Spending over a month doing restoration work on Little Woolden Moss has inspired me to find out more about the ecology of peat bogs – what they are, how they come to life, how they function, how exploitation by humans has led to the deaths of 80% of them, and how they are currently being revived.
The information that follows has been gleaned almost exclusively from the IUCN Peatland Programme’s Conserving Bogs: The Management Handbook, which can be downloaded for free HERE.
What is a Peatland?
A peatland is ‘first and foremost’ a ‘wetland’. Water is essential because peat does not form when the land is dry. Peat is ‘a wetland soil composed largely of semi-decomposed organic matter deposited in-situ, having a minimum organic content of 30% and a thickness greater than 30cm’ and a peatland ‘an area with or without vegetation but possessing a naturally accumulated peat layer at the surface.’
The EU Habitats Directive distinguishes between ‘active bog’ as ‘a system that supports a significant area of vegetation which is normally peat forming’ and ‘non-active bog’ which does not. It is my intuition that a peat-forming bog is a living bog and a bog that is not forming peat is dead.
The most common type of peat bog is raised bog ‘accumulations of peat’ ‘within a non-peat landscape’ or ‘a fen-peat landscape’ mainly found in the lowlands. Such bogs form distinct domes that rise above the surrounding landscape by as much as 10 metres.’ The ‘classic type’ is basin raised bog – ‘formed within a shallow lake that has then infilled through terrestrialisation’.
Other types include floodplain raised bog, estuarine raised bog, and the evocatively named Schwingmoor ‘swinging bog’ which forms as ‘a floating raised dome’ ‘over a deeper lake basin’.
A rarer type of bog is blanket bog which forms as mantles of peat in the uplands. Types include watershed bog, saddle bog, valleyside bog, spur bog, minerotrophic bog, and ladder fen.
How Do Peat Bogs Form?
The UK’s ‘classic’ basin raised bogs developed over basins created in the aftermath of the last Ice Age by the melting of dead ice, surrounded by sediment, which was left when the glaciers retreated. These basins filled with water and became lakes which were ‘colonised by a fringe of fen vegetation’.
As this vegetation decayed the lakes were filled in with ‘fen peats and sediment’. The plants at the centre were ‘cut off from the nutrients at the lake margins.’ These nutrient poor, waterlogged conditions were perfect for sphagnum mosses to come to dominate, forming a thick carpet.
As the sphagnum began to slowly decompose it resulted in an accumulation of peat. When this reached a thickness of half a metre above the original level of the lakes the bogs were separated from the groundwater and became ombotrophic (rain-fed) so dependent on precipitation alone. Slowly they expanded, escaping their lake basins, to become the lowland raised bogs we know of today.
Blanket bogs formed in a similar way in waterlogged areas of our uplands such as saddles and spurs.
The Structure of Living Peat Bogs
All living peat bogs have a ‘diplotelmic structure’ (telm is Greek for marsh, pool, standing or stagnant water’). The acrotelm (from Greek acro ‘topmost’) is the surface of peat-forming vegetation and the catotelm (from Greek cata ‘down’) is the ‘inert, permanently waterlogged peat store’.
This structure is created, mainly, by sphagnum mosses. Their densely packed heads, known as capitula, form a thick carpet around 2cm deep. Sphagnum leaves are specially designed to cope with water logging due to their ‘large and empty hyaline cells’ ‘sandwiching smaller photosynthetic cells’. This provides a ‘large surface area for cation exchange’ – absorbing ‘nutrients dissolved in water’.
Their swapping of scarce nutrients for hydrogen ions acidifies their surroundings making them more favourable for sphagnum colonisation and unfavourable for decomposer micro-organisms. This leads to conditions that are favourable for peat formation. Sphagnum mosses also release a ‘pectin-like substance’ called sphagnan which coats the upper parts of the plant and the water and ‘inhibits nitrogen uptake in decomposer bacteria’ further slowing the processes of decomposition.
(The anti-microbial properties of sphagna led to the use of packs of sphagnum as dressings in World War I).
Sphagnum mosses ‘grow from the top of the plant – the apices – and die at the base’ at around 10cm. The stems and branches collapse, flatten, and break down to form peat. This is the point of transfer to the catotelm. The living layer of the acrotelm can be between 10cm and 40 cm deep. The catotelm can extend up to several metres (the deepest peat bog, in the Netherlands, is nearly 7m deep).
