How Wildfire Fuel Load Increases under Unmanaged Conditions

Members are now asking us how much fuel loads will increases if burning and cutting cease on upland moors. To help answer this, we have produced a summary of the relevant evidence on fuel accumulation rates.

These figures are drawn from peer-reviewed studies and national research programmes including Davies et al. (2008, 2016), Glaves et al. (2020), Belcher et al. (2021), Heinemeyer et al. (2025) and the James Hutton Institute Fire Danger Fuels Report (2023).

Vegetation type	Typical management time cycle	Annual biomass accumulation	Change in total fuel load if unmanaged	Key sources
Calluna vulgaris (heather)	8-20 years (managed burning / cutting)	1.5-2 tonnes of dry matter per hectare per year	After 15-20 yrs unmanaged, total live and dead fuel load increases 4-6 times compared to recently burned stands	Davies et al. (2008, 2016); Glaves et al. (2020); Belcher et al. (2021)
Molinia caerulea (purple moor grass)	5-10 years (cut/burn)	2-3 tonnes per hectare per year	Within 5-7 years of no management, fine dead fuel layer doubles; standing litter depth rises from 3 cm to over 10 cm	Heinemeyer et al. (2025); JHI Fire Danger Fuels Report (2023)
Peatland shrubs and bog grasses	Not usually burned when rewetted	Variable	Once drained/re-vegetated and unburned, above-ground fuel load typically increases by 50-100% within a decade, especially where Molinia dominates	JHI (2023); Davies & Legg (2016)

Key Findings

• On a moor which is traditionally burnt in the winter, about 10-15% of the area is in early (low-fuel) stages each year, resulting in an average fine-fuel load of 2-4 tonnes per hectare.

• However, after 20-25 years without management, live and dead heather form dense woody stands with continuous canopy and accumulated litter. This raises the total fuel load to 10-15 tonnes per hectare.

• This represents an approximately four-fold increase in fine-fuel availability within two decades of non-management.

At the Winter Hill wildfire in 2018 the unmanaged vegetation fuel loads reached a magnitude higher fire intensity (10,000 kW/m), while Davies et al. (2008) measured fuel load increases from 3 tonnes per hectare, five years after burning, to over 12 tonnes per hectare after 20 years.

Summary for Policy Use

Peer-reviewed data shows that where prescribed burning or cutting ceases, fine and coarse vegetation fuels on heather-dominated uplands increase four to six-fold within 15-20 years.

This raises potential wildfire intensity from less than 1 megawatt per meter (so controllable, short flames at the fire line) to over 10 megawatts per meter (a full-blown wildfire that creates its own weather and impossible to tackle with ground crews).

So the removal of management represents a shift from fires that can be safely controlled to fires that cannot be suppressed.

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