Fuel Load and Flame Length on Heather Moorland: What the Science Tells Us
- Rob Beeson
- 3 days ago
- 5 min read
The Science Behind Safe Controlled Burning
For generations, land managers have used controlled burning, also known as cool burning or muirburn, as a vital tool to manage heather moorlands. This long-standing practice creates the mosaic of habitats needed to support red grouse, as well as grazing for sheep and deer. While the practical experience of gamekeepers and estate managers is invaluable, scientific research can provide new insights to make burning safer and more effective.
Here we share key findings from a detailed scientific study, "Fire Behaviour and Impact on Heather Moorland." This includes the direct and predictable link between the amount and type of heather - the 'fuel' - and how a fire behaves, especially its flame length.
Understanding this relationship can help us better predict fire behaviour and conduct burns with greater confidence and control.
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Understanding Fuel: How Heather Changes as It Grows
In fire science, 'fuel load' is a simple term for the amount of burnable material in a given area. For practical purposes on a moor, this primarily refers to the available fine fuels, the foliage and small twigs that ignite and burn readily, as the larger woody stems often don't burn completely in a management fire.
As heather ages, its structure and fuel load change significantly, following what ecologists call the 'Calluna cycle' (pioneer, building, mature, and degenerate phases). Older, taller heather has a higher proportion of thick, woody stems and a greater accumulation of dead material within its canopy. This results in a much higher fuel load than is found in younger heather.
In the study, heather stands were categorised into three groups that correspond to this life cycle and provide a useful framework for managers. Being able to visually identify these stages on your ground is the first step to predicting how a fire will behave.
Low Fuel Load: Typically younger, shorter heather with less dense canopies (pioneer phase).
Medium Fuel Load: Usually building-phase heather, which forms dense, continuous canopies.
High Fuel Load: Older, taller, and often more open or 'gappy' heather stands with the greatest amount of woody material and overall fuel (mature and degenerate phases).
Fuel Load and Fire Behaviour: What the Study Found
These findings come from a replicated experiment where fires of a consistent size were lit across different fuel types and weather conditions, allowing for reliable comparisons. The study's burns established clear and predictable links between these fuel load categories and the key characteristics of fire behaviour.
How Fuel Load Affects Fire Spread and Intensity
The primary finding was that fires in High fuel load plots (older heather) spread significantly faster and were much more intense, meaning they produced more heat, than fires in either Low or Medium fuel plots.
We must treat wind as a critical multiplier in these conditions. The research showed that fires in older, taller heather are much more sensitive to changes in windspeed. This is because the open, airy structure of old heather allows wind to penetrate the canopy, supplying more oxygen and driving the fire forward with greater force.
This means that on a day that might be considered safe for burning young heather, the same breeze could turn a fire in old heather into an uncontrollable run.
Flame Length and Fire Intensity
For a land manager on the hill, flame length is one of the most important visual clues for assessing a fire's intensity, potential danger, and controllability. Longer flames mean greater radiant heat, which pre-heats fuel further ahead of the fire front, accelerating its spread and increasing the likelihood of it spotting across firebreaks.
The study directly measured flame lengths and confirmed that they increase dramatically with fuel load. The typical flame lengths observed in the experiments were:
Fires in Low fuel stands typically produced average flame lengths of around 1 metre.
Fires in Medium fuel stands produced average flame lengths of around 1.5 to 2 metres.
Fires in High fuel stands produced the most variable flames, ranging from just over 1 metre to over 2.5 metres, with an average of around 2 metres.
These longer flames are a direct result of higher fire intensity, which in turn is driven by the combination of higher fuel loads and increased windspeed.
The Dangers of Old, Rank Heather
The primary management implication is clear: burning old, tall, high-fuel-load heather is inherently riskier. These fires are hotter, spread faster, and produce longer flames, making them much more difficult to control.
The fire intensities observed in the 'High' fuel plots often approached or exceeded 1000 kilowatts per metre (kW m⁻¹). Crucially, five of the experimental fires in the study reached this threshold.
International studies have identified this as a critical point beyond which fires are generally considered uncontrollable by firefighters using hand tools. This finding scientifically validates the caution that experienced managers have always applied when dealing with old, rank heather.
Why a Mosaic Matters for Fire Safety
The traditional practice of creating a patchwork or 'mosaic' of different-aged heather is not just a habitat management technique, it is a critical fire safety strategy. The research reinforces this principle, showing that this practice is a multi-purpose strategy for both habitat maintenance and proactive wildfire prevention.
Patches of younger, low-fuel heather act as effective natural firebreaks. Because fire behaviour is so much less intense in these areas, they can significantly slow or even stop the spread of a fire burning in an adjacent patch of old heather. By preventing succession to woodland and breaking up large, continuous areas of high-hazard fuel, a well-managed mosaic is one of the most effective ways to prevent large, intense, and potentially devastating fires.
The Takeaway: Key Principles for Safe Burning
This research reinforces many of the core principles of traditional controlled burning with scientific data. The key takeaways for any manager planning a burn are:
Fuel Load Governs Fire Behaviour: The age, height, and density of your heather directly determines how a fire will spread and how long its flames will be. Older, taller stands produce more intense and less predictable fires.
Wind is a Multiplier in Old Heather: You must be extremely cautious when burning older stands. Higher windspeeds will have a much greater effect on fire spread and intensity in old, gappy heather than in younger, denser stands.
Manage the Mosaic to Manage the Risk: Maintaining a patchwork of different heather age classes is the most effective way to break up large, continuous areas of high-hazard fuel and limit the potential for large, uncontrollable wildfires.
Follow the Guidelines: The research reinforces traditional best practice. Sticking to a managed rotation that avoids letting large areas become old and degenerate is the safest approach for both habitat management and fire control.
This research puts hard numbers and clear principles behind the traditional skills of moorland management, providing an additional layer of confidence when making decisions on the hill.
A scientific understanding of how fuel load affects fire behaviour empowers land managers to use muirburn more safely and effectively, protecting both the unique moorland landscape and the people who dedicate their lives to looking after it.
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