the spreader were made to ensure good distribution of the seed. The fields were then rolled with a Cambridge roller to ensure good contact with the soil.  The approximate cost for this (in 2008) was just under £900 per hectare, split between herbicide (17%), seed (76%), dried sand (2%) and labour and diesel (5%).

Early establishment and management

In the first year, the sites looked very patchy, although many chalk grassland species were showing by midsummer, such as salad burnet and small scabious. The fields were topped once in early May, timing the cut to catch most of the black grass and other unwanted plants such as thistles and docks before seed set.

Patchy coverage in the first spring after autumn sowing

An early cut avoids topping more than 25% of the yellow rattle, which is a key species in the arable reversion process. Yellow rattle is semi-parasitic on grasses and clovers, suppressing grass growth and increasing the light available for other wildflowers to prosper.

The side shoots from the yellow rattle and the other chalk grassland species were then allowed to flower and set seed and a further cut was made in July. The arisings from this July cut were collected and removed to prevent mulching and help drive down the nutrient levels, allowing the flowering species to compete effectively with the grasses.

Yellow rattle helps to keep the sward open

Sheep were introduced in the Autumn/Winter to create a short open sward with occasional tussocks, which created some habitat diversity in an otherwise flat field. In the following two years, hay crops were taken to further reduce soil nutrients levels, with aftermath sheep grazing from autumn to early spring. Supplementary feeding was avoided to prevent the import of nutrients.

Assessing for habitat quality

By the third year after sowing the plant community was typical of early grassland creation, with a large diversity of species present - 37 species were recorded inside the assessment quadrats, with a further 21 species elsewhere in the sward. These included species which are characteristic of chalk grassland and typical of nutrient-poor conditions. Analysis of the species makeup showed that there were five 'dominant' or 'abundant' wildflower species: yellow rattle, ribwort plantain, black medick, self-heal and white clover. Of those recorded at lower frequencies, several were chalk indicator species, including harebell, perforate St. John's wort and field scabious, as well as those listed in the Natural England ‘species indicators’ table below. The woodier perennials like greater knapweed, oxeye daisy and wild marjoram were establishing well.

Of the typical grasses present, sheep's fescue was dominant, with quaking grass, upright brome, yellow oat-grass, meadow oat-grass and common bent also recorded.

Year three - a herb-rich sward is establishing

The following table shows the criteria for success set out by Natural England for this type of habitat and how the reversion fields measured up against these criteria:

The importance of soil fungi and organic matter

We found that some species were slower to establish than others. This is likely to be due to the abundance and diversity of mycorrhizal fungi present in the soil. Intensive arable agriculture reduces this diversity, and some plant species will not be able to exist on the land until the correct fungi arrive. It wasn’t until about four years after the start of the reversion that the first signs of fungal colonisation, the ‘fairy rings’, could be seen and species such as dropwort and devil’s-bit scabious began to appear. It was more than five years after sowing before any orchids were spotted, which is likely to be an indicator of the developing populations of mycorrhizal fungi.

Some good indicator species like saw-wort, have still not been recorded, although we know the seed was sown. This could be due to the soil conditions or plant associations not yet being right for them to get established.

Fairy rings show signs of mycorrhizal fungi

The levels of organic matter in the soil were extremely low to begin with, at around 2%, which is consistent with the land having been in long-term continuous arable cultivation. The establishment of permanent grassland will cause the organic matter levels to increase over time, especially where the land is grazed. The soil analysis results for 2017 showed that the levels in the early reversions had generally gone up to around 5-9%. The increase in soil organic matter goes hand in hand with bacterial and fungal diversity and with the locking up of carbon, making semi-natural grasslands like chalk grasslands in our landscapes a natural way of helping to tackle climate change.

Achievements for fauna

We saw an amazing transformation from cultivated land to herb-rich sward by year three, and even by the second summer after sowing there was visibly and audibly more insect life in the new grasslands. The most obvious insects are of course the butterflies, and from the second year we saw meadow brown, common blue, small blue, dark green fritillary , marbled white, ringlet and small skipper.

Other insects, such as moths, beetles, bees and flies had also colonised the grasslands, although no formal assessment was made in the early years to compare with the baseline survey taken at the start of the reversion process. However, in an invertebrate survey undertaken in 2016, one new BAP Priority species (the bumblebee Bombus ruderatus) and eight new Nationally Scarce species were recorded, including the six-belted clearwing moth and the picture-winged fly Merzomyia westermanni.

