The Plain Truth About Wildfires

 

Chalk downland has three available trajectories, and only one preserves the habitat as designated. It can be grazed to a standard. It can burn. Or it can be under-grazed, in which case succession returns it to coarse grass, scrub and ultimately wood — though the route to wood runs through a long phase of high fire hazard, so the third option tends to collapse into the second. The largest chalk grassland in north-west Europe has been demonstrating what the second produces for the better part of a century.

The habitat is semi-natural, a product of clearance and sustained grazing rather than undisturbed succession. The sheep-and-fold system held the downs open by grazing the hill and folding the flock on valley arable, exporting nutrients from hill to field year on year. That impoverishment is the basis of the botanical interest: low herbs persist because nothing vigorous can establish to shade them out. The habitat therefore has no equilibrium of its own.

Prescriptions have shifted over the last two decades from a uniformly short sward with zero tolerance of scrub toward structural mosaics, on the sound grounds that short turf serves a narrow suite of species and little else. Marsh fritillary and Duke of Burgundy, both notified features of Salisbury Plain, require tall tussocky vegetation and successional scrub margins. Structural diversity, however, means standing dead material — upright brome and tor-grass, unpalatable past early summer, accumulating a dense basal layer annually, and favoured by nitrogen deposition into a system whose interest depends on being nutrient-poor. In spring the taller sward retains moisture; in drought the same material cures into fuel.

The uncontrolled experiment

Salisbury Plain Training Area holds roughly 390 square kilometres of chalk under military control since 1897, with the SSSI, SAC and SPA covering 19,690 hectares and notified for twenty-five features. Grazing is logistically constrained: stock must be enclosed and moved around firing schedules, and research on the Plain has largely had to work on ungrazed swards. In the impact areas the constraint is absolute. Unexploded ordnance excludes the fire service from range danger areas, and the standing position on impact-area fires is to suspend firing, monitor, and let them burn out; water cannot be dropped directly, since low flying risks detonating ordnance by heat. Figures obtained under Freedom of Information put wildfires linked to MoD training sites at 1,178 between 2020 and August 2025.

Fire as a management agent

Natural England's Integrated Site Assessment of the SSSI (2014–15) records that wildfires in the Central Impact Area help produce species-rich swards in places by removing thatch build-up, and that Centre for Ecology & Hydrology work there has shown pronounced cyclicity in devil's-bit scabious — the marsh fritillary's foodplant. The feedback is straightforward: undergrazing accumulates dead material, dead material carries fire, fire removes it, herbs recover. The revegetating shell craters produce short-sward grassland supporting rare plants including early gentian, and are thought to act as fire refugia for marsh fritillary. The ordnance provides the firebreaks.

Fire and grazing are not equivalent instruments. Grass fires remove biomass without discrimination — work on grassland invertebrates on the Plain characterised burning and mowing as indiscriminate in what they take, against the selective defoliation a grazing animal provides. Fire takes tussocks and anthills along with the thatch, and anthills do not resprout, nor do the twelve nationally rare and scarce bryophytes for which the Plain is notified, nor the lichens. Woody growth, by contrast, is only top-killed: hawthorn and blackthorn survive at the root and return.

What the regime selects for

The same assessment describes the fires as an unreliable and potentially hazardous management tool that possibly promotes tor-grass. It records tor-grass as having increased substantially on the Central Impact Area since the mid-1990s, with no control available other than grazing and vehicle compaction along tracks; a CEH aerial-photograph study commissioned alongside it found the extent of Brachypodium rupestre significantly expanded there over the preceding decade. Natural England classes tor-grass as a negative indicator species.

Tor-grass is rhizomatous, unpalatable, litter-forming and a vigorous resprouter — a grass optimised for precisely this regime. The trajectory in the impact area is therefore not toward scrub but toward a fire-maintained monoculture: structurally open, botanically impoverished, and self-sustaining, since each burn returns the ground to the species best placed to reoccupy it.

This is the grass–fire feedback operating as expected. Fire is a weapon of grasses, implicated in the expansion of open grassy biomes through the late Miocene, but a weapon for grasses carrying the requisite traits — and on the chalk the grass holding it is the one under suppression. The prairie and steppe analogy fails for a further reason: those systems are grassland because the climate will not carry closed forest. The Wessex chalk sits within the woodland envelope, which is why the downs require management at all, and lightning in oceanic Britain largely arrives with rain. The chalk flora carries no fire-adapted traits of consequence — no serotiny, no fire-cued germination. Against the 2002 condition assessment, the proportion of grassland parcels passing all mandatory attributes rose from 43% to 62%, an improvement Natural England attributes principally to scrub management rather than to any natural process.

Prescription, not abandonment

The term for the third trajectory matters. Withdrawal of grazing on the chalk is not usually abandonment in the economic sense; it is frequently prescribed. On the Plain, grazing runs through Farm Management Plans with some forty-five tenant farmers, which require long grass margins, specify maximum rather than minimum stocking levels, and in special-restriction areas limit grazing to a mapped proportion of the area per year. The basic stocking rate on the chalk grassland is 0.3 livestock units per hectare per year. Natural England's own assessment notes that the plans emphasise avoiding over-grazing but do not adequately emphasise avoiding under-grazing or litter removal — and it identified under-grazing as the principal threat to the chalk grassland, flagged on 83 of the 282 parcels surveyed.

The standing fuel load is therefore not simply what happens when farming retreats. It is in substantial part a consequence of prescription, and graziers reducing numbers are often doing what they have been asked to do.

The transitional hazard

Reduced grazing is no safer a route than fire, for structural reasons. Succession from open sward to closed canopy passes through a phase carrying both fuel types at once: a continuous fine fuel bed of rank grass and litter, and above it establishing hawthorn, blackthorn and dogwood holding fine dead material. Fine fuel governs ignition and rate of spread; woody fuel governs intensity and residence time. The transition holds the maximum of both, and the hazard falls away only at canopy closure, when shading suppresses the sward and fine fuel continuity breaks. On chalk that window opens once scrub establishment is underway, which is the phase under-grazed downland and much recent rewilding now occupies.

The general finding is well supported: withdrawal of grazing drives woody encroachment, encroachment increases flammable biomass, and increased fuel load raises both wildfire probability and intensity, with herbivore grazing identified as the corresponding mitigation.

The choice

Grazing to a standard maintains the notified interest, at cost and with continual attention. Under-grazing leads to woodland, but only across two decades of elevated hazard during which the transition is liable to be interrupted — and interruption returns the ground to the fire-adapted grass. Fire holds it open indefinitely and without expense, and converts it to something structurally similar and botanically far poorer.

Fire is not a third route to preserving chalk grassland but a third outcome, and the one that arrives by default. That it arrives partly by prescription is the part worth examining.