Heather burning

Heather burning in actionHeather burning has a long history of use in managing moorland. It encourages the growth of new shoots, which are more palatable than older, woody heather to grazing livestock and grouse. Currently, however, heather burning, especially over blanket bog stimulates an active debate about the possible effects on water quality, flood risk and carbon storage. These are complex issues, on which the science is not always available or clear, with a range of findings and interpretations.

Q: What is prescribed heather burning?
A: Prescribed heather burning refers to setting a fire on moorland that follows the methods laid out in law and codes of practice. The activity is also referred to as muirburn, rotational burning or managed burning. It is typified by planned burns of small patches of older heather (in some settings around 30x30m but sometimes larger), aiming for a low intensity, quick, ‘cool burn’ to remove the heather and grass canopy without damaging the underlying peat or soil layer.

Q: Where is heather burning used?
A: Managed burning is widely used across the UK uplands as part of vegetation management for livestock and red grouse, as well as for conservation. Concerns about its possible effects are greatest when prescribed heather burning is carried out over blanket bog (defined as peat more than 40cm deep in England and more than 50cm deep in Scotland) though its use on dry heathland can also provoke some concern.

Q: Why is it done?
A: As heather becomes older, it becomes less palatable and nutritious. The process of burning small areas removes the older growth and allows the plants to regenerate afterwards. New heather and grass shoots grow, and these provide food for red grouse, deer, mountain hares and livestock. Burning small areas of heather in different years leads to a patchwork, with heather and other vegetation of different ages and heights. This mosaic provides red grouse with areas that are suitable for feeding, breeding and cover. Burning also suppresses tree and scrub spread and the eventual progression of moorland to woodland cover. Prescribed burning is also sometimes used to create firebreaks on moorland.

Q: When is heather burning carried out?
A: The law only allows burning to be carried out between October and mid-April in most of the UK (the end of March in Wales). Most burning occurs in the spring when the plant material has dried out, allowing it to burn, while cold, damp conditions underfoot mean the fire is most easily controlled. Burns are not performed in summer because birds and other animals are breeding, the daily temperatures are warm, and underlying peat may have become drier.

Q: Are there guidelines or legislation that land managers must follow?
A: Yes. Westminster and the devolved governments and countryside agencies set out and manage the rules for safe burning and can prosecute those who do not burn in line with them. For example, the Muirburn Code is produced for the Scottish Government by Scotland’s Moorland Forum. The Code provides good practice guidance for burning and cutting of vegetation, as well as statutory restrictions that must be followed. Land managers receiving public payments must also meet Good Agricultural and Environmental Condition (GAEC) requirements or face loss of financial support.

In areas which are designated as an SSSI, consent must be applied for in order to burn heather. Natural England does not as a rule allow prescribed burning on blanket bog habitats within designated areas; its position is that burning on blanket bog is generally considered to be harmful, but in exceptional circumstances it may be appropriate to carry out a one-off burn for the purposes of restoration. Where other habitat management options have already been explored, a consent for burning can be applied for as part of a blanket bog restoration project.

Q: Are these guidelines followed?
A: Evidence looking at this is limited, but a case study on a moor managed for grouse shooting and sheep grazing in the Peak District showed that burning was within the widely accepted best practice guidelines. The authors of this paper do note that, while burning was found to be following the guidelines, the results apply only to that site and they “do not claim that the management of this moor is characteristic of UK moors in general”.

Q: Is heather only burnt on grouse moors?
A: No. Although it is often used on grouse moors, heather burning is also carried out for livestock grazing on moorland, as well as other types of heathland. A recent study in Scotland looked at 26 estates and found that heather burning occurred on 23 of these, although grouse shooting was only the main land use on ten. The others stated their predominant management was for deer stalking, sheep grazing or conservation, with the specific conservation objectives set by the individual estates. Those estates that managed for grouse shooting had 15% of land managed by burning per year, compared to 5% of land on other estates.