When farmland is taken out of cultivation there is a risk that some bird species associated with arable cropping might decline, so it was important to ensure that the key requirements of these species were maintained. To help ensure seed food availability for farmland birds on the farm, some strips of wild bird seed mixes were sown alongside natural features such as hedges. Farmland birds are regularly seen in the floriferous arable margins and the bird seed plots, and whilst the figures from the Common Bird Census recording took a little while to show real signs of benefit, the ongoing provision of suitable habitat has reaped rewards. Local populations of farmland birds such as skylark, yellowhammer, linnet, lapwing and corn bunting, all of which are currently species on the Red List of Conservation Concern, have either increased or been maintained, as the chart below shows.

Lessons learnt

We have learnt a couple of important technical lessons regarding nutrient levels and seed sowing as a result of our reversion projects.

  1. Where we started the reversion process with a phosphate index of around 2, it sometimes produced a thick grass-dominated sward very quickly, crowding out some of the sown wildflowers and emphasising the need for a lower initial phosphate index. Although haymaking can be used as a way of reducing nutrient levels in the soil once the wildflower seed mix has been sown, we have found that growing barley for a couple of years without a compound fertiliser prior to starting the reversion is a more cost-effective way of reducing the levels of key nutrients, especially available phosphorous. These crops are best grown with the application of ammonium nitrate fertiliser; this helps the barley pull up the phosphates from the soil without contributing to the nutrient load, as nitrates will leach readily from chalk soils.

 

  1. We did not mix the seed with any other agent at first and found that the seed clumped together in the machinery (termed 'bridging'). Mixing the seed with the kiln-dried sand improved the flow, but we have found in other reversion projects that it is even better to use a fertiliser spinner, which broadcasts the seed through a wider aperture.

 

All photos supplied by Patrick Cashman, site manager RSPB Winterbourne Downs

For more information, contact: patrick.cashman@rspb.org.uk

 

 

Case Study: Adding Value to Chalk Grasslands: Creating Chalk Banks to benefit butterflies and other insects.

Author: Lynne Roberts . Farm: The RSPB’s Manor Farm, Newton Tony, Wiltshire

Aims and setting:

Manor Farm is a 296 ha working farm strategically placed between two of the largest tracts of semi-natural chalk grassland in the British Isles – Salisbury Plain and Porton Down. The RSPB purchased Manor farm in 2006 and have been reverting former arable land back to species-rich chalk grassland to create a landscape-scale stepping stone between these two areas.

Whilst this reversion has been very successful in establishing flower-rich grassland, even after several years the ex-arable fields still have unsuitable soil conditions for the plant species typical of very thin, nutrient-poor chalky soils. The fields are also rather flat as a result of historical cultivation, lacking the humps and hollows of natural grassland which help to create a variety of microclimates for a wider range of plants and insects.

For these reasons, in 2013 it was decided to create two large mostly south-facing banks to support viable populations of species characteristic of thin, chalky soils. Butterfly banks and scrapes can provide ideal conditions for butterflies, with sheltered sunny spots and bare ground for basking, alongside a mosaic of nectar-rich flowers for foraging adults and specific foodplants for growing caterpillars.

Funding for the creation of the banks was provided by the SITA Trust (now SUEZ) “Conservation of the small blue butterfly at RSPB Winterbourne Downs” project, and the Biffa Award “Saving a special place for Wiltshire’s endangered butterflies” project. Appropriate permissions for the work were also obtained.

Construction:

The site for the butterfly banks was flat terrain with a 20cm layer of flinty loam over chalk. Two large banks measuring 150m x 12m and 180m x 12m were constructed in two different fields. The loam topsoil was bulldozed into an S-shaped mound, oriented so that most of its length faced south. The mound was then covered with the chalk from the beneath the soil, leaving a wide 'scrape' area of bare chalk alongside.

The S-shape ensures that there are sides facing all possible directions, creating a variation in topography and diverse range of micro-climates. The idea is that this helps to increase the resilience of both plants and insects to the more extreme conditions predicted as a result of climate change. In the mornings, butterflies, bees and other warmth-loving insects can warm up on the east-facing side of the banks, which catches the early morning sun. During the main part of the day, they can use the warm, sunny, south-facing side. If it gets too hot on the south side, the insects can retreat to the north-facing side. As expected, during the middle part of the hottest July day on record in England in 2015, all the bee activity was on the banks’ north-facing slopes.

Construction begins

The first bank was initially seeded with a generic chalk grassland and wildflower mix as well as kidney vetch and horseshoe vetch, which are vital food plants for small blue, chalkhill blue and Adonis blue butterflies. On the second bank, only the kidney vetch and horseshoe vetch were introduced. The banks could have been left to regenerate naturally, but the butterfly foodplants were not growing in the vicinity and therefore seeding with specific species was preferred to ensure that the right plants established.

