Cors Geirch: Fen on the LLyn

The third and final day of the LIFE conference took us on a meandering bus trip to the sleepy Llyn peninsular fen site Cors Geirch! 

Geological origins

Cors Geirch is the largest of the LLyn peninsular fen SAC sites and is located between Nefyn and Pwlleli in North Wales. In contrast to the Anglesey fen sites visited previously, groundwater discharge from  a calcareous post-glacial retreat sand and gravel deposit springs and seepage lines feed the fen and not limestone influenced spring-water experienced at the Anglesey sites. Alike the Anglesey sites, however it is formed within a broad, shallow valley head fen system set within intensely farmed agricultural catchment.  The whole site has undergone partial drainage in the past leaving some parts of the peatland to be classed as mesotrophic grassland. 

 Cord Geirch in Red ion the map

 

The constructed wetland approach to nutrient management

Cors Geirch has acted as somewhat of a reference site for calcareous and alkaline fen restoration in Wales. In 1993, some 30 cm of enriched rush dominated topsoil peat was stripped from 5.5 ha of the reserve and used to landscape a nearby landfill site, leaving a bare peat scrape in its place. The bare scrape was re-colonised successfully by fen species with the help of some planting, hay spreading and re-connection of fen to spingwater. The success of the is project is shown by the enhancement in biodiversity  and specifically species richness of the site which jumped form 11 in 1993 to over 100 in 1999. The LIFE project replicated this 1993 management model on another part of Gors Geirch fen recently, creating a new peat scrape and reconnecting the land with diverted spring-water.

The old scrape, restored in 1993, this is what the bare peat scrapes below will hopefully look like after successional development.

          The new scrape constructed in 2011

              New scrape from the car park

One of the major conservation issues not previously addressed at this site was enrichment from nutrient run-off from adjacent dairy farm upland. This problem is now being addressed by the installation of a constructed wetland to treat and reduce nutrient loading into the fen system but save the calcium rich water from the sand and gravel deposits and direct the 4 spring fed steams previously intercepted by drainage ditches onto the fen. Unlike the constructed wetlands installed at Cors Erddreiniog and Cors Bodeilio based on a clay substrate, a membrane was used to line the depression to avoid water loss through the permeable peat. 

The constructed wetland with a bunded feeder drain at the inlet, the spring water emerges from the wooded upland area in the background. 

It is hoped that the wooded area will be converted to peatland as arial photography has shown that this was previously covered by wetland vegetation as far back as 1945. 

A peat dammed drainage ditch was used as a preferred site for the constructed wetland here, the ditch was transporting the spring water away from the fen via a main drainage channel. A peat bund feeder drain was created to raise the water levels and control the water flow through the constructed wetland to ensure no flooding of the surrounding area. The constructed wetland seepage face outlet was designed with digital levelling lazer technology to create a shallow gradient of 0.5 degree so the water could gently ooze over the fen. Water levels are being measured across the site by dipwells, this is important as raising the water levels in one area can affect the water flow in another area. The wetland has been designed so that if there are major pollution incidents from slurry spills there is a valve which can re-direct water into the main drain or an overspill drain and out of the fen.

This was the overflow channel leading from the constructed wetland to the car park

According to the site manager, had the spring-water not been plumbed into the constructed wetland whilst the vegetaton was going through successional development, some of the early nutrient loading could have been avoided. It is possible that as a result of this an aggressive species (flote-grass) has started to establish on the scrape, however this is being monitored. The fen is being lightly grazed in more established such as the 1993 scrape by Welsh mountain ponies.

Grazing and vegetation adaptations

The fen is lightly grazed by Welsh mountain ponies in the more established area's such as the old scrape.  Such grazing will help to control scrub encroaching onto the fen, this is coupled with strimming in patches where very rare species occur. Ponies will create a diverse range of micro-habitats of short ‘lawn” grass, longer untouched grass and bare ground for colonisation by pioneer species. 

      Ponies grazing the drier  mesotrophic.             grassland near the old part of the site.

Due to waterlogged conditions, plants contain specially adapted systems such as called aerenchyma cells (air spaces) in their roots and stems which enable oxygen diffusion to the root zone as wetland plants struggle to obtain oxygen form the soil.