Rewetting Protects the Peat. Controlled Burning Protects the Moor

A moorland fire is really two fires — one underground, one on top. They are different problems, and they need different tools. Here's why a resilient upland needs both.

Quick summary

• Moorland wildfire comes in two forms: fast flames above ground, and slow smouldering within the peat itself.

• The smouldering fire does the lasting damage — burning for months and releasing carbon stored over thousands of years.

• A hot surface fire is the fuse: it dries out and ignites the peat below it.

• Rewetting protects the deep peat; cutting and burning tackle the surface fuel — two problems, two tools.

Most moorland wildfires look like this: a wall of orange running through the heather, smoke rolling off the hill, crews beating at the edge of it. That fire is real, and it is frightening.

But it is not usually the one that does the lasting damage. The fire that destroys a peat bog is the one nobody can see — a slow, flameless burn creeping through the ground itself, sometimes weeks after the visible flames have gone.

Telling these two fires apart is the single most useful thing anyone can do before joining the debate about how we protect our uplands, because they are two different problems, and they need two different answers.

The fire you see

A surface fire is fast. It burns the living and dead material above the ground — the heather canopy, purple moor-grass, gorse and the dry litter between them — and it is driven hard by wind and by how dry those fine fuels are.

In heavy, unmanaged stands the flames can reach several metres and burn hot, with peak temperatures around 800°C in mature heather.

But a surface fire is also brief at any one spot. Because the heat radiates upwards and outwards as the front moves through, the ground beneath it is only heated intensely for a matter of minutes — typically on the order of ten to fifteen.

Pass a thermometer under a well-behaved fire and the soil a few centimetres down barely warms. This is why a fast surface fire can scorch the canopy and still leave the seedbank, the moss layer and the peat below largely intact. It looks dramatic, and it can be dangerous to people and property, but on its own it is a shallow event.

The fire you don't see

The fire that matters for carbon is smouldering combustion — and it behaves in almost the opposite way. Smouldering is flameless, low-temperature, incomplete burning of the organic soil itself. It creeps rather than runs, advancing perhaps a centimetre or two an hour, two orders of magnitude slower than surface flame.

What makes it so destructive is not speed but persistence. A smouldering peat fire can burn for months. It can survive heavy rain, over-winter beneath the snow and re-emerge the following spring to set light to the surface all over again.

The cost is measured in centuries. Burnt peat can take hundreds of years to build back, where heather regrows in around twenty — so on any human timescale a peat fire is a near-permanent loss.

And the losses are large: recent peer-reviewed research found that peat fires, though they account for only about a quarter of the UK's burned area, have caused up to 90% of the country's fire-driven carbon emissions since 2001, with the 2018 Saddleworth Moor fire alone emitting an estimated 24,000 tonnes of carbon.

The visible fire burns this year's growth; the invisible one burns thousands of years of it.

How one lights the other

Here is the link that ties the whole debate together. Smouldering does not usually start on its own. Wet, mineral-rich peat resists it — and ground made up of more than about 80% mineral content cannot sustain a smoulder at all.

What sets a peat fire going is a surface fire sitting on top of it for long enough to dry out and ignite the upper layer. The bigger the fuel load above ground, the longer and hotter that surface fire lingers in one place, and the greater the chance it drives enough heat down to overcome the moisture in the peat.

In other words, the flaming fire is the fuse and the smouldering fire is the charge. We cannot switch off the weather or stop every ignition — but the fuel above ground is the one part of this we can directly manage, and it is the fuel that decides how long and how fiercely the fuse burns.

Two fires, two jobs

This is exactly why rewetting is essential, and exactly why it cannot be the whole answer. The two tools we argue about are not competitors. Each does a job the other cannot.

Rewetting is the defence against the deep fire

Raising the water table is a powerful way to stop the slow, smouldering burn that destroys peat.

Wetter peat is far less able to sustain a smoulder, and over time a healthy surface of living Sphagnum moss — which holds water at many times its own dry weight, keeping its own moisture far above the point at which the moss layer could burn — becomes both a poor fuel and a reservoir.

A recent study across 27 Scottish wildfire sites found that wet blanket bog and wet heathland genuinely resisted severe burning where the ground had become wet enough.

That resistance is real, and it is one of the strongest reasons to keep restoring peatlands.

What rewetting takes years to decades to achieve, though, it cannot deliver overnight — and it does nothing to the fuel standing above the water.

Managing the fuel is the defence against the surface fire

The canopy dries according to its own moisture, not the water table metres below it: fine heather fuel ignites readily once its own moisture drops below around 60%, and that can happen on a bright, breezy spring day even over saturated or frozen ground.

As Natural England's chief scientist put it in evidence to Parliament, a wet peat substrate does not necessarily stop a wildfire taking the surface vegetation.

A rewetted bog still carrying rank heather and dense moor-grass litter still has a continuous, flammable fuel bed — and, as we have seen, that surface fire is how the deep peat gets lit in the first place.

Cool controlled burning and cutting, where possible, are the tools that break up and reduce that fuel.

Wetting the ground and managing the fuel are answers to two different questions.

Why this matters

Wet ground and managed fuel are not competing philosophies but two tools for two jobs — one protects the ancient carbon locked beneath the moor, the other protects the living surface that keeps laying it down — and a resilient upland needs both, not one held on principle at the expense of the other.