Development over time and ongoing management

The vetches were the first plants to establish and were flowering in the first summer after autumn sowing. Over the following two years other species, such as small scabious, ribwort plantain, harebell and quaking grass appeared, the coverage developing into a patchwork of abundant flowering species interspersed with bare areas - the ideal structure for butterfly habitat.

Vetches establish quickly on the bare chalk

The vegetation on the banks has become denser over time but has required little ongoing management as the tough conditions prevent many unwanted species from colonising. Sheep are our management tools, grazing from August onwards, when most flowering is over. This helps us to keep on top of any scrub encroachment and keeps the sward open, with some disturbance of the ground surface to create germination opportunities. Cattle would probably cause damage to the banks, so are avoided.

Where necessary, brush cutting is carried out in the summer and the arisings removed and taken to other areas of the farm which are species-poor and could benefit from wildflower sowing.

Brush cutting the coarser vegetation

Achievements:

Colonisation of the banks by breeding small blue butterflies was impressively quick - within the first three years, as the kidney vetch established readily. The abundance of suitable foodplants is particularly important for sustaining viable populations of butterflies, so the fact that small blues were seen on the banks in August, probably a second brood from eggs laid in June, was a good indication that suitable habitat for breeding had been achieved.

Small blue on its larval foodplant, kidney vetch

Other chalk grassland butterflies have also been recorded on the banks in the last few years: common blue, brown argus, marbled white, dark green fritillary and, excitingly, the marsh fritillary, a species in severe decline which has been attracted to Manor Farm by the abundance of scabious plants in the grassland. Good numbers of these species are now being seen on the farm as they are moving in from the neighbouring strongholds on Salisbury Plain and Porton Down. We have yet to record chalkhill blue, but the Adonis blue was a new record in 2018, so hopefully it will just be a matter of time before the chalkhills arrive.

The diversity of flora species continues to develop, with some of the less common chalk specialists such as devil’s-bit scabious, starting to appear on the banks.

In addition, we have shown that the fields containing the banks can still be used as grazing land, albeit in a carefully managed way.

Our experience and lessons learned:

Five years on we found that some plants had been harder to establish on the banks than others. The pioneer species kidney vetch and horseshoe vetch, two of our key butterfly food plants, established readily from seed and plug plants were not required. However, we didn’t have the same success with common rock rose, the food plant of the brown argus and cistus forester moth. This was easier to establish from plugs, although the sown seed may germinate eventually when conditions are favourable and the seed coat has weathered a little. Plugs are more expensive, so a pragmatic approach may be to try sowing seed first and then supplementing with a few plugs after a couple of years if the seed has not germinated. Germination rates of common rock rose can be improved by scratching the seed surface (scarifying) prior to sowing.

Where we sowed the generic mix, some of the taller plants have become dominant, greater knapweed particularly, shading out some of the foodplants, such as horseshoe vetch. We have therefore had to manage some of the less desirable species in order to help the key plants thrive.  This was not such issue on the second bank, where we had just sown the foodplants, with just the odd thistle to deal with.

We suspect that by creating the base of the banks from the flinty loam topsoil, the longer rooted chalk grassland species may be accessing nutrients from this base and gaining an advantage. Ideally, the bank would be created from pure chalk and the removed soil utilised elsewhere. A chalk-only bank would create the harsher alkaline conditions which favour the more extreme chalk specialists, including the kidney vetch and horseshoe vetch, and would slow the rate of encroachment by scrub species.

Although our butterfly banks are very large it is important to note that banks of any size can make a big difference to the survival of butterfly populations - even just moving soil around to create variation in the topography of reverted arable fields or pasture can help. Butterflies exist in small populations which are linked to create larger ‘metapopulations’, so even quite small areas of suitable habitat could provide a vital link in the chain.

All photos supplied by Patrick Cashman.   
For more information, contact: patrick.cashman@rspb.org.uk

Further examples of butterfly banks

Organisations such as Butterfly Conservation and Buglife have been constructing butterfly banks on several sites, both rural and urban, as a way of creating breeding habitat suitable for a number of different butterfly species. For further information see the following links:

Butterfly Conservation Fact Sheets

Case Study: Bare Ground for butterflies and moths

Case Study: Using hay strewing as a technique to create species-rich grassland

Author: Jennifer Palmer

Farm: High Burnham Farm, Epworth

Aims

High Burnham is a large (+300ha) arable farm.  As part of the RSPB’s Axholme and Idle Farmland Bird Initiative¹ (Lincolnshire), an opportunity was identified to revert an arable field corner to a species-rich meadow.  The 1.7ha field corner sits within the base of a large L-shaped arable field.  Because of the clay-based soil type, the field corner lay wet so was deemed unsuitable for arable cropping hence it was left out of production for four years.

