Peatlands and carbon credits: natural and anthropogenic threats to the carbon stock

Abstract

“Only by examining the recent palaeoecological record does the effect of increased dust loading on the delicate nutrient balance of this ecosystem, and consequently, its ability to sequestrate carbon, become clear.”

Keywords:

• peatland
• bog
• carbon credits
• peat accumulation
• Kyoto Protocol

Peatlands are important stores of terrestrial carbon and play a key role in the biosphere's carbon cycle. The peat carbon stock has accumulated over millennial timescales and in the mid to high latitudes much of this store has developed during the Holocene. Considerable uncertainties exist in the estimates of carbon in saturated peat soils; however, a recent study suggests that northern peatlands alone contain approximately 500 ± 100 (approximate range) Gt of carbon [1], and globally peat soils may account for as much as one-third of the earth's organic soil carbon [2].

Carbon accumulation occurs in peatlands when primary production exceeds organic decay in anoxic, waterlogged conditions. Plant litter accumulates because of the suppression of microbial decay. However, even in the waterlogged catotelm slow peat decomposition occurs, producing CO₂ and CH₄. These GHGs are produced naturally from peats in variable amounts depending upon the peatland type and surface hydrological conditions. In intact peatlands with a near-surface water table, the less soluble gas, CH₄, tends to be preferentially emitted via outgassing (ebullition).

While the net ecosystem carbon balance (NECB) of peatlands is currently considered to be positive at a global scale [3], locally and regionally peatlands are vulnerable to both natural and anthropogenic disturbances, particularly those that alter the ecohydrological conditions of the peatland surface. Gross-scale disturbances to water table levels, such as those produce by ditching, burning and peat mining, can lead to peatlands becoming net sources of carbon. For example, Wetlands International estimates that drained peatlands, covering a mere 0.3% of the global land surface, are responsible for some 6% of total global anthropogenic CO₂ emissions [101]. Likewise, deposition of loess and volcanic ash can change the nutrient balance of peatlands, altering the composition of plant communities, again turning mires temporarily from sinks to sources of carbon [4].

Management of carbon stocks in peatlands, both through the conservation of existing peat deposits and through the enhancement of peatland carbon sequestration, has the potential to be an effective way to reduce the emission of GHGs to the atmosphere [5,6]. Despite this potential, peatlands were not included in the provisions of the Kyoto Protocol's first commitment period (2008–2012). The original rules of the land-use sector of the Kyoto Protocol (‘Land Use, Land-Use Change and Forestry’, LULUCF) required that signatory countries must account for GHG emissions and removals via afforestation, reforestation and deforestation (Article▒3.3), with voluntary accounting of forest management, cropland management, grazing land management and ­re-vegetation activities (Article 3.4) [102].

Recently, following approval at the Seventeenth Conference of the Parties (COP17) in Durban in 2011 [103], Article 3.4 was amended to include an additional ‘wetland drainage and rewetting’ category. As a result, for the second commitment period (2013–2017/20), countries can submit any emissions savings achieved through peatland restoration towards the commitments of the Kyoto Protocol [7].

The peatland amendment to the Kyoto Protocol has the substantial benefit that it places an economic value on the restoration of damaged peatlands, aiding the conservation of wetland biodiversity at the same time as managing GHG emissions. However, for the amendment to be most effective, peatland management needs to be based on a sound understanding of both modern peatland functioning and the factors affecting the long-term net ecosystem carbon balance in peatlands. Consideration must be given to past, present and future climate change and changes in burning regime, as well as responses to the deposition of nutrients and pollutants such as soil dust, nitrogen and volcanic ash.

In most instances, ecological monitoring extends back just a few decades. As a result, our understanding of ecosystem responses to external anthropogenic and natural stressors can be limited. Palaeoecology provides a long-term insight into ecological responses to past disturbances, which can be used to inform management decisions [sensu 8]. Similarly, ecological modelling studies can be equally important in predicting future peatland carbon change.

In this paper, we highlight four examples where palaeoecological and ecological modelling studies provide insights into peatland carbon accumulation of relevance for modern carbon management in anthropogenically modified peatlands. Note that these examples do not represent an exhaustive list of the influences affecting carbon accumulation in peatlands.

Examples of key influences on long-term carbon accumulation in peatlands

Climate change

Peatland distribution is closely coupled with climate, typically occurring in regions where annual precipitation totals exceed 500 mm. Different types of fens and bogs exist in the peatland bioclimatic zone and these distributions are also strongly controlled by the prevailing climatic regime, as well as by differences in topography and other local factors.