Sphagnum are the dominant species of mosslands and our lowland raised bogs include species such as S. magellanicum, S. papillosum, S. capillifolium, S. tenellum, and S. cuspidatum. Other plants that can survive in these conditions include common cottongrass (Eriophorum angustifolium), hare’s-tail cotton-grass (Eriophorum vaginatum), cross-leaved heath (Erica tetralix), bog rosemary (Andromeda polifolia), and round-leaved sundew (Drosera rotundifolia). Sundews are carnivorous bog plants ‘which gain extra nutrients (particularly nitrogen) by catching and absorbing insects.
Ombotrophic bogs gain their water not only from rainfall but from ‘occult precipitation’ – mist and fog.
Exploitation and Death
Humans have been exploiting peat bogs for fuel for many centuries. This began with domestic peat cutting. ‘Turves’ were cut by hand with a spade from banks 10m to 100m long, then stacked nearby to dry. The banks were reached by ‘peat tracks’ and carried by horse and cart or in creels or baskets
Traditional cutting has been replaced by ‘tractor-driven machines’ that ‘cut slits beneath the surface’ to extract ‘sausages’ of peat that are left on the surface to dry before their use as burning briquettes’.
Commercial peat extraction for horticulture and energy is currently taking place on a far greater scale and is far more damaging. Peat accumulates at ‘2mm a year’ and ‘a hundred times that depth’ is removed each year by modern extraction methods such as peat milling. This begins with the drainage of the land by ‘regularly spaced deep drains’. Once the upper peat has dried ‘the surface vegetation is removed’ to create ‘bare black fields’ then the top layers are ‘rotovated’ or ‘milled’ to allow further drying before the peat is ‘bulldozed into long ridges’ then bagged or taken to power stations.
Drainage of the land for human benefits has deadly results. The immediate effect is water loss from, and the drying out of, the acrotelm, leading to the decline in sphagnum and other peatland species.
Loss of water from the catotelm is slower (up to one million times slower than the speed of a snail!). However, water in the spaces between peat fragments seeps into the drain. This causes peat adjacent to the drain to collapse and shrink. This process is called ‘primary consolidation’.
The ‘drier catotelm peat adjacent to the drain itself becomes a heavy load on the peat beneath because the drained layer no longer floats buoyantly within the bog water table.’ It ‘compresses the peat beneath it and squeezes more water from the peat into the drain, causing the bog surface to subside still further.’ The entire depth of the catotelm subsides. This is ‘secondary compression’.
Drainage ‘allows oxygen to penetrate the catotelm’ which is usually prevented by waterlogged conditions. ‘Preserved plant material is thus lost in the form of carbon dioxide (CO 2 ), leading to further subsidence as the peat material itself vanishes into the atmosphere.’ This is ‘oxidative wastage’.
‘The three drainage processes – primary consolidation, secondary compression and oxidative wastage – cause the peat to subside progressively and continuously across an ever-expanding area. Drainage in effect continually widens the dimensions and impact of the drain.’
‘Where drainage leads to the loss of the peat-forming layer, carbon sequestration may cease’ and there are increases in the production of dissolved and particulate organic carbon (DOCs and POCs).
A peat bog in this state is termed haplotelmic. Without an acrotelm it is no longer living.
Sadly ‘80% of UK peat bogs now lack an active living surface as a result of human impacts’ and ‘therefore now have little or no capacity for resilience in the face of future climate change.’
Reviving Peat Bogs
Peat bogs can be revived by blocking drains and building dams and bunds to prevent water loss and begin the long process of silting up with vegetation that will lead to the development of sphagnum.
Bare peat can be re-vegetated by planting hardy species such as common cottongrass and hair’s tail cottongrass.
Sphagnum mosses can then be planted amongst the common cottongrass to form new hummocks and once these have formed species such as cross-leaved heath and bog rosemary can be planted on them.
Doing this research has provided me with a greater depth of understanding of the workings of peat bogs and the impact of my work on Little Woolden Moss. It has shown me how restoring a living acrotelm of sphagnum is essential to reviving a bog and reinstating the process of peat formation which sequesters carbon in the depths of the catotelm rather than releasing it into atmosphere.
I look forward to revisiting the mossland in years to come and watching its return to life.