The low-lying field corner lends itself to a pastoral management and is less than 200m from Rush Furlong Meadow SSSI.

This will be the only parcel on the holding managed as grassland.  It is anticipated that hay will be cut by a local contractor used by the Lincs Wildlife Trust and aftermath grazing will be carried out by a local grazier.

Management

Verbal advice and a written proposal were provided to the landowners on species-rich grassland establishment and management.  The RSPB’s Hay Meadow and Arable Reversion topic sheets were used to supplement this verbal and written advice.  The landowners understood the principles of grassland management through knowledge of a local grassland SSSI.

Hay strewing is a tried-and-tested method for enhancing the botanical diversity of species-poor grassland² and can also be used to create diverse grasslands on arable land³.  It entails taking freshly cut ‘green hay’ from a local donor site and, on the same day, strewing (spreading) it onto a suitable receptor site.  It is a cost-effective method and ensures that the received seeds are of local provenance.

Two donor sites were identified and Lincolnshire Wildlife Trust kindly donated and cut the hay from Sedge Hole Close, a damp meadow (MG4 National Vegetation Community) containing cowslip, great burnet, lesser knapweed, oxeye daisy and cuckooflower.  Natural England consent would have been required for using SSSI hay.

Loading a trailer with green hay at East Lound with Matt Cox Lincs Wildlife Trust

Technical advice was followed to ensure the receptor site’s soil was suitable, through testing phosphorus (P) levels.  The soil sample results showed a P Index of 1 (low) so was deemed acceptable.  The farmer prepared the site by spraying off weeds using herbicide and creating a create a fine, firm and level seedbed, avoiding looseness at depth.

The site has no historical significance.

Because the donor site is an NVC MG4 vegetation community, containing abundant great burnet, we followed advice from a floodplain grazing meadow conference (attended by Helen Norford of Yorkshire Wildlife Trust) to spread the hay at reasonable depth (up to 10cm).

Once strewn, in the first year the vegetation growth should be cut up to 4 times and then the grassland should be treated as a traditional hay meadow thereafter.

Because the donor site was smaller than the receptor site (0.9 and 1.7ha respectively) we found that we had a deficit of green hay for the receptor field and a ratio of 1:1 (as recommended by Dr Duncan Westbury) would have worked better.  Partners therefore plan to revisit, survey and repeat if necessary next year.

Black grass growth will also be re-sprayed off this year.

The cost of the green hay was free as it was donated by Lincolnshire Wildlife Trust.

There was a minimal fuel and labour cost incurred by the farmer transporting the two loads of green hay from the donor site to the receptor site.

Achievements

Breeding skylarks have been recorded in adjacent fields during RSPB bird surveys and breeding skylarks are also recorded at Rush Furlong SSSI so the parcels should attract skylarks.  The parcel also offers potential lapwing nesting habitat, providing the additional scrape excavation works are undertaken. There are records of yellowhammers in the hedges and reed buntings nesting in the adjacent OSR crop.

Sitting within the Humberhead Levels NCA, the project meets multiple NCA priorities - the creation of lowland meadow (biodiversity priority) and permanent grassland (landscape priority).

Advice for other farmers

Don’t be tempted to miss out the soil testing step! If phosphorous index is anything above low, species-rich grassland creation may not be viable for your site at this moment in time.

Ground preparation is really important and if strewing onto established grassland, really open up the sward so that lots of bare ground is showing.

Orchids may take several years before they appear so don’t be disheartened if nothing happens for the first few years.

Expect to have to consider repeating the method to ensure a diverse sward.

Additional information

¹ The Axholme and Idle Farmland Bird Initiative covered the river catchment area in the Idle Valley and Isle of Axholme, an area of Lincolnshire, South Yorkshire and Nottinghamshire recognised as being nationally important for its farmland bird assemblage.  The project area was one of the RSPB’s Farm Advice Focus Areas and ran from 2012 to 2018 combining farmland bird monitoring and farm conservation advice.
² Natural England Technical Information Note TIN063, Sward enhancement: diversifying grassland by spreading species-rich green hay.  Also through own experience on land owned by the Malvern Hills Trust, following advice from Dr Duncan Westbury of Worcester University.
³ Visit to arable reversion hay strewing site led by Professor Ian Trueman, 15-18 June 2012, FSC Shropshire Wildflower Weekend.
⁴ Natural England Technical Information Notes: TIN035, Soil sampling for habitat recreation and restoration and TIN036, Soil and agri-environment schemes: interpretation of soil analysis.