Our peatlands are more than a source of fuel, they act as very efficient carbon sinks. But they are proving vulnerable to new turf-cutting methods, with potentially dangerous results
IN THE MONTY PYTHON film The Life of Brian, there is a classic, oft-quoted scene, where the question is posed “What have the Romans ever done for us?” Similarly, our view of peatlands in the past was often as dismissive. Bogs were seen as wet, brown deserts, with no value to man and of little benefit to beast. However, in recent times, there is now a much wider appreciation of peatlands, for example in the areas of biodiversity, flood control and, most importantly, carbon storage.
The recent assessment report from the Intergovernmental Panel on Climate Change highlighted the fact that the concentration of carbon dioxide in the atmosphere has increased by about 35 per cent over the past 200 years, primarily due to the burning of fossil fuels. Peatlands, by actively removing carbon from the atmosphere over the past 10,000 years or so, have thereby played a significant role in the regulation and maintenance of the global climate.
So how do they do it? For a peatland to be a carbon store, more carbon must go into the ecosystem than will exit. Key to this process is the position of the water table within the peat. The peatland plants take in carbon dioxide through photosynthesis and release some of it back to the atmosphere through respiration. The remainder is used by the plants to produce new leaves, stems and roots. As these parts die, they are deposited into the peat. In intact or undamaged peatlands, the water table level tends to be at or close to the surface. Under these anoxic, waterlogged conditions, decomposition of the plant litter occurs very slowly and relatively little carbon dioxide is produced by the myriad of microbes that feast on the detritus. Under these conditions, the undecomposed litter begins to accumulate, the carbon contained within the litter is stored and the peatland grows and expands.
However, if the water table begins to drop (this can occur during periods of low rainfall or from damage to the peatland), the plant litter deposited into the peat is now subject to decomposition by a different range of microbes that thrive under higher oxygen concentrations. In this oxic zone, decomposition rates are typically 10,000 times faster than below the water table, carbon-dioxide emissions increase dramatically and the peatland now begins to lose some its hard-gained carbon.
Researchers have estimated that the peatlands of the northern hemisphere alone contain about 450 billion tonnes of carbon (the atmosphere contains about 750 billion tonnes of carbon). To this figure can be added the substantial quantities stored in tropical peatlands, currently under extreme threat from illegal logging, fire and climate change. In Ireland, peatlands cover about 17 per cent of the land cover and store about 1.2 billion tonnes of carbon. For centuries, the peat has been used as a source of fuel. Traditionally, the peat was hand cut from a bank face with a sleàn. This produced a landscape of varying peat depths, characterised by banks and low-lying areas where the peat had been removed.
In recent times, hand cutting has been superseded by large-scale industrial milled peat extraction or by the use of tractor-mounted peat extractors operating at a much smaller scale. The impact of peat extraction on the carbon cycle is most keenly felt on the wide vistas of exposed brown peat that are characteristic of milled peat extraction.
In order to facilitate the use of machinery to extract the peat, drainage ditches are installed at 15m intervals throughout the bog. This leads to a severe drop in the water table and an increase in emissions of carbon dioxide. The vegetation at the surface is removed and with it the ability of the peatland to capture carbon. Over the economic lifetime of the bog, these conditions are maintained and the bog becomes a large annual carbon source.
The impact of small-scale peat extraction is also acutely damaging. Two methods have been generally employed to extract the peat or turf. In the first, the peat is extruded onto the surface of the peatland from narrow openings made in the peat by a chain cutter (sausage machines). This practice has a number of adverse effects on the peatland. Firstly, the vegetation is damaged. Secondly, as the tractor passes over the surface, the peat becomes compressed, leading to changes in water movement within the peat. Thirdly, it creates deep crevices within the peat that function indirectly as drainage ditches, leading to a severe drop in the water table and a rise in carbon-dioxide emissions.
While this practice is now prohibited on peatlands that are designated as Special Areas of Conservation (SACs), the effects of previous actions are still evident in many peatlands. The second method of extraction, the hopper method, involves the block cutting of the peat at the margins of the peatland. The harvested peat is placed into a tractor-mounted hopper and the peat is extruded on the surface. This also damages the vegetation and severely scars the periphery of the peatland.
Work by the Irish Peatland Conservation Council has suggested that less than 20 per cent of Ireland’s peatlands are in an intact condition. Long-term monitoring by Prof Ger Kiely’s research group in University College Cork has shown that intact peatlands in Ireland are still actively fixing and storing carbon. If these results (and results from other studies) were extrapolated to all the intact peatlands in the country, it would result in about 60,000 tonnes of carbon being removed from the atmosphere and stored below ground.
UNFORTUNATELY, THERE IS a sting in the tail. The results from the EPA-funded BOGLAND project suggests that the magnitude of carbon losses from the rest of Irish peatlands (ie those that are damaged to some extent) is astonishing. About 2.5 million tonnes of carbon is released each year from Irish peatlands, equivalent to the amount of carbon emitted by cars on Irish roads every year.
A large amount occurs with the combustion of the four million tonnes of peat burned in power stations every year. Considerable emissions are also associated with the peat extraction fields and from domestic turf burning.
Most worrying is the indirect impact of small-scale domestic peat extraction, which is seldom confined to the margins where it takes place. In reality, the damage caused at the margins radiates deep into the intact part, leading to further drops in the water table, increased carbon dioxide emissions and ultimately to the subsidence or collapse of the whole bog – literally “death by a thousand cuts”.
In regard to preserving the carbon store within Irish peatlands, the decision by Minister for the Environment, Heritage and Local Government John Gormley to ban turf cutting on the 32 peatland SACs is a long-overdue first step. A recent report from researchers at Trinity College predicted that about 40 per cent of Ireland’s peatlands (and the carbon contained within them) could disappear by 2050 as a result of climate change. Intact peatlands are reasonably robust in dealing with climatic changes – they contract in times of drought and expand with high rainfall.
However, when ecosystem functioning is impaired through human actions, then peatlands become extremely vulnerable to any kind of disturbance, and the effects of climate change are likely to be greatly exacerbated in those peatlands. To future Irish generations, the question is not likely to be “What have peatlands ever done for us?” but rather “What exactly was a peatland?”
Dr David Wilson is a researcher at the school of biology and environmental science, UCD
The Irish Peatland Conservation Council is holding a nature craft camp next Saturday at the Bog of Allen Nature Centre to celebrate World Wetlands Day. See ipcc.ie