The Role of Fire in the Ecology of Heathland
in Southern Britain
(IFFN No. 18 – January 1998, p. 80-81)
Heathlands dominated by ericaceous dwarf shrubs are widespread on acid nutrient-poor soils throughout those regions of north-western Europe with an Atlantic climate. They developed c. 4000 years ago following forest clearances and have been maintained by grazing, burning, turf (sod) cutting, and the gathering of vegetation for fuel. These activities arrest succession to scrub and woodland (Webb 1986). Heathland is burnt to improve the forage for grazing stock but some fires are uncontrolled and wildfires may occur.
Heathlands show some of the characteristics of a fire climax (Gimingham et al. 1979). The dominant species Calluna vulgaris regenerates freely from the stem bases when burnt and the germination of its seeds is promoted by heat. Regular burning reduces the floristic composition of heathland and species which regenerate from underground organs or rapidly from seeds (eg. Calluna vulgaris, Erica tetralix, Vaccinium myrtillus, Deschampsia flexuosa, Molinia caerulea, Scirpus cespitosus and Pteridium aquilinum) tend to dominate. Species such as Juniperus communis are eliminated by regular burning (Gimingham 1972).
During heathland fires about 95% of the nitrogen and 20-30% of the other principal plant nutrients in the standing crop and accumulated litter are lost from the system. Losses of Na, K, Ca and Mg can be replenished from precipitation within a few years but the losses of P and N cannot. Phosphorus is held in the soil organic matter, and where the adsorption capacity is low, some of the P released during the fire is lost through leaching. It takes c. 20 years for P to be replaced (Chapman et al. 1989). The nitrogen budget is not fully understood. Because losses during a fire cannot be made up through rainfall, nitrogen-fixing plants (Ulex spp.) may be important (Chapman and Webb 1978).
Where the fire has not been too hot Calluna regenerates from the stem bases and within three years enters the building phase. If the roots are killed regeneration is from seed giving a true pioneer phase, but taking longer before the building phase is reached. Over the 30-40 years of heath growth production increases but declines from about 20 years and onwards. The structure of the vegetation affects the microclimate during this process. At first, the canopy is open and the soil and litter surface dry with extremes of temperature. During the mature phase when the canopy is more or less complete, humid, still conditions with small fluctuations of temperature prevail. As the canopy opens during the late mature and degenerate phases more extreme conditions occur again.
Heathland managed by burning consists of stands of a uniform age where the bushes all have the same structure. Other forms of management create mixed-age stands where bushes of different structure grow side by side. The invertebrate fauna is dependent on the structure of the Calluna bushes and there is a positive relationship between invertebrate diversity and the structural diversity of the vegetation. For some ground living species (eg. ants; Hymenoptera: Formicidae) this relationship may be negative as the developing vegetation canopy reduces insolation. After a fire there is a well-marked succession of species, with some species being characteristic of the early stages and others characteristic of the mature and degenerate phases. Spiders (Araneae) and ground beetles (Coleoptera: Carabidae) are typical examples. The soil fauna is dependent on soil moisture and the presence of plant litter. After a fire the development of the fauna is closely associated with the recovery of the heathland vegetation (Webb 1994). During a fire soil temperatures are <45° C because of the good insulating properties of the litter layer. Few animals are killed by the fire; however, populations decline rapidly once the vegetation canopy has been removed because the litter becomes very dry and blows away. A new layer begins to form only when the plant canopy closes c.10 years after the fire.
Heather moorland in the north of England and Scotland is burnt every 12-15 years to provide nutritious young heather shoots for sheep and grouse. Grouse moors are burnt in strips because grouse require young heather for food and taller old heather for nesting and a supply of invertebrate food for their chicks. The management aims for these moors are very clear, but for lowland heathland conservation Chapman and Webb (1978) have suggested burning every 20 years. This cycle matches the replacement rate of nutrients, particularly P, by rainfall. Although the 20-year cycle matches nutrient inputs, too large a fuel load develops causing hotter fires and affecting plant succession. Fire temperatures depend more on conditions (moisture, rainfall, wind) at the time of burning, than on fuel load (Allchin et al. 1996).
Accidental fires are common with peaks in their numbers during holiday periods in April and August. Most of the accidental fires occur near the urban areas and because of this there has been a reluctance to use controlled burning for heathland management. Strict fire protection measures have been implemented over the last 20 years and in Dorset the area burnt has declined from 1071 ha in 1978 to 451 ha in 1987. Until recently, when grazing has been introduced, fire has almost been the factor controlling succession. Because of the decline in burning the extent of scrub increased by 15% between 1978-87 (Webb 1990).
In the very hot dry summer of 1976 eleven percent of the Dorset heathlands was burnt. A landscape scale analysis over the period 1978-1987 (Bullock and Webb 1995) showed that neither the extent nor the composition of the principal heathland types was affected by these fires. The only long-term effect was in the species composition of scrub. The fires had conserved the dynamic mosaic of the heathland vegetation types by preventing the succession of heathland to scrub and by reducing the cover of woodland. At large temporal and spatial scales the heathland landscape remained stable despite catastrophic disturbance at specific locations.
Allchin, E.A., P.D.Putwain, A.M.Mortimer, and N.R.Webb. 1996. Burning heathland for management: fire temperatures and vegetative regeneration. Aspects of Applied Biology 44, 407-412.
Bullock, J.M., and N.R.Webb. 1995. Responses to severe fires in heathland mosaics in southern England. Biological Conservation 73, 207-214.
Chapman, S.B., R.J.Rose, and M.Basanta. 1989. Phosphorus adsorption by soils from heathlands in southern England in relation to successional change. J. Applied Ecology 26, 673-680.
Chapman, S.B., and N.R.Webb. 1978. The productivity of Calluna-heathland in Southern England. In: The Ecology of some British Moors and Montane Grasslands (O.W.Heal and D.F.Perkins, eds.). Springer-Verlag, Berlin.