The Science Doesn’t Add Up: No Evidence That Controlled Burning Causes Flooding

A major scientific review critically examines ten common claims regarding the effects of prescribed heather burning on peatlands in the UK, with “prescribed heather burning causes flooding " being the eighth claim reviewed.

The authors, including Dr. Andreas Heinemeyer, consider the claim unsubstantiated and lacking robust scientific evidence. The review critiques this assertion as part of a broader review of ten common claims about heather burning on UK peatlands, highlighting significant gaps and weaknesses in the supporting arguments.

Claim 8 and Its Narrative

The claim directly links the practice of prescribed rotational burning on peatlands to increased flood risk, suggesting it should cease as a management method. This narrative is often tied to the popular perception of peatlands acting like "sponges" that store large amounts of water and alleviate flooding naturally. Arguments supporting this claim, as noted by organizations like the IUCN, typically focus on two main points:

• Loss of vegetation cover: Burning removes vegetation, which is thought to regulate water flow by intercepting rainfall and promoting infiltration. The argument is that its removal leads to faster and increased surface runoff.

• Damage to peatland properties: It is asserted that burning alters the physical properties of the peat's top layers (the acrotelm) and causes micro-erosion, which reduces the peatland's ability to absorb water, thereby increasing runoff and potential flood risk.

Critiques of the Evidence and Underlying Assumptions

The review article systematically questions the validity of these arguments and the evidence used to support them, asserting that the link between prescribed burning and flooding is not clear.

• The "Sponge" Analogy is Misleading: The authors reference Bacon et al. (2017) to argue that the "sponge" analogy is an oversimplification. The idea that peatlands can soak up most rainwater to reduce downstream flood risk is not the reality in many cases. This analogy overlooks crucial site-specific variables like topography and existing vegetation that affect a peatland's susceptibility to flooding.

• Lack of Robust Experimental Evidence: A central criticism is the absence of studies with an appropriate Before-After-Control-Impact (BACI) design conducted at a meaningful catchment scale. Such studies are necessary to separate the effects of burning from other confounding factors and establish a clear causal link to flooding.

• Critique of Key Studies: The review points out that a key study often cited to support the claim (Holden et al., 2015, part of the EMBER project) has significant experimental limitations. Specifically, it lacked an adequate control and compared catchments in geographically and environmentally distinct locations, creating a high risk of bias. Furthermore, that study did not actually investigate flooding-related aspects.

• Scaling Up from Plot to Catchment: The authors argue that even if burning causes localized effects on water flow, there is no concrete evidence linking these small-scale changes to large-scale flooding events downstream. It is considered unlikely that small, managed burn patches, which constitute a minor proportion of a landscape, could influence the entire catchment's flow enough to cause downstream flooding.

The Need for Better Methodologies

In the broader context of the ten claims review, the critique of Claim 8 aligns with a recurring theme: the need for more rigorous, well-designed research. For the claim on flooding, the authors recommend:

• Implementing BACI studies at the catchment scale to compare burnt vs. unburnt areas over appropriate timescales.

• Conducting multi-factor trials to properly identify causation and disentangle the effects of burning from other variables like drainage, grazing, and climate.

• Moving beyond generalizations and conducting site-by-site assessments to understand how a particular peatland's state and management affect flood risk.

In summary, the sources position Claim 8 as an assertion that has been integrated into policy-informing documents, such as the IUCN's position statement on burning, despite being based on weak, experimentally flawed, or misinterpreted evidence.

The authors conclude that the claim is unsubstantiated due to the lack of robust data and the presence of confounding factors that have not been adequately addressed in existing research.

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