Heath and grassland fires – causes and effects
Like 1976, the exceptionally long, hot dry summer of 2022 saw unusually large and extensive heath and grassland wildfires in many parts of the country. With climate change promising that this hotter and drier weather is likely to become more frequent, it is to be expected that measures to both avoid and limit wildfires will become more pressing. Natural England has undertaken a detailed assessment of the causes and prevention of wildfires (Glaves et al, 2020).1 Its review focusses mainly on safety aspects with little on the ecological effects and utility of fires. Yet fire is potentially one of the most important and effective ways of manipulating ecosystems. A lack of sufficiently intense management of grassland and heathland areas has very likely been a factor in causing the recent wildfires. Grazing and mowing are often not enough to prevent the development of rank grass, leggy heather and scrub which are fuels ripe for ignition in prolonged dry weather. The increase in projects directed towards rewilding may create such conditions and result in a big problem for land managers (Pereira & Navarro 2015).2
Natural fire episodes and natural succession
Wildfires are nothing new. Charcoal in peat bogs shows they have burned since as far back as Mesolithic times. Even during the subsequent forest climax they were likely a factor maintaining the more open landscapes attributed to grazing animals by the Dutch ecologist Frans Vera. These fires occur because heathland and grassland plants are fire adapted. Their standing crop and straw litter is easily set alight and they can recover readily from burning. All over the world fire is a perfectly natural occurrence in these disturbance-dependent ecosystems. Twenty-three fires due to lightning were recorded in Galloway on two days in June 1970 (Thompson, 1971).3 When in the past there were vast unbroken tracts of heaths and grasslands, the likelihood was relatively high, both of a dry lightning strike coinciding with and igniting suitably dry vegetation, and also of it being followed by the fire then spreading. The small size and isolation of inflammable ecosystems now makes both the chances of lightning ignition and of the spread of such natural fires much lower. Fire return intervals at any particular place are thus much longer than they would naturally be, if indeed some places burn at all. In the USA prairie fires once ran for hundreds of miles. Now, with farmlands acting as firebreaks isolating remaining prairie fragments, like many small British heathland and downland nature reserves and open spaces, they are scrubbing over and changing to woodland (Sharpe 1989).4 Grazing and mowing can obviously, to some extent, control this natural succession. Yet, in the absence of burns, these agencies by themselves may not be enough to control either fire risk or the loss of wildlife in these ecosystems. Some of the reasons for this are argued later.
The lost art of swaling
Most fires now are either deliberately or accidentally set by people. Controlled management burns have a long history. It is speculated that the Mesolithic fires were made to concentrate game and facilitate hunting. American Indians certainly used fire to herd game; the Blackfoot tribe were named for their feet blackened by walking through the ashes of their burns. In Britain controlled firing of heaths and downs, or “swaling”, has long been traditional management practice to provide fresh “bite” of grass for sheep, grouse or other stock. Gilbert White5 (1760) records the firing of Woolmer forest:
“The plea for these burnings, is, that, when the old coat of heath, etc., is consumed, young will sprout up, and afford much tender browse for cattle; but, where there is large old furze, the fire, following the roots, consumes the very ground;”
Here White recognised that fire can have very different effects. Big fuel loads make intense fires and they often result from long intervals between fires. It is clear from the literature (Nicholson 1968)6 and I know from personal experience, that fire intervals of more than 15 years or so can make old, woody heather fires, very fierce and hard to control. Gorse fires must be far worse.
“Men are burning the gorse on the down’s shoulder; A drift of smoke glitters with fire and hangs, and the skies smoulder, and the lungs choke.” Up on the Downs. John Masefield.
Such hot fires are liable to kill buried heather stems, roots, seeds and soil animals, limiting regeneration. On heaths, purple moor grass, bracken, gorse, birch and pine then commonly invade the bare ground. If they do not, then erosion can ensue (Maltby,1980).7 On the other hand artificial fires which take place more frequently than natural fire episodes can prevent species obtaining dominance which would otherwise do so (Moll et al, 1980).8 In Britain heather swards can be thus converted to acid grassland.
In addition to fire frequency, the season of burning can also make big differences in the effects of a fire. Most accidental burns take place the spring or summer. The ground is then likely to be dry, the fire hot and regeneration poor. Controlled management burns are usually made in early November or late February and March when the vegetation is dry, but the ground still damp. The underground parts of plants, and seeds and animals which are then in the soil are relatively unaffected by such burns. Temperatures beneath a centimetre of soil can be as low as 45 degrees in an 800 degree fire (Webb, 1986).9 Regeneration of the original vegetation is usually rapid and provides the succulent new growth which pastoralists have always sought. In removing accumulated plant straw and letting in warm sunlight, fire creates space for many weakly competitive plants and animals. In some parts of the world, notably in South Africa, astonishing displays of wildflowers with bulbs and corms result after fires. In Britain silver studded blues, pale dog violet and marsh gentian populations do best in the open, warm conditions after heathland fires (Chatters, 2021, Thomas, 2007).10, 11 On the Akiyoshidai plateau protected area in Japan firing grasslands is specifically employed as a conservation measure to maintain some high brown fritillary butterfly populations. Their pupae survive in the soil with other animals and plants.