Climate change can be expected to shift the optimum geographical regions for specific mire types. For example, a recent study has modelled the likely impacts of predicted future climate change on blanket peats, which occur in hyper-oceanic regions, finding that this bioclimatic space is likely to shrink under many predicted future global warming scenarios [9]. The study concluded that blanket peats falling outside the optimum bioclimatic zone are likely to come under stress from climate change and that this mire type is unlikely to continue forming peat and could potentially become a carbon source. Further research is required to examine how other important climatically sensitive peatland types, such as lowland raised bogs, might fair under similar climate change scenarios. This baseline information will be vital for future carbon management of many peatlands.

With rising global temperatures in the 21st century, it is widely assumed that rates of peat decay will increase in northern peatlands ‘… causing a positive feedback to climate warming and contributing to the global positive carbon cycle feedback’ [10]. To test this assumption, 24 carbon accumulation datasets spanning the last 1000 years were collated from across the northern hemisphere in a recent study [10]. The timeframe was chosen to span the Medieval Climate Anomaly (AD 1000–1425) and the transition into the Little Ice Age (AD 1425–1850). The study therefore included a climatic warming and cooling cycle with a magnitude of approximately 1°C. Contrary to expectations, the results revealed a small negative carbon cycle feedback from past changes in long-term peat accumulation rates and the research concluded that

… total C accumulation over the last 1000 yr is linearly related to contemporary growing season length and photosynthetically active radiation, suggesting that variability in net primary productivity is more important than decomposition in determining long-term carbon accumulation. [10,▒p.▒930]

Whilst other factors are undoubtedly important in peatland carbon accumulation, such as changes in moisture status, peatland distribution, fire regimes, nitrogen deposition, permafrost thaw and GHG emissions, the effects noted in this study suggest that northern hemisphere peatland carbon sequestration rates may increase with future global warming of the order of 1°C and changes in cloudiness. Greater rates of warming may have the potential to produce net carbon release from peatlands if temperature rises are sufficient to cause surface drying and accelerated peat oxidation.

Peatland burning

Peatland fires vary in frequency and severity but have the potential to burn deep into the bog surface for weeks, resulting in catastrophic carbon release. The burning of peatlands, whether anthropogenically or naturally induced, can release carbon directly through combustion, but also indirectly over longer timescales via enhanced peat decomposition and failure of vegetation to re-establish [11]. Human impact on peatlands can increase fire risk through drainage and drying of the upper strata. Equally, there is some evidence that minor fires on peatlands can block the pores of the peat, causing subsequent flooding of the peat surface [12]. Consequently, peatland fires might be expected to cause phases of variable apparent carbon accumulation in disturbed mire systems – but what is the relationship between fire and carbon accumulation in pristine or uncut mires?

A recent carbon accumulation study of intact boreal peatlands in Quebec, Canada, found no consistent relationship between carbon sequestration and fire regime on millennial timescales in the Holocene [11]. However, evidence based on the comparison of burnt and unburnt areas in 10 stratigraphically intact Alberta bogs suggests that a negative correlation between peatland fires and carbon accumulation may have existed over the last 100▒years [13].

The recent rapidity of climate change may be altering the relationship between burning and carbon accumulation, compared with that experienced during the Holocene. Furthermore, based on future climate predictions [14], fire regimes may change considerably over the next century, both in intact and damaged sites. For example, increased fire incidence is predicted in Boreal regions [13]. Future carbon management of peatlands for carbon credits will need to understand and account for likely changes in fire regime, which may be regionally variable.

Volcanic ash loading

Recent palaeoecological research into the affects of volcanic ash loading of intact ombrotrophic peatlands shows that moderate (>2 cm ash deposition) to high loading (>10 cm ash deposition) can result in a shift in plant communities from Sphagnum to monocotyledon domination in sites from Alaska [15] and Hokkaido, Japan [4]. In both studies, the switch to sedge communities was accompanied by increased humification of the peat surface and, in the Hokkaido study [4], it was shown that this palaeoecological shift was also accompanied by a sharp but short-lived decline in peat carbon accumulation. Here, the recovery of Sphagnum communities in the centuries following heavy ash deposition led to a major acceleration in carbon accumulation that more than offset the initial decline in carbon accumulation after c. 300 years. Analysis of nitrogen and phosphorous levels in the peat showed that the acceleration in carbon accumulation was coincident with a radical switch in mire nutrient cycling, most probably caused by the interaction of Sphagnum magellanicum with leachates from the volcanic ash. Leaching and washing of the deposited volcanic glass surfaces most probably increased the availability of phosphorus and potassium. At the same time, Sphagnum colonization reduced the pH and redox potential of the peat surface. These conditions inhibit the microbial immobilization of phosphorus to organic forms, while nitrogen availability declines through efficient uptake by Sphagnum. Under conditions of high phosphorus and potassium but low nitrogen, Sphagnum litter decay may be very slow and this can sharply alter the net ecosystem carbon balance of a peatland in favour of carbon accumulation▒[4]. Therefore, management of peatlands to maintain a healthy cover of ash-tolerant Sphagnum species may be a very effective means of increasing the long-term resilience of peatland carbon stocks in volcanically active regions.

