Repairing Bare Peat
The blanket bogs of the Peak District and South Pennines are characterised by evidence of significant long term decline, caused by a range of natural and human induced factors. The most significant feature of this decline is the large area of bare and eroding peat and associated damaged vegetation. This land has reduced capacity for delivering the ecosystems services that the area has always supplied: clean drinking water released slowly over a long period following rainfall, storage of carbon from old vegetation in the form of peat, raising animals (sheep and grouse) and extensive areas for recreation.
There are two main reasons for the formation of these areas of bare peat. Firstly, atmospheric pollution since the start of the Industrial Revolution caused the loss of sphagnum mosses, the building blocks of these bogs, which are very susceptible to the sulphur oxides emitted by burning coal. Secondly, without these mosses, the bogs start to dry out and they become more susceptible to summer wildfire. These can remove all of the vegetation, including the roots and seeds, from the bog and set light to the peat itself. Once this happens, the peat shrinks, forming cracks, pipes and holes, which makes the peat dry out further. In addition, gullies form, either from collapsing peat pipes, or through the surface erosion of the drying bare peat which dries the bog surface even further.
These conditions are exacerbated by the weather conditions on the summits where blanket bog forms. Blanket bog formation requires high levels of cloud and rainfall. In addition, because of the altitude that these bogs form at in the Peak District and South Pennines (approximately 600m), they are subjected to regular periods of freezing and thawing. These factors, together with the summer drying of the peat, cause extensive lamination of the bog surface, which was built up in layers. Combined, these erosive forces lead to the loss of approximately 2.5cm depth of peat from the whole of the bare peat area, a highly mobile surface which is not conducive to the establishment of vegetation. This problem is exacerbated by further factors:
· the atmospheric deposition has lowered the pH to around 3.5, which most plants, even moorland plants, cannot tolerate;
· the areas are extensively grazed and the vegetation is generally unpalatable and nutrient poor. The most palatable vegetation types are the extending areas of young cottongrass and the flowering heads of grasses and sedges; removal of these leads to the loss of sources of new vegetation.
In order to protect the remaining active blanket bog (a nature conservation priority habitat) and to prevent the further loss of peat, the surface of the bog needs to be revegetated. The ideal situation, in order to reduce the risk of wildfire in the future, to reduce the drying of the peat and to reinstate the ecosystem services outlined above, is to establish a sward dominated by sphagnum mosses.
Stabilisation of bare peat
In order to allow vegetation to develop, the hostile surface conditions of the blanket bog need ameliorating. We aim to form a skin of vegetation over the peat, reducing the erosion. The first step is to form a network of fibres. For this we use a combination of heather brash (heather cut from other moors) on slopes of up to about 50o and geo-textiles (used where the slope is between about 50o and 60o). Heather brash is the preferred material as it contains heather seeds, mosses, lichens and fungi, which brings these species onto the site. Both of these materials prevent the peat from eroding very effectively.
The second step is to further bind the skin together. For this, we develop a sward of grasses. We generally use standard amenity grasses as these are very cheap and allow us to apply seed at a high density for a reasonable cost. In order to get these species to grow, we need to ameliorate the soil chemistry and so, approximately 6 weeks before sowing the grass seeds, we apply granulated lime fertiliser. This raises the pH quickly (to approximately pH 5), needed to get the grasses to establish. Once the grasses have germinated, we treat them with an agricultural fertiliser, which is high in phosphate to encourage root growth. In order to protect these grasses, we re-apply lime and fertiliser, when soil chemistry conditions require it, for a further two to four years. During this time, as the sward continues to develop, moorland plants will come in and, after four or five years, the sward will be stable enough to allow the grasses to die off. This is inevitable when we stop treating with lime and fertiliser, as the amenity grasses cannot tolerate the reducing pH or nutrient levels.
This work has a significant impact on preventing sediment loss, with Particulate Organic Carbon (peat or POC) being reduced by 95% within two years of work starting. The work has little effect on water tables and there is currently little evidence of the benefits to Dissolved Organic Carbon (DOC and water colour).
Some moorland plants will come into the sward through material present on site (such as heather, mosses and lichens from the heather brash, cotton-grasses and wavy hair grass by seed and vegetative expansion). These are facilitated by the addition of lime and fertiliser, which increases flowering and vegetative expansion. However, many other moorland species (e.g. bilberry, crowberry, cloudberry, cross-leaved heath) are much slower colonisers. These species, which are significant structural species, growing at different depths in the peat, need further interventions in order to colonise. These can be added as plug plants, grown by micro-propagation from material collected locally.
In order to reduce DOC, drying out of the peat needs to be reduced. The main mechanism for doing this is by blocking the erosion gullies. The methods used can be split into two categories: those designed to stop sediment loss, which allow water to pass through and those designed to prevent water movement, which do not. Both types will affect the water table, with impermeable dams raising it significantly more. Examples of permeable dams include: timber planking, stone, logs and heather bales whilst impermeable ones include: peat, plastic and plywood. The method used will depend on a range of factors: angle of slope, width of gully, material at gully base, contractor experience and machine access.
Sphagnum moss reintroduction
The most important group of species to re-introduce are sphagnum mosses, both biologically and structurally. In order to get sphagnum to establish, there must be adequate surface water for most of the year. In lowland raised bogs, this can only be achieved by raising the water table; however, in upland blanket bogs, which have formed where rainfall and cloud cover are very high, raising the water table is less important. In the Peak District and South Pennines, the main reason for the absence of sphagnum is historical, leading to a loss of material for colonisation. In order to re-establish sphagnum, material needs to be brought in from elsewhere; this can be undertaken using sphagnum rich heather brash and we have also worked with a contractor to produce material by micro-propagation.
Further information can be found on the following pages.