No Evidence That Heather Burning Reduces Sphagnum or Peat Formation

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 reduces Sphagnum moss abundance and peat formation" being the third claim reviewed.

The authors, including Dr. Andreas Heinemeyer, consider the claim unsubstantiated and too generic.

The review critiques the fundamental assumptions behind this claim, particularly the definitions of "peat-forming" species and the role of Sphagnum mosses in that process.

Critiquing the Role of Sphagnum as the Primary Peat-Former

A central issue with the claim is the widely held but poorly supported assumption that Sphagnum species are the primary or sole peat-formers in UK blanket bogs.

• Challenging the "Sphagnum-centric" View: The review argues that the idea of Sphagnum being the main contributor to peat formation is not supported by evidence. Instead, it suggests that any plant species can form peat under the right conditions, such as waterlogging and acidity.

• Palaeoecological Evidence: To support this, the review points to multiple palaeoecological studies across the UK that show sustained and often high rates of peat accumulation dominated by non-Sphagnum plant remains, including heather (Ericales) and especially sedges, throughout the Holocene period.

• Global Context: The argument is further strengthened by noting that major peatlands in other parts of the world, like Indonesia and the Amazon Basin, do not contain Sphagnum moss species at all, yet have formed extensive peat deposits.

• Unverified Claims: A key technical report by Lindsay (2010) is cited for claiming that over 99% of peat mass is formed by Sphagnum, but the review notes this claim is based on unverified model decomposition values rather than actual data. In contrast, a peatland model for a Canadian bog estimated that vascular plants (shrubs and sedges) accounted for about 36% of the remaining plant matter.

• Conflicting Information: The IUCN has presented conflicting information, at one time listing heather as a peat-forming species but more recently claiming it is not.

The Effect of Burning on Sphagnum Abundance

Contrary to the claim, several studies indicate that prescribed burning can actually increase Sphagnum cover under certain conditions, particularly by reducing competition from dense, older heather.

• Increased Sphagnum Post-Burn: Studies in the North Pennines found that Sphagnum cover was, on average, five times higher in plots that had been burned 8–10 years earlier compared to unburned control plots. This finding confirms similar results from other long-term experimental sites.

• Heather Suppression of Sphagnum: The evidence suggests that a dense, aging heather canopy can suppress the growth of Sphagnum moss. Prescribed burning can open up this canopy, creating conditions favorable for Sphagnum to recover and thrive.

Redefining Terminology and Causality

The review proposes a more nuanced understanding of peat formation to move the debate forward.

• "Peat-Formation Enhancing" Species: Rather than labeling species as "peat-forming" or "non-peat-forming," the review suggests more appropriate terms like "peat-formation enhancing" or "supporting". This terminology avoids the misconception that only certain species, like Sphagnum, can create peat.

• Circularity of Argument: The authors highlight a potential circularity in arguments about Sphagnum's role. It is often difficult to determine from peat core records whether periods of rapid peat growth were caused by Sphagnum abundance or if favorable conditions (like high water tables) promoted both peat formation and Sphagnum growth simultaneously.

Recommendations for Future Research

To resolve the debate around Claim 3, the authors recommend specific improvements in definitions and research methodology.

• Better Definitions: There is a need for clearer, universally agreed-upon definitions for terms like "peat-forming" species, with suggestions to use "enhancing/supporting" instead.

• Improved Methodology: More