Atmospheric dust input and nutrient loading

Increased aeolian deposition of atmospheric dust, associated with anthropogenic landscape disturbance, represents a considerable threat to carbon accumulation in peatlands. Cumulative nutrient loading, accompanying dust deposition, has the potential to lead to significant ecological changes and even state-shifts in nutrient-poor peatland ecosystems, primarily through changes in competitive dynamics, leading to shifts in vegetational composition. Such changes, in turn, affect rates of peatland primary production and decomposition – the key components that determine carbon accumulation. For example, in a kettle peatland in Pennsylvania, USA, evidence was found for increased dust input and associated nutrient loading (nitrogen, phosphorus) concurrent with European settlement and landscape degradation in the area from c. 1850, as identified by significant increases in Ambrosia spp., an anthropogenic pollen indicator [16]. At this point, Sphagna were largely replaced by vascular plants in the palaeoecological record, a change consistent with increased nutrient availability. Carbon accumulation was found to decrease considerably at this point as a result of reduced peat accumulation, typically associated with the disappearance of ‘peat-building’ Sphagna and increased microbial decomposition. The latter factor was reflected in faunal composition changes in the testate amoebae palaeoecological record.

Critically, it was noted by Ireland and Booth that the dramatic ecological changes in the kettle peatland were not immediately obvious on initial inspection of the bog surface, as Sphagna have begun to re-establish [16]. Only by examining the recent palaeoecological record does the effect of increased dust loading on the delicate nutrient balance of this ecosystem, and consequently, its ability to sequestrate carbon, become clear. This underlines the importance of palaeoecological records for the development of conservation and management strategies [sensu 8], particularly in the case of peatland restoration and the award of carbon credits. By reducing dust input from deforestation, agriculture and broader landscape degradation in regions with high concentrations of restored and restoring peatlands, and encouraging the re-establishment of Sphagna, carbon accumulation in these systems can be maximized.

Conclusions

The use of peatland restoration to gain carbon credits, while worthwhile, has associated risks. The studies presented in this paper highlight several examples of key threats to peatland carbon stock security. These threats, and other similar ones, need to be thoroughly understood if the award of carbon credits under Article 3.4 of the amended Kyoto Protocol (COP17) is to be fully successful.

This paper emphasizes how threats to peatland carbon security can originate beyond the boundaries of the mire system. From global and regional climate change to nutrient deposition, anthropogenic activities exert negative externalities on peatlands. Strategies to manage the whole landscape for the benefit of peatland carbon accumulation are required. Such strategies now assume greater importance in a regulatory regime that uses peatland management as a tool for atmospheric carbon management.

Some threats to peatland carbon accumulation are much harder to control than others. Where threats are natural events that cannot be reduced or removed, such as volcanic ash deposition, management strategies should aim to increase the resilience of peatlands to these impacts; for example, through the conservation or re-establishment of ash-tolerant Sphagnum species.

Palaeoecoloigcal and modelling studies provide ways of tracking past and possible future trajectories of ecosystem change, respectively. Such studies should be considered an integral part of the future carbon management of peatlands. However, users of palaeoecological evidence need to be aware that the emergence of novel ecosystems [8] and novel combinations of ecosystem drivers mean that the relationship with the past needs to be examined with caution. Likewise, the quality of input data and our understanding of processes limit ecological models. More research is required to refine both of these modelling elements with respect to carbon accumulation in peatlands.

The peatland amendment to the Kyoto Protocol, made at COP17, is welcome and timely; however, this update does not encourage the protection of carbon stocks in intact and pristine peatlands. Many such peatlands remain under threat of anthropogenic modification in developed and developing regions, particularly following the displacement of peatland exploitation from regions where environmental legislation has been tightened. Given their importance for carbon storage, biodiversity and other ecosystem services, preservation of intact peatlands should be a global priority.

Financial & competing interests disclosure

The authors thank the Natural Environment Research Council (NERC) for funding support under grant NE/D006899/1.

Additional information

Notes on contributors

PDM Hughes

TP Roland

D Mauquoy D