Q: Is there an alternative to heather burning?
A: Moorland vegetation can be maintained as a patchwork of heights and densities by burning, grazing and/or cutting. Grazing alone can sometimes be difficult to manipulate between too little and too much, but it is an important management technique used alongside burning or cutting. Cutting requires low slope angles and smooth terrain to avoid machinery damage or damage to the vegetation.

Where access is possible cutting may be a valuable tool in areas of high fire risk or fire impact, but care is needed to avoid compacting the ground with machinery. Where rainfall levels are high and there is little risk of tree and scrub encroachment, heather can spread naturally by a process called layering, where stems touch the ground, root and produce a canopy of younger shoots without intervention.

Q: Why can grazing be difficult to control?
A: Livestock tend to congregate in certain areas on moorland, for example more sheltered areas in bad weather, or those where the available vegetation is more palatable. This means that these areas can become overgrazed, leaving other areas under-grazed. Grazing is best used for vegetation management when livestock is well shepherded, or fenced, into the areas which need to be grazed at that time.

Heather burning and biodiversity

Q: What effect can heather burning have on moorland biodiversity?
A: A 2013 report by Natural England (NE) examined much of the scientific literature related to burning on peatlands, to which we refer heavily in this section, along with a comprehensive report to  the Scientific Advisory Committee of Scottish Natural Heritage (SNH) published in 2015. Most studies examined in the NE report indicated an overall increase in species richness or diversity when burning was considered at a whole moor level. Because burning takes place in small areas leaving the majority unburnt in any given year, a mixture of habitats is produced which can support a wider variety of species.

An article examining moorland sites in Scotland over 44 years concluded that without burning, plant diversity decreased and stated “to maintain diversity, timely burning is recommended”. However, as with any management technique, it is important that heather burning is done responsibly, according to best practice. The report to SNH noted that much of the conservation benefit from burning depends on local site management and conditions.

Q: Can burning affect bird numbers?
A: As with any land management intervention, heather burning influences the habitat. Food quality, structure and composition are affected, therefore so are the number and diversity of species that live in the area. Some species are more abundant where there is burning compared to where there isn’t. Curlew have been shown to be more abundant as the percentage of recently burnt ground increases, though this effect isn’t clear in all studies.

Golden plover prefer to nest in areas of short vegetation typical of sites where burning has recently occurred. Benefits have also been shown for capercaillie where burning was used as a management tool in open pine stands with ground vegetation that includes bilberry, but is dominated by heather. While some species seem to benefit from burning, those preferring longer, older heather, scrub or grassland may be disadvantaged, for example stonechat, whinchat, hen harrier, merlin and skylarks.

Q: What would happen to the vegetation if the heather were not burnt?
A: It depends on the environment. Moors tend to be grouped together when discussing these complex issues, whereas in fact there are a range of different habitat types, including blanket bog/deep peat, and heather-dominated dry heathland. For example, if heather was not managed on dry heathland, it would become old, degenerate and ultimately be lost. Scrub and tree regeneration would gradually occur, and would progress to a vegetation community of shrubs, bushes and trees.

Over blanket bog, this succession may be slower or not occur at all. Each moorland site is different, and vegetation responses depend on many factors, including altitude, rainfall, typical wind conditions, and grazing management. On sites where heather is flattened by heavy rain, wind, snow or gravity, natural layering may occur, which can allow other plant species to grow up through the opened heather canopy. The amount of grazing on the moor will also play a significant role in how the habitat might change.

Heather burning and peat formation

Q: What is blanket bog?
A: Areas that experience high rainfall and low temperatures, usually at high altitude on shallow gradients, with ground that is waterlogged for most of the year, can produce areas of blanket bog (sometimes known as blanket mire or blanket peat) where a significant peat layer covers the landscape. Blanket bog has a particular mix of plants, usually found where the peat is saturated. In England, blanket bog is defined by a minimum peat depth of 40cm; in Scotland this cut-off is 50cm. Many of the concerns around the effects of heather burning focus on the impacts on blanket bog and its ability to store both water and carbon.

Q: How is peat formed?
A: All vegetation can become peat in the right conditions, but certain plant species appear particularly good at forming peat. These include certain mosses and sedges, which grow well in waterlogged conditions. The low oxygen content of the saturated soil on which these plants grow prevents their dead material from rapidly decomposing. Instead, the plant remains are slowly compressed as more dead material falls each season.

These layers of matter build up and eventually turn into peat. The peat is deepest where wet conditions are maintained (deeper areas of peat take thousands of years to form), and shallowest where the climate is drier and ground conditions more free-draining, factors which vary with relation to slope.

Q: Does heather burning prevent peat formation?
A: No. Recent research has shown that peat formation can still continue when managed burning is used on moorland and heather burning may be beneficial to some species that are considered peat-forming. Three recent papers suggest that where the interval between prescribed burning is shorter (10 years), the extent of Sphagnum cover increases. Ground under a ten-year burning rotation was found to have similar amounts of Sphagnum to areas that had not been burnt for 60 years, both of which had more Sphagnum than sites under a 20-year rotation.

One paper concludes that it found “no evidence to suggest that burning is deleterious to peat-forming species; indeed, it was found to favour them”. However some studies indicate that the rate of peat accumulation may be slower where managed burning is used and others have found burned areas had less Sphagnum and more heather, although the difference was small. Any burning will cause damage to the vegetation, but it is the nature of that damage and the vegetation response to it which is important. Some evidence shows that where moorland is wetter, this can mitigate any damage caused to the moss and litter layer by prescribed burning. The evidence is conflicting and further research is needed.

Q: How can heather burning encourage Sphagnum?
A: Though burning can slow Sphagnum growth in the short term, fire removes the dense heather, grass or sedge canopy. As long as conditions are suitable, for example the ground being wet enough, Sphagnum can then thrive because of the increased light and reduced competition.

Q: Why is peat burnt?
A: Burning peat is never the intention of moorland managers. Prescribed burning of moorland is sometimes mistakenly or deliberately called peat burning in the media. In controlled muirburn (which aims for a ‘cool’ burn) the plant canopy is burnt, with little impact on the ground layer and underlying peat. Peat maybe damaged by a ‘hot burn’, where conditions are more typical of unmanaged or wild fires. If the peat itself is burned, it can cause very severe damage and loss of carbon. The Scottish Fire Rescue Service (SFRS) is now actively promoting heather management, including the use of controlled burns to reduce the fuel load and prevent wildfires that can cause more damage.

Q: What do you mean by ‘cool’ and ‘hot’ burns?
A: ‘Cool’ burns pass quickly over the ground, burning the above-ground vegetation but having little impact on the humus or litter layer that sits on top of the peat, or on moss growing on the surface. The temperature at ground level remains low and typically is barely raised below the soil surface. Achieving a cool burn requires knowledge and experience. Practitioners need to ‘read’ the ground conditions – how much burnable fuel there is, the wetness of the vegetation and ground layer, the wind speed and direction, slope angle, sun strength and air humidity to ensure that the burn passes over the vegetation quickly, has minimal impact on the ground layer, and can be contained within the area of habitat to be burnt.

‘Hot’ burns occur where the fire is slow moving, there is a large amount of fuel or the weather is hot and dry. These burns deliver more heat to a patch of ground for longer, affecting both growing plants and plant debris. This can result in damage to or ignition of the underlying peat, temperatures becoming higher still, and greater difficulty in controlling the fire. Such hot burns can result from managed burns being done in the wrong conditions or being mismanaged, or from wildfires breaking out.

Heather burning and carbon storage

Q: Why is peat important for carbon storage?
A: Peatlands store around 30% of the world’s soil carbon, with UK moorlands alone storing about 3,000 Megatonnes. 13% of the world’s blanket bog is found in the UK. Introducing fire can affect the carbon dynamics of any system, so it is important to improve our knowledge of the effects on carbon storage when heather burning is carried out over blanket bog.

Q: How does peat store carbon?
A: The growth of moorland plants takes carbon dioxide from the atmosphere and incorporates that carbon into the vegetation. When this vegetation dies, if the plant forms layers of peat instead of decomposing (the process of rotting would release much of the carbon again), this carbon is retained and stored within the peat.

Q: Why is this important?
A: The amount of carbon that we release into the atmosphere is an important factor in global climate change. Carbon dioxide emissions, and how carbon can be captured from the air and locked away are increasingly important in our efforts to combat human-induced climate change.

Q: Does heather burning release carbon?
A: All fires release carbon into the atmosphere. Each heather burn will release a small amount of carbon. However, reducing the available fuel load with the use of controlled fire also reduces the risk of wildfire, which carries the much greater chance of a large carbon release.

Q: Does heather burning prevent carbon storage on moorland?
A: No. Recent research shows that carbon is still taken up and stored on moors on which managed burning is undertaken. This paper supports others which have found that carbon uptake was slightly lower when comparing areas of no burning with areas that had been burned six times in the 60 years, but the difference was small.

Q: What is the overall effect on carbon storage?
A: The “carbon budget” of moorland (the total amount of carbon taken up/released, giving an overall estimate of whether the area captures or releases carbon) and the effects of land management on this remain unclear. This is an area of active research. One study found that the carbon released as a result of prescribed heather burning over a cycle of 15-20 years accounted for less than 10% of the total carbon lost from the system over that time. Although carbon is released with heather burning, the authors concluded that careful burning management at that site did not have a major detrimental effect on the overall carbon budget for the moor.

This known limited loss of carbon associated with prescribed burning may reduce the risk of wildfire, which carries a much greater chance of huge carbon releases. However, another study also on blanket bog in the North Pennines found that areas managed with prescribed burning release less carbon overall than those which were unburnt. This is a complex area of science and there are many other considerations, for example the role of producing charcoal through prescribed burning. Charcoal is harder to break down than peat or vegetation, which may mean it can have a positive effect on long-term carbon storage, but this remains a contentious area. Research into carbon storage is ongoing.

Heather burning and water

Denbigh bog flushQ: Why is water quality important?
A: Upland areas provide around 70% of the UK’s drinking water. As well as all of us wanting to drink clean water, the EU has set standards for water quality that go beyond its purity and safety to include its colour. Water from peatlands is naturally discoloured as a result of draining through the peat, so water companies must treat water from peatlands to meet these standards.

Q: How does heather burning affect water quality?
A: This is still being studied, with different pieces of evidence suggesting different effects. There is evidence that in England burning may be associated with increased water colour, with some sources equating this to an increase in dissolved organic carbon (DOC) in the water. However, one study showed that DOC did not rise in response to burning, and the colour of water is not always a good indicator of DOC. Another showed that DOC in the nearby lake fell following a wildfire. The picture is not clear.

Q: Does heather burning on grouse moors increase flooding?
A: Although there are relatively few studies available, the authors of a Natural England report could not find any evidence that burning increases flood risk, and state that: “No evidence was identified specifically relating to the effect of burning on watercourse flow or the risk of downstream flood events. If there are any effects, these are likely to be highly site specific”.

A recent study examining the effect of rotational burning on deep blanket peat sites drew several conclusions. These include that the lag time to peak runoff is increased on burnt sites for most rain conditions – meaning that the movement of water is slowed down across areas managed with burning – and that, for the heaviest 20% of storms, the lag time is the same but that the peak flow is higher from burnt compared to unburnt catchments.

Although this study is often cited, there are flaws in the experimental design which have led to the findings being questioned by other academics. Once more, the evidence base regarding a possible impact of prescribed burning on flood risk is very limited and inconclusive, particularly when considered in the context of moorland drainage, the impact of which on flood risk is also very little studied.

Q: Why is the impact of burning on water not yet fully understood?
A: Results differ depending on the length of time since burning, and the scale at which the studies are performed. Effects may be different at smaller, local scales, compared to the larger, catchment or landscape scales. The possible effect of burning on water quality and amount of run-off is also complicated by interactions with other upland management, such as woodland expansion and grazing. These interactions have been relatively little studied.

Heather burning and wildfires

Q: What causes wildfires?
A: Evidence looking at the causes of vegetation fires is very limited. The Fire Service Incident Recording System does not include cause or source of ignition, unless an investigation is conducted, which is very rare for vegetation fires. Therefore, the relationship between the use of prescribed fire and the frequency and extent of wildfires on moorland remains unclear. This is an area that needs more research. However, evidence does show that increased visitor numbers significantly increase wildfire risk in the Peak District National Park, with more fires starting at the weekend or on bank holidays than on weekdays.

Q: What are the differences between prescribed fire and wildfire?
A: Prescribed burning is carried out in winter or early spring, and aims to achieve a ‘cool burn’. However, wildfires tend to occur in spring or summer and are mostly accidental or caused by arson. They may cover large areas and burn with far greater intensity and severity, sometimes consuming all the available fuel above ground (fuel-load), as well as significant amounts of underlying peat.

Q: How does prescribed burning affect wildfire risk?
A: The evidence is mixed, with one study finding that heather burning can reduce wildfire risk in the Peak District, where grouse moor management is associated with a lower frequency of wildfire. However, there is also evidence that sometimes prescribed burns are not adequately controlled and can lead to wildfires. There is evidence across the world for the benefits of prescribed burning in reducing wildfire risk, but there are not enough studies specifically referring to the UK moorlands, and experts call for more research.

Prescribed burning can play an important role in wildfire management. By decreasing the accumulation of old, woody heather, which can build up to a large stock of potential fuel, it can reduce the likelihood and intensity of wildfire. In this way, prescribed burning is used to create fire breaks, which hinder the spread of wildfire. Specifically, it may:

  • Reduce the risk of a fire starting, for example around areas with lots of public access.
  • Prevent a wildfire spreading, by breaking up extensive areas of high, dry vegetation.
  • Protect particular landscape features that may be damaged by wildfire, again by using controlled fire to create firebreaks around them.

The Scottish Fire and Rescue Service supports land management that reduces fuel load, such as burning, cutting or grazing. The growing fuel load in the countryside is an increasing concern for them, as any fire that does occur will have a ready source of fuel and will spread quickly and exhibit extreme fire behaviour.

Q: What are the consequences of wildfires?
A: The impacts of wildfire are not always predictable. Severe (slow, hot and very large) wildfires can lead to huge carbon release, peat damage, reduced air quality and habitat destruction. There can be a cost to the public purse with extensive and prolonged use of the fire and emergency rescue services in difficult to reach areas and a high demand on volunteer control efforts from local moorland managers. However, less severe wildfire can have little or no lasting impact on habitat, environment or wildlife. Public policy suggests it is best to prevent fires with a combination of controlling fuel loads and a variety of social measures.

Q: Is there a consensus amongst scientists regarding the role of fire for moorland management?
A: No. In a complex and controversial field, there are inevitably differences of opinion. There are many reasons for this, including the interplay of different factors, and the complexity and variety of habitats under consideration. A simple answer to questions about the effects of heather burning is rarely available, and a balanced review of the facts often reveals a more complex picture.

This was highlighted by a series of papers in 2016, the first of which was by 13 authors calling for informed, unbiased debate on the role of fire in moorland management. This prompted two response papers, and then an answering paper from the original authors. The scientific debate continues.

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