Field Event Reports

29th April: Lye Valley Nature Reserve, Oxford

Board walks provided good access tthrough the fen area.

Leaders: Judy Webb and Rod D'Ayala

A group of nine members gathered at the entrance off the Slade to the Lye Valley Reserve where we were met by our primary host Judy Webb. While the weather was initially dull and cool, steady rain soon ensued and continued throughout our visit – a shame as this was the first significant rain for several weeks.

Judy started by explaining the the history and geology of the site and the general restoration measures being undertaken and planned. Lye Valley Reserve is a Local Nature Reserve (LNR) known as the North Fen with the central section an SSSI (Site of Special Scientific Interest - of national importance).

Habitats at present range from dry limestone flowery banks to bramble, hawthorn and blackthorn scrub, woodland and most importantly, wet spring-fed calcium-rich alkaline fen areas (now the rarest habitat in all England). This last habitat is also extremely rare in all Europe. Small ponds and shallow pools of all sizes are within the fen area. The last four years have seen big changes with habitat restoration stimulated by the BBOWT Wild Oxford Project, working in association with landowners Oxford City Council & Friends of Lye Valley.

Our first stop was at the The Slade Entrance. A few years ago this dry limestone soil entrance was dominated by nettle, bramble, scrub, mile-a-minute climber and horseradish. Three years of work from the Friends Group on removal of all this and re-seeding with native local limestone-loving wildflowers have resulted in the attractive flower mix favoured by a variety of pollinators. One cherry plum bush had been left because this is favoured for egg-laying by the rare brown hairstreak butterfly.

Using the boardwalk that extends down through the site, we entered the SSSI Fen Areas biodiversity hot spots of international importance. Here we frequently stopped for descriptions of the habitat and key plants of special interest. On the valley sides all the way down the Fen is a spring line at the junction of the porous limestone above and the Oxford Clay below. Alkaline spring water high in calcium has been emerging for thousands of years and in places 1m depth of peat and tufa (lime) has built up produced by the continuous flow. Originally grazing by horses maintained the biodiverse short flowery fen, but when this stopped 100 years ago reed, scrub and trees took over. Now the reed is controlled by cutting and raking several times a year by the council and volunteers. The SSSI has 22 plant species rare in Oxfordshire including three species of orchids, the most important being the large population of marsh helleborines found in only two other Oxon fen sites. A vast diversity of insects breed in the fen, with many rare species, especially the spectacular soldier flies with their bright patterns like uniforms.

At the south end of the SSSI Fen are a series of larger ponds, which were dug in the late 1980s. However, they became heavily invaded by reed, effectively disappearing. Work by the Freshwater Habitats Trust has enabled the removal of reed and choking vegetation from five of them resulting in open water again. Recent volunteer work has involved removing shading scrub and trees and cutting the surrounding vegetation short. Drainage pipes have been removed and dams all along the boardwalk enable the ponds to retain a healthy high water level. They are now home to a large population of frogs, smooth newts, toads and are a great place to see damselflies dragonflies and even a kingfisher!

At this point we met Rod 'the Beaver' D'Ayala, renowned for his expertise in controlling and modifying water flow in such habitats. He explained how they had introduced numerous dams and bank reinforcements along the Lye Brook, which controlled the flow, creating slower moving areas that reduced erosion of the banks, in particular at times of high flow, and critically raising water level, rewetting the fen. Dams were made of logs and sods of vegetation, while bank shuttering comprised woven twigs and thin branches, all recovered from the site.

We then walked out of the main site through a wooded area, still beside the Lye, then out onto a rough meadow, land owned by the Churchill Hospital. What immediately stood out were numerous large mounds of rocks, branches and earth. Judy explained that these had been put in as refugia for a colony of mainly slow worms, but some grass snakes and common lizards, that had been translocated here from another site that had been redeveloped. There was some doubt that introducing some 500 extra slow worms had overloaded the already stable population.

In summary, the problems that face the Lye Valley Reserve mostly resulting in the drying out of the fen included:

  • Flash flooding from storm road run off drains, which produced fen peat bank erosion. As well as the damming and bank reinforcement already mentioned there is a plan to install a limestone leaky bund across the brook at the valley head to stop flash flooding and recreate natural shallow stream.
  • Trees shading and sucking water from fen, which are being progressively removed from the spring-line.
  • Reduced flow of the springs due to catchment loss and climate change.
  • Water pollution of springs possibly from leaky sewers in nearby housing.
  • Habitat isolation – the site being too small to support species populations.
  • Plant thugs. There is a continual battle to hold back and remove dominant plant species that threaten to overrun sensitive areas, including field horsetail, marsh thistle, Japanese knotweed and pendulous sedge.
    Future projects and plans include:
  • Building more small natural log dams in the brook.
  • Continuing restoration of eroded fen along the banks of the brook.
  • Creating more small shallow warm pools for fen invertebrates.
  • Extending the boardwalk and fen restoration.
  • Reducing isolation by restoring pockets of fen between two SSSIs (north and south fen).
  • Improving footpath access down from Peat Moors and top path, Churchill side.

Finally we retraced our steps to the Slade entrance and a rather sodden group departed having given sincere thanks to Judy for an interesting and stimulating visit that had given us considerable insight into the management and restoration of the site, not the least the problems and issues being continually faced.

Much more detail is available on the Friends Group website at

Report: Graham Bateman (heavily based on notes provided by Judy Webb)

Sally Ainslie, Sally Gillard, Margaret Abel, David Guyoncourt, John Killick, Julian Annells, Jackie Hudson, Michael Booth, Graham Bateman