Field Event Reports

Tuesday 25 April: Nettlebed Common

Leader: Rod D'Ayala (Environmental Consultant)

The group enjoyed a fascination tour of the area

A group of 12 AbNats members and visitors met near the Old Kiln, Nettlebed on a bright morning, but the temperature had retreated from Spring to Winter. Here we were met by our leader for the day, Rod D'Ayala, environmental consultant for the site. Rod first introduced us to the site specifically and to the Chiltern Commons in general. In summary:

The long history of clay extraction for brick- and tile-making at Nettlebed has created a unique habitat. It is usually accepted that there are around 100 ponds on the site, although Rod reckoned there were up to 150, and it was clear from later that the activities of Rod and his volunteers were regularly creating new ones. Most of the ponds are the left-over flooded excavations from the quarrying, although some had been a source of water for local residents. The clay base of the ponds is impermeable, so water collects, a unique feature in the Chilterns where rain normally drains away quickly into the chalk. The acidic nature of the water makes it nutrient poor, again unique features for the this part of the Chilterns.

The party then took a short walk into the woods to reach the first ponds. It is worth pointing out that the definition of 'pond' to the layman was very broad. Some were typically what might be said to be 'village' pond size, a few larger, but very many were small scrapes, sometimes no more than a few metres square, very shallow and quite often ephemeral. Some of the 'larger' ponds were the spawning sites to five species of amphibian (European Frog, Toad, Smooth, Great-crested and Palmate Newts), but the population had declined and some, notably the large pond by the cricket pitch, were virtually devoid of life - more of that later. One problem were the Mallards that ate the spawn, and although the duck population is declining we still saw a number including ducklings.

It is not possible here to talk too specifically about particular ponds or complexes as Rod's descriptions were highly informative and detailed. While Rod had worked on the site to a lesser extent for several decades it has only been in the past three years that resources, funding and his time has enabled considerable restoration work to be undertaken, as the scrubby woodland had virtually swamped the area - just natural succession. A key boost to his work had been the use of chain saws and a big digger to undertake heavy work. Essentially many of the trees had been removed, bunds had been constructed to divert and control water flow and levels, and numerous new small ponds and scrapes created. The opened up habitat benefited the wetland vegetation. Clearly Rod's philosophy was that rules are there to be tested and broken and new ideas be experimented with- like his trial to see if he could create peat by swamping dead vegetation in one of the ponds. Local volunteer work parties are a critical part of the restoration work, for example the removal of the aggressive Pendulous Sedge that had invaded some areas.

We were joined on the walk by local wetland expert Judy Webb who was testing the water quality in many of the ponds for nutrients (Nitrates and Phosphates) - this added an extra dimension to our walk as several attendees had not seen before the simple technique used for this testing. As expected every sample came back nutrient poor, except one. This shallow scrape was the only one in which filamentous algae were growing and had a high phosphate content - probably the result of leakage from the drainage of the house just above it on a ridge.

Having spent considerable time at the first main area of ponds and the location of Rod's recent restoration work, the party then walked through another area of wood (mostly mature beech and oaks) that was devoid of ponds until we reached a single large pond that was going to be the next area for Rod's attention. While once having been rich in life it was now effectively 'dead', which needed to be investigated.

We then progressed to another completely different and unique area of major geological significance - Priest Hill, an Area of Special Scientific Interest (SSSI). This small area of some one hectare was mainly covered in bracken, heather and gorse and is the home of Common Lizards, Grass Snakes and Slow worms. It's unique character is created by the underlying acid gravels (Nettlebed Gravels), which date from the Pleistocene Ice Ages and are thus some of the oldest Thames deposits known.

Our final walk brought us out onto Nettlebed Cricket pitch, where we stopped for another chat - perhaps some wondered why as there was not a pond in sight, just a pristine 'lawn'. The point Rod explained was that a large pond (some might say a small lake) that was hidden from sight by trees on one side of the pitch, had once been a thriving site for amphibians and covered in water lilies, but in very recent years had become effectively dead. There was speculation that some seepage of grass treatments on the pitch into a drain that entered the pond was the cause. Our last sight of water was this pond, whereupon we walked back to the cars. Thanks were conveyed to Rod for his highly informative and at times thought-provoking visit, always conveyed with a unique sense of humour. Not the least we all also had a splendid nearly three-hour walk through wonderful spring woods, frequently surrounded by a carpet of bluebells.

Report: Graham Bateman

Attendees:
Graham Bateman, Felicity Jenkins, Sally Ainslie, Chris and Bridget Biggs, Elizabeth Henderson, John Killick, Tony Rayner, Richard Lewington, Susan Bowditch, Jackie Forty, Judy Webb (water quality survey)