More on Purple Moor Grass – Molinia and water levels

In my blog yesterday I wrote about the the rise in abundance of purple moor grass (Molinia caerulea) on Dartmoor, the challenges of suppressing its dominance along with the role that aerial pollution may be playing in encouraging it – see here. Today I am looking at the performance of Molinia in situations where the water table has been deliberately raised so as to restore and rewet degraded peatlands.

Molinia ‘is most abundant and grows most vigorously on sites where there is ground water movement, good soil aeration and an enriched nutrient supply’ according to Taylor et al (2001). One might suspect therefore that on sites where the water table is high – reaching the surface, and constant Molinia might be at a competitive disadvantage to other wet loving species.

Peat

Over the centuries many peatlands in the UK have been drained either for agricultural purposes or for peat cutting. Today conservation organisations are attempting to restore many of the remaining drained fragments of these original peatlands. Thomas (2015) described such an approach in the Manchester Mosses Special Area of Conservation (SAC). It should be noted that the Manchester Mosses are lowland raised bogs and not upland blanket bogs such as we get on Dartmoor. Nevertheless these remaining fragments of peat had been drained and were dominated by Molinia. The restoration work carried out  on a variety of reserves within the Manchester Mosses SAC was carried out by the Cheshire Wildlife Trust, the Lancashire Wildlife Trust and Warrington Borough Council. At all three sites it  became impossible to maintain raised water levels as the adjacent land had been fully drained and all the sites had ditch networks which further facilitated drainage. The restoration work at these sites involved blocking ditches and installing plastic piles around the perimeter to stop the water from escaping. The three sites combined totalled 193ha and therefore such an approach although expensive was feasible. Once the water levels had been raised it had a dramatic impact on the Molinia dominated swards – in all cases the Molinia declined and was replaced by such plant species as Sphagnum mosses and cottongrass.

Anderson (2015) describes work carried out on the North Yorkshire Moors and the Peak District. Both these areas have been severely impacted by sulphur dioxide pollution (from heavy industry since the Industrial Revolution), this has led to acidified soils which ended up killing many of the sensitive plant species e.g. Sphagnum mosses. In addition these areas attract huge numbers of visitors and as a result these Moors have suffered from a large number of accidental and deliberate wild fires. These two factors combined with heavy sheep grazing led to erosion and gulley formation (which drained the otherwise wet peat) on these moors on a very large scale. Restoration work has been going on for several decades now in the North Yorks Moors and the Peak District to try to undo the damage caused by pollution, overgrazing and wild fires. This has involved blocking gulleys with thousands of small wooden dams and in many cases the aerial re-seeding of the Moors with heather. Prior to the commencement of this restoration many of these areas had become dominated by Molinia.  The dams in many places have resulted in the raising of the water table which in turn has seen the Molinia decline and wetland species such as Sphagnum return.

Examples from the uplands and lowlands showing that where water tables can be raised sufficiently the rise and expansion of Molinia can be halted so that sites of conservation importance can start their journeys back to favourable condition.

The Exmoor Mires Project aims to deliver multiple ecosystem service outcomes by restoring peatland areas of Exmoor previously drained as part of historic conversion to agriculture or during peat cutting activities. Since 2006 the Project has restored around 2000ha of peatlands at over a dozen sites. Smith et al (2014) describe the results of their vegetation monitoring of the Project. On sites where water levels were successfully raised they have clear evidence which shows that the original Molinia dominated communities have been replaced more species rich wetland ones. At several sites the Molinia swards prevailed after the restoration work had been carried out but the authors blame failures in the ditch blocking and bunding works i.e. the water levels were not raised sufficiently to remove the Molinia.

screen-shot-2017-01-12-at-15-53-06

More recently on Dartmoor via The Dartmoor Mires Project,  a series of pilot projects have been carried out to rewet small areas where erosion had been taking place. The situation on Dartmoor regarding moorland erosion is very different from the situation described earlier in the Peak District and the North Yorkshire Moors. Dartmoor did not receive very high levels of sulphur dioxide from the Industrial Revolution on account of its south westerly location and the direction of the prevailing winds. Unlike Exmoor, the Commons of Dartmoor did not see the level and extent of agricultural drainage in the second half of the 20th century.

The pilot Dartmoor Mires Project has however successfully demonstrated at least at some of the sites that raising water levels via small scale dams in localised areas of erosion can lead to a  reduction in the area of bare peat (at Blackabrook Head, Blackabrook Down and South Tavy Head ) and that the process of paludification re-commences, i.e. new peat is formed, along with an increase in ‘beneficial mire species’ (at Blackabrook Head, Blackabrook Down and Winney’s Down Area 1). However to date there has not been a decrease in the abundance and distribution of Molinia. In the control site (where no rewetting was carried out) however the abundance of Molinia did increase (Lunt 2015). It is however fair to say that the Dartmoor project has not been running for very long and when the sites are re-surveyed in the future it could be expected that Molinia will start to decline as has happened on the other sites discussed.

At the moment work on the Dartmoor Mires Project has been halted whilst an evaluation of the results to date is carried out. It will be interesting to see whether following the pilot scheme, work will recommence on a large scale which may then have a bigger impact on parts of the Molinia dominated moor.

References
Anderson P. (2015) Molinia – the importance of controlling water and other management techniques. In Meade (2015) pp39-54.
Lunt P. (2015) Dartmoor Mires Project Vegetation Analysis 2015. Download here.
Meade R. (ed) (2015) Managing Molinia. Proceedings of a 3-day conference 14-16 September 2015, Huddersfield, West Yorkshire, UK. National Trust. https://www.nationaltrust.org.uk/marsden-moor-estate/documents/managing-molinia.pdf
Smith D.M.,  Barrowclough C., Glendinning A.D.  &  Hand A. (2014) Exmoor Mires Project: Initial analyses of post restoration vegetation monitoring data. In the Bog Conference September 2014. Download  here.
Taylor K., Rowland A.P. & Jones H.E. (2001) The Biological Flora of the British Isles: Molinia caerulea. Journal of Ecology 89: 126-144.
Thomas P. (2015) Problems with Molinea caerulea in the restoration of lowland peat bogs – Manchester Mosses Special Area of Conservation (SAC). In Meade (2015) pp127-133.

2 thoughts on “More on Purple Moor Grass – Molinia and water levels

  1. I continue to be amazed at the huge amount of knowledge you have st your fingertips about the natural world and, in particular, Dartmoor. I thoroughly enjoy reading your blog, your photographs are wonderful to see – I just wanted to acknowledge and thank you for sharing this with me and all your other avid recipients of posts and images.
    Warm regards
    Stella

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