Molinia and the Restoration of Dartmoor’s Blanket Bogs

Tony Whitehead

The deep peats of Dartmoor’s Commons support internationally important areas of blanket bog. However, they are in a much diminished condition. Last month, Natural England published new assessments that described this habitat as being in unfavourable condition and, in some cases, in decline.

The key reason for this poor condition is the degraded state of the peat on Dartmoor. It has suffered historical cutting and drainage, military activity, and, for decades, intensive burning (known as “swaling” on Dartmoor) tied to managing land to support high numbers of livestock, both cattle and sheep. This resulted in a drop in the water table across the high moor and the drying and erosion of the peat.

This drop in the water table has also changed the vegetation's nature. Peat-forming sphagnum mosses are a vital component of any blanket bog. But they can only thrive in waterlogged conditions when the water table is at or very near the surface. Once it drops, they lose their competitive edge to deeper-rooted plants still able to access water. This is where the Purple Moor Grass, Molinia, comes in.

Molina is a perfectly natural component of a healthy bog. However, it is balanced with other species. Once the peat degrades, the Molinia is given an advantage; its roots are able to reach down to the lowered water table, which other plants cannot reach. Not only that, when a blanket bog is regularly burned, the sphagnum is irreparably damaged.  Like most grasses, Molinia thrives after fires – it regrows from its base into a space now cleared of competitors.

As if this isn’t enough encouragement, animals graze around the Molinia, further removing competition.  The palatability of Molinia is something we’ll return to in a moment. The icing on the cake for Molinia was, from the 80s, the huge increase in atmospheric nitrogen – in short, it started to rain fertiliser on Dartmoor, a final boost for the grass.

On the peripheries of the high moor, high livestock numbers in the 80s and 90s may have suppressed the Molinia’s vigour for a while in the days when favourable subsidies supported numbers of animals way in excess of sustainable limits.  Cattle and ponies will eat the grass, but it is only palatable in Spring; its nutritional value declines rapidly as it dies off in autumn (it is a deciduous grass). They will also trample it. However, the central and more inaccessible parts of the high moor may have long been dominated by the knee-deep tussocks so well known by hikers – archival photographs suggest this.

From the 2000s, much-needed controls on the excessive grazing, combined with a fall in stock numbers post foot and mouth, may have allowed the Molinia to become more vigorous, but by this time, the plant's hold was established.  This is an important point – the plant did not suddenly gain dominance due to a fall in livestock numbers, as some on Dartmoor have it – often those with a vested interest in increasing grazing.  Its rise to dominance has been a much longer process, and is a symptom of degraded peat.

Fortunately, the symptom points to the cure. Raising the water table and restoring the peat is the only way to restore the blanket bogs and restore balance to the system, where Molinia is in healthy balance with other plants.  In this way, the habitat can be returned to favourable condition, with all the societal benefits that flow from healthy bogs - carbon sequestration, flood control, clean water, etc. Rewetting the peat will, of course, also increase the peat’s resilience to wildfire. The South West Peat Partnership is doing some great work on Dartmoor in this respect.

Crucially and perhaps controversially, livestock have no useful purpose in restoring and maintaining Dartmoor’s blanket bogs.  Some argue that cattle on the high moor in Spring and Summer can be used to suppress the Molinia.

First, there’s the simple point that even if grazing was useful, the cattle to do this are probably not available, and even if they were, there are practical problems about how commoners can keep cattle on the high moor. I imagine nothing more galling than to take a herd to the central high moor to find the animals back at the moor gate the following day because that’s where all the nice grass is!  Remember that the central parts of moor are very poor grazing land because the vegetation underpinned by the peat lacks the minerals necessary for a healthy diet.

Second, cattle grazing Molinia might cause more damage than they cure by trampling sphagnum and other plants that survive in places beneath it. If you look at the Molinia-dominated parts of the moor, on many of the slopes, it is something of a monoculture, but on the flatter, still-wet bits, you will often find cotton grass and moss around the Molinia, just awaiting restoration to flourish and spread. 

In relation to trampling, we also need to consider the potential damage caused by commoners accessing their herds using 4WD vehicles and quad bikes (note that the peat restoration teams deliberately use low ground pressure, wide track machines to avoid damage).

Third, perhaps also controversially, while awaiting restoration, the Molinia can still provide a valuable role even where it is dominant. It protects the underlying peat from exposure to the atmosphere, and it may slow the flow of water off the moor. It’s not ideal, but it’s not useless either.

Given this, it’s high time we looked again at the role of commoners in relation to the restoration of the blanket bogs.  Currently, they are paid through Higher Level Stewardship to graze Molinia-dominated areas.  For the above reasons, this may not be necessary. What we need to do is rewet the bogs. Without this, to be blunt, grazing the Molinia will achieve nothing. Then, once rewetted, we need to maintain the restored area, but here again, grazing is unnecessary, as a recently published Natural England report on the favourable condition of blanket bogs stated (my emphasis): 

Large areas of the English uplands, including areas of blanket bog, are subject to livestock grazing. There are fewer grazing animals now than before headage payment subsidies ended in 2005. Future trends in livestock numbers are uncertain but grazing management is not required to maintain blanket bog, so further reduction or cessation of grazing is not a threat to this habitat.

The suggestion, therefore, is to stop paying for grazing the Molinia through Higher Level Stewardship and instead direct the money towards providing opportunities for commoners to become more involved in the peat restoration work. We need to level up the amount we are doing, which requires materials, knowledge of the land, and willing hands with practical skills. There’s also a role possibly in the aftercare of the works – dams built to hold back water need checking and maintenance.

In summary, the restoration of Dartmoor’s blanket bogs is vital and dependent on rewetting.  Grazing has no role in restoration and maintenance, but there are new opportunities for the profitable involvement of the local commoning community and others.