Are We Basing Policy on Faulty Science? A Critical Look at Heather Burning Claims

A scientific review by Dr. Andreas Heinemeyer and colleagues critically examines ten common claims regarding the effects of prescribed heather burning on UK peatlands, often made by governmental, non-governmental organizations, media, and scientists.

The review concludes that much of the evidence base supporting these claims is insufficient, contradictory, or unreliable, often suffering from methodological limitations, site-specific biases, and misinterpretations of temporal and spatial scales.

The authors advocate for a re-evaluation of current policy, arguing that an outright ban on prescribed burning is not evidence-based. Instead, they recommend a more robust, long-term, multi-site, and scientifically rigorous approach (e.g., BACI studies) to understand the actual impacts, benefits, and risks of various management options, including burning, cutting, and no management.

A key takeaway is the need to safeguard existing carbon stocks in peatlands rather than solely focusing on their potential as "climate saviours."

Claims and Key Findings

The review addresses ten specific claims, providing a detailed assessment of the available evidence for each:

1. Claim: Prescribed heather burning causes a net peat carbon loss and contributes to the climate crisis.

Finding: This claim is unsubstantiated. The evidence is limited, often from single study sites, and highly uncertain. While immediate biomass combustion occurs, potential long-term net carbon gains from rejuvenating biomass and the conversion of biomass to charcoal (which resists decomposition and may reduce methane emissions) are often overlooked. Studies often fail to assess the entire management cycle (~20 years) or all elements of the Net Ecosystem Carbon Balance (NECB).

Quote: "there is no clear evidence to support this statement and, until there is a shift in the approaches used to assess the effect of prescribed burning on carbon budgets... the evidence remains limited by incomplete assessments as long as only part of the management and recovery cycle and not all major NECB elements are being considered."

Fact: Lowland agricultural peatlands are estimated to account for ~60% of total UK peatland greenhouse gas (GHG) emissions, significantly more than blanket bogs managed by burning.

2. Claim: Fire and heather dominance are a result of recent management changes. 

Finding: This claim is unsubstantiated. Palaeoecological evidence (peat cores showing pollen counts and charcoal layers) indicates that both heather dominance and fires (wildfires or intentional burning) have been features of UK upland peatlands for approximately 6,000 years, well before the intensification of grouse moor management in the last 200 years.

Quote: "the palaeoecological record tells us that both fire (either wildfire or prescribed burning) and heather dominance are unlikely to be recent phenomena within UK peatlands and that the key drivers of historical and more recent changes to peatland vegetation remain largely unknown."

3. Claim: Prescribed heather burning reduces Sphagnum moss abundance and peat formation.

Finding: This claim is unsubstantiated. The assumption that Sphagnum is the sole or primary "peat-former" is not consistently supported by evidence; other plant species, including heather and sedges, can also form peat and are found substantially in peat cores. Some studies show increased Sphagnum cover after prescribed burning, suggesting that dense, older heather cover can suppress Sphagnum, which burning may address.

Quote: "it is clear from these case studies that Sphagnum as a species required for peat formation is a false/misleading terminology (and equally that heather is a non-peat-forming species) as claims of nearly all peat matrix to consist of Sphagnum matter are unsubstantiated and heather and other species are found often substantially throughout many peat cores."

4. Claim: Rewetting reduces heather dominance and thus protects peatlands against wildfire.

Finding: This claim is unsubstantiated. There is no directly applicable evidence to support that rewetting generically reduces existing heather dominance or increases peatland resilience to wildfires in UK upland peatlands. Heather can thrive in wet conditions, and unmanaged, ageing heather has been shown to dry out surface peat, increasing wildfire risk due to higher fuel loads.

Quote: "there seems to be no directly applicable evidence to support such generic assumptions, especially in relation to reducing existing heather-domination, and previous claims remain unevidenced."

Fact: Heather becomes flammable when moisture content drops below 60%.

5. Claim: Cessation of heather burning results in wetter peat, less heather cover, and no need to burn.

Finding: This claim is unsubstantiated. Similar to Claim 4, the relationship between wetter conditions and declining heather cover is often based on opinion or studies not applicable to the context. Long-term studies show mature heather can be tallest and have high cover on wet sites. Cessation of burning can lead to significant increases in above-ground biomass, increasing fuel load and potential wildfire intensity.

Quote: "The relationship... seems purely based on opinions... or studies not applicable to the context."

6. Claim: Seventy-five percent of global heather moorland is found in the UK.

Finding: The 75% figure is unsubstantiated, based on uncertain data from a limited number of European countries, and predates modern classification systems.

Quote: "this statement lacks robust calculations of percentage coverage in the UK and globally."

7. Claim: Prescribed heather burning causes water colour and quality issues. 

Finding: This claim is unclear and lacks robust data. Direct evidence linking prescribed burning to water color and quality issues in the UK is limited and often contradictory. Confounding factors like acidification, drainage, and grazing make it difficult to isolate the impact of burning. Laboratory findings often do not translate to field observations, and drainage is suggested as a major cause of increased dissolved organic carbon (DOC).

Quote: "We lack clear evidence on direct impacts of heather burning on water colour and quality within the UK... and further research is required."

8. Claim: Prescribed heather burning causes flooding.

Finding: This claim is unsubstantiated. Reviews indicate little evidence to support a clear link between prescribed burning and widespread flooding. The "peatland as sponge" analogy is often misleading. Reported localised effects from loss of vegetation cover or altered peat properties are not concretely linked to catchment-scale flooding, especially considering the small proportion of land typically managed by burning.

Quote: "the perception created by the sponge analogy, that peatlands can soak up most rainwater and thereby reduce downstream flood risk, is not the reality in most cases."

9. Claim: Peatlands offer huge carbon sequestration potential and are climate change ‘saviours’.

Finding: This statement is too simplistic and potentially misleading hyperbole. While peatlands are vital long-term carbon stores (containing ~30% of global soil organic carbon), their immediate carbon sequestration potential is site-dependent and often limited, especially in older blanket bogs compared to degraded sites. They also release methane, a potent GHG. The primary importance lies in safeguarding existing carbon stocks, as future gains are uncertain due to climate change impacts like wildfire.

Quote: "it is the claim of their value as a ‘saviour’ in the fight against climate change that is too simplistic." and "It seems more important to safeguard peatlands’ existing carbon stocks and prevent further losses than to promote their anticipated future carbon gains as part of a climate ‘saviours’ storyline, which might literally ‘go up in smoke’ if we continue to ignore key evidence."

10. Claim: Prescribed heather burning causes loss of biodiversity.

Finding: This claim is ill-defined and lacks sufficient generic support. The evidence indicates mixed results on biodiversity impacts, varying by experimental and site context, timescales, and species groups. There is no evidence to support a generalization of overall negative impacts. Prescribed burning, when carefully planned, can promote positive biodiversity aspects and benefits to beta diversity (at the ecosystem scale), which can increase gamma diversity (at the landscape scale).

Quote: "There is certainly no evidence to allow a generalisation of overall negative impacts, even on Sphagnum moss... often reported negative impacts are short-lived and need to be seen as part of a patchwork across larger scales (catchment/landscape)."

Policy and Research Recommendations

The authors provide critical recommendations for future UK policy and research:

1. Re-examine Evidence Base: Government agencies and policymakers must critically re-examine the strengths and limitations of the evidence base and be cautious of generalizations concerning management needs and options for heather-dominated peatlands, particularly for prescribed burning.

2. Account for Site-Specific Differences: Researchers need to fully account for potential site-specific and pre-management differences, as well as limitations in temporal and spatial scales. Systematic reviews are urgently needed.

3. Compare Management Scenarios Robustly: Future work should adequately and robustly compare all major alternative management scenarios (burning, mowing, no management) and assess their short-term (disturbance) and long-term (trajectory) impacts across appropriate landscape scales. This will reliably identify management effects (benefits and risks) on ecosystems, their functions, and services to inform policy.

4. Improve Definitions: Establish clear, agreed-upon, and universal definitions for terms like "heather moorland," "peat-forming species," and "degraded/intact" habitat conditions. Condition assessments should link to ecological functions, not just species presence/absence or management.

5. Enhance Methodologies: Experimental studies must improve methodologies, including using multi-site analyses, "Before-After-Control-Impact" (BACI) approaches to account for confounding factors, and examining alternative management regimes for direct comparisons. Limitations must be acknowledged.

6. Consider Scale: Evidence reviews must consider temporal and spatial scale limitations. Short-term plot-scale studies should not be generalized to long-term, landscape-scale impacts. "Gold standard" plot-to-catchment scale studies with long-term monitoring are crucial.

7. Conduct Multi-Site Studies/Meta-Analyses: To obtain robust and generalizable findings, multi-site studies or critical meta-analyses incorporating site and historical information are necessary.

8. Avoid Precautionary Bans Without Evidence: Legislating against or banning prescribed burning on precautionary grounds is not an informed or evidence-based solution, especially when less is known about the consequences of alternatives.

9. Review Official Publications: Reviews commissioned by statutory conservation agencies and NGOs should be critically assessed and rewritten to correct or acknowledge any false, misleading, or unevidenced claims in online content and publicity material, ensuring unbiased and sound evidence for policymakers and land-users.

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