A further factor affecting the outcome of a fire is the wind. Fire managers choose conditions as calm as possible when to burn. Hot air rising from fires sucks in its own wind and in some situations like coombes and gullies which focus this phenomenon, the rapidly rising air can make burns very difficult to contain. Whilst downwind fires like this are hard to manage, they are generally not too hot and, because they skip rapidly over the ground, they tend to leave unburned patches which may be important for the survival of many species. Wildfires are usually like this and even summer ones can sometimes have surprisingly little, if any, damaging effects on heathland ecosystems (Bullock and Webb,1996, Nicholson 1968).12,6 Backburns into the wind are much easier to control, but are slower, hotter, and consume the sward more completely. To control fires on hilly ground, they should be ignited at the top to backburn downhill into the updraft.
Recovering downland diversity by fire management – a Kent example
In the 1970s the dense tor grass swards on Wye Downs in Kent were clearly suppressing the rich downland flora, despite licensed sheep grazing by local farmers. The farmers were naturally primarily concerned with the condition of their stock rather than that of the sward, so it was difficult to bring sufficient grazing pressure to control the rank grass. A warden, an old countryman, persuaded me that the grass needed to be burned. He argued that not only was this part of the traditional management system, but that if an area with such recreational pressures was not burned deliberately in the winter, it would sooner or later be burned accidentally in the summer. We thus burned small areas and successfully restored more species-rich short swards. Later, on with the Wye College estate, along with farm staff, I was involved in burning another part of Wye downs and able to make more complete records of the effects (Green, 1983).13
Parts of the thick tor grass sward were burned downwind in March followed by light grazing; the sheep mainly confining themselves to the more fertile grassland over superficial deposits at the top of the chalk slope. In August the burned swards bore only a quarter of the above ground standing crop of the unburned, most of the difference being attributable to the almost complete consumption by the fire of the dead tor grass leaf litter. The regrowth of the tor grass was also less vigorous, with the height of the sward only half of that formerly. Nearly twice as many vascular plant species (40) were recorded in three metre squared quadrats as before burning (24). Both oat grasses, glaucous sedge, thyme and ground thistles made particularly notable increases in frequency. The one species which declined was sheep’s fescue. Its vulnerability to fire has been recorded elsewhere (Lloyd, 1968).14 Further burning, with and without grazing, maintained the open sward leading to the reappearance of orchids not seen for many years and to increases in abundances of butterflies as their food plants like horseshoe vetch and bird’s foot trefoil spread. Chalk hill blue butterflies were successfully reintroduced.
Burning is an effective management technique, not only because of its physical removal of litter and setback of scrub and woody species, but also for the losses of plant nutrient elements it brings about. Up to 95% of the nitrogen and 26% of the phosphorus in the standing crop can be lost as oxides in the smoke (Chapman,1967).15 Potassium in the ash is soon leached from porous downland and heathland soils. This is important, for heaths and downs are naturally infertile ecosystems. Nutrients naturally accumulating in them from rain, dust and dung drive the succession to faster growing, bigger, more nutrient demanding and more competitive plants like rank grasses, bracken, scrub and trees (Green,1972).16 Currently elevated nitrogen levels in rain from pollution are speeding this process.
The key to managing heaths and our little western outliers of the great steppe grasslands is thus to maintain their infertility. In the past this was done incidentally by transhumance. Stock were grazed on the heaths and downs by day, then at night folded on to fallow fields to dung and fertilise the soil for the next year’s crop. As well as these nutrient drains and those in fires, nutrients were also pumped from these outfields in meat and wool and in harvested turf, peat, thatch, bracken, gorse and wood. These then were resources vital to the rural economy:
“Lo! where the heath, with withering brake grown o’er, lends the light turf that warms the neighbouring poor;” The Village. George Crabbe.
Downland turves were also cut and burned to fertilise the croplands by “denshiring”. In the Netherlands heathland turves were similarly used to fertilise “plaggen” fields. Some heaths and grasslands were exploited to near deserts. Their characteristic species survived, not despite this thoroughgoing destructive treatment, but because of it. For this sort of rough activity and disturbance, one needs to look no further than the armed forces, so it is no coincidence that some of our best heaths and grasslands survive on MoD training areas. The gentle, enclosed grazing, or cutting, of most heath and downland areas by timorous nature reserve managers today rarely has the same effects; it generally increases, or recycles, rather than removes nutrients. In very large areas it may work, because stock then tend to concentrate nutrients in restricted latrine areas. The bright green of nettles, thistles and brambles marking old rabbit warrens still stand out in some places. The more intuitively satisfying use of feral rather than farm stock is probably less effective, for their biomass and its nutrient load are not generally extracted for food or fibre.
Before myxomatosis I remember one evening disturbing a field of hundreds of rabbits that scampered back to their burrows like a brown carpet being rolled back. They left stretches of sparsely vegetated white chalk gleaming in the moonlight. When rabbits were almost eliminated in many places by myxomatosis, the bare white downs shown in Tansley’s photographs in The British Islands and their Vegetation soon recovered to species-rich swards in which orchids and other poor competitors like juniper exploited the open conditions. As the swards closed, many disappeared again. A period of very heavy grazing like this to open up the sward, followed by a year or two with light stocking is what is probably required to allow the establishment and survival of many species. Mowing can sometimes be effective, provided the cuttings are removed; but in large areas their collection is difficult and disposing of them problematic. Mowing moreover, like grazing, rarely removes much thatch from old swards and so lacks the cleansing effects of a burn. A study in the New Forest showed that fire was much more effective in maintaining heathland biodiversity than swiping, or even baling the cut material.
Distinguishing controlled burns from accidental fires
Conservation managers are reluctant to burn protected and amenity areas for many good reasons, including the following:
First, fire is almost universally and justifiably, seen as destructive and feared: there is also the worry of burns getting out of control; of fires being seen as vandalism; or of encouraging others to arson.
Second, some losses of animals, especially invertebrates like butterfly pupae from taller swards, are inevitable.
Third, fire adds to atmospheric carbon dioxide.
Careful rotation burning of small patches, surrounded by firebreaks and with adequate numbers of firebeaters attending, can overcome most of these problems. Wildfires consuming big standing crops of vegetation are likely to release more carbon dioxide than small managed burns. Perhaps though, what most discourages the use of prescribed burning, is a failure to see that its results are very different from those of uncontrolled wildfires and that it is an important safety measure. Crucially fire eliminates dead plant litter and other accumulated combustible material so that it takes some time before a second fire is likely.
Fighting fire with fire
It is ironic that many conservationists’ fears of fire has led not only to its sad neglect, but also to additional legal regulation of fire management, just at a time when it is needed more than ever as the most effective way of preventing accidental burns. Climate change is already causing disastrous fires everywhere, which, without protective measures, can only increase. Controlled burning is also the easiest, most effective and most economical way of managing heathlands and grasslands. Whilst in Britain careful prescribed burning is still standard management practice in some places, notably in the New Forest and on grouse moors, elsewhere lack of experienced staff is likely another reason it is not practised. In the past reserves and amenity lands were generally managed by trained people who had come from farming or forestry. They tended to take a rather more robust whole ecosystem approach to management than the pussyfooting of many managers today, perhaps too preoccupied with rewilding and habitat gardening recipes for particular species. Instead of fearing and condemning prescribed burning, countryside managers should perhaps be seeking to learn from experienced fire managers, training courses are available through NatureScot, the University of Exeter and private organisations. Those who have used and studied heath and grassland fires have no doubts about their efficiency:
“A heath long unburnt tends to be a very dull place and heathland managers who will not use burning for management bear a heavy burden of guilt for declining biodiversity in our heathlands. This is as true of the heathland animals as it is of the plants. ….Mowing is no substitute and grazing alone does not have the same effect.” Proctor (2013).17
“In the absence of grazing periodic spring fires in grassland communities have the effect of maintaining the floristic diversity of the communities and checking scrub invasion. Only rarely do fires appear to be detrimental to the communities in which they occur.” Lloyd (1968).15
“Grassland managers, from private ranchers to agency directors, should be “encouraged” to experiment with fire and learn to use it as an effective tool. It is time that we realise that ‘playing with fire’ will not necessarily lead to getting burned and just might help us to relearn a lost art and gain a powerful and natural tool.” Vogl (1974).18
“Burning still remains the simplest and most effective way of maintaining heathland.” Webb (1980).19
A report published by the Future Landscape Forum as this paper was being processed for ECOS is reported to have come to similar conclusions as this article.
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I am grateful to Clive Chatters and Rick Minter for their helpful comments on this paper.
Bryn Green is Emeritus Professor of Countryside Management, the University of London. Previously Nature Conservancy Council Regional Officer SE England, and Lecturer in the Department of Botany, the University of Manchester.