Peat – mired in myths? (Part 1)

Written by Henrietta Appleton, GWCT Policy Officer (England)

Over the course of a series of blogs this year I will be considering the current science on peatland management and restoration with a view to challenging some of the ‘accepted truths’. These are limiting our ability to adapt our approach to peatland management and restoration, which is an important policy component for achieving net zero in the agriculture and land use sector.  Ironically this blog is due to be posted on the day that Defra has announced a proposed extension to the ban on burning on deep peat New proposals to ban heather burning on peatland to protect air, water and wildlife - GOV.UK

I thought I would begin by revisiting a 2017 article in Mires and Peats (an academic journal) by the ‘Leeds Peat Club’, which consisted of a number well known academics in the world of peatlands[1].  This questioned 10 common assumptions about peatlands (although I will only be covering 9 of them as one related to tropical peatlands which is not within the GWCT’s remit).

Over the course of the next two blogs, I will follow the approach of the 2017 authors and consider each assumption in turn “and, where appropriate, identify where the assumption may be misleading and where focused research may usefully help resolve any misunderstandings or lack of understanding” (University of Peat Club, 2017). This last point is particularly important in helping to dispel aspects of peatland science that have become ‘accepted truths’.

1. Will the northern peatland carbon store shrink under a warming climate?

As reviewed in the 2017 article, the evidence is equivocal.  There is the likelihood of CO₂ emissions from thawing permafrost with peatlands, but this could be compensated by the accumulation of peat (and carbon) due to greater plant productivity in response to warming – and wetter conditions where the permafrost has thawed. 

Concerns about increased CO₂ emissions from peatlands challenging our ability to meet net zero are driving the focus on protecting intact peatland carbon (C) stores and restoring degraded or degrading peatlands. But rewetting temperate peatlands as part of restoration (such as in the UK) in combination with increasing temperatures and or pH may increase methane (CH₄) emissions (Abdalla et al. 2016, Heinemeyer, David & Pateman, 2023), a gas that is 28 times more potent over a 100-year timescale than CO₂.

The balance between CO₂ and CH₄ emissions is often overlooked as climatic effects are strongly time dependent and interact (e.g., wetter and warmer increases methane). Whilst research suggests that increased CH₄ emissions from rewetting maybe short lived (although the timescale is site dependent e.g. Darusman et al. 2023, Vanselow-Algan et al. 2015), methane emissions should nevertheless be a necessary policy consideration given that degraded peatlands may not return to their ‘natural’ condition including carbon cycling functions and so more research is needed.

2. Are peatlands fragile ecosystems?

Peatland formation consists of a complex interaction between hydrological, ecological and biochemical processes.  These processes result in a mire system rather than a single “state” with different components depending upon the relative contributions of each process (see diagram below after Lindsay 1995).  These complex systems are vulnerable to degradation by both natural erosion, once an accumulation tipping point (maximum) has been reached, and through activities which will lower the water table, particularly drainage, but also climate change i.e. hotter, drier summers. Drier peatlands are also very vulnerable to wildfire.

Today’s evidence varies little from the original review in 2017, but what has perhaps been increasingly overlooked is how timescale could be important.  Peatlands go through millennial-scale cycles of growth and erosion and so can be resilient to climatic and land-use changes which occur over long timescales but are vulnerable to short periods of rapid transition often anthropogenic in nature such as through drainage (a government policy post-war).

The impact of drainage and pollution on peatland health has resulted in a policy focus on peatland restoration to minimise the on-going loss of deep stored carbon. Many commentators will include burning in the list of anthropogenic actions.  But there is little distinction in these references between intense hot wildfires and prescribed cool fires.

The detrimental impacts on peatland functions are due to the intensity of wildfires as they burn into the peat whereas managed burns merely remove the surface vegetation, resulting in ground temperatures remaining below the level at which peat combustion occurs.  In addition, peatlands go through millennial-scale cycles of growth and erosion with recent research showing that internal mechanisms can initiate peatlands naturally reforming (Milner et al. 2020, Chadburn et al. 2022).  Such research demonstrates that the internal mechanisms that govern peatland systems can adapt.

 3. Do wet peatlands have greater rates of net carbon accumulation?

The idea that “wetter is better” (for C storage and wildfire resilience) has become an ‘accepted truth’.  However, the 2017 article already stated that “the assumption that wetter peatlands accumulate more C may be overly simple and even misleading”.  Current policy is focussed on raising water tables (through rewetting) at a time when climate change predictions suggest that key peatland areas such as the Peak District and moorland in the south-west of England will be outside ideal conditions for peatland formation (Ritson et al. 2024). Hotter, drier summers are likely to affect some Sphagnum moss species (bleaching, dessication), in particular those recently planted (and Ritson et al. 2024), whilst warmer, wetter winters are likely to increase plant growth leading to both increased fuel loads and water loss from evapotranspiration, the end result being drier peat and greater risk of wildfire.  As the 2017 review concluded “.where drainage [has reduced water tables], increases in wetness [..] are likely to be beneficial to net C accumulation. Beyond this threshold, other factors [..] (e.g. temperature) are likely to have a greater role [..], and further increases in wetness may actually cause a decrease in rates of net C accumulation”.  This is likely due to plant growth, and therefore carbon uptake, being limited under constantly waterlogged conditions. In fact, the optimum mean annual water table depth for carbon accumulation is not at the peat surface but around 5-13 cm below it (considering emissions inventory data in Evans et al. 2022). Research is needed to determine the height of water table in upland peatland and associated management approaches that best balance a) climate mitigation objectives with b) flooding and c) biodiversity ambitions.

4. Is peat a single soil type?

This is an interesting question as the definition of peat is often based on its organic content and depth. As the 2017 review says, this suits some classification needs such as agriculture but doesn’t help understand the finer scale details that will support peatland ecosystem function.  A critical review of peat definitions (Lourenco et al. 2022) highlighted the significant disparity between definitions but given the variation in peatland environments and therefore peat formation mechanisms one definition is inappropriate. A point often missed in the drive for restoration via rewetting is that the surface topography and underlying geology are important.  For example, the Peak District has permeable geological foundations (e.g. limestone in the White Peak) that mean only about 30% could be rewetted.  The formation of the surface peatlands in these areas is most likely to have been the result of high rainfall levels and reduced evapotranspiration through forest clearances creating the anaerobic conditions necessary.  These peatlands are classified as ombrogenous as they receive water from precipitation alone.  Consequently, they have a naturally drier limit to how wet they can be and are more likely to be damaged by the changing climatic conditions as outlined above than the valley or topogenous bogs such as in the border mires.  The 2017 review concluded that the variation in several key properties means that just focussing on organic matter and regarding peat as a single soil type risks ignoring factors vital to peatland function and protection.

5. Do peatlands behave like sponges?

This first blog concludes on what is perhaps one of the most commonly stated ‘accepted truths’ – and a myth that needs dispelling.  As the 2017 review stated “The analogy … may lead to misinterpretation of the hydrological functioning of peatlands”. Peat is naturally saturated (with often about 86-94% water content by volume).  But unlike a sponge, peat does not readily release water; in fact it can retain the water even in dry periods. The idea, therefore, that peat will absorb water during high rainfall events and then slowly release it, minimising flood risk, is too generic and overall, a fallacy.  As the 2017 review observed, rain is likely to be shed quickly as there is little additional capacity in a healthy peatland, even if the water table is below the peat surface.  Therefore, only those peatlands that are in depressions (or floodplains) are likely to have a flood reduction function.  It is therefore a concern that peatland restoration via rewetting is seen as a flood mitigation exercise.  Clearly the revegetation of bare ground will slow the flow; and dwarf shrubs may actually increase the storage potential by reducing the water table in their rooting zone (Heinemeyer et al. 2023).

It is interesting that the points the Leeds Peat Club felt were important back in 2017 remain pertinent today.  Where the frustration lies in re-visiting this article is the lack of progress made in dispelling the myths and ‘accepted truths’ that pervade the policy debate and how political ideology remains a barrier to effective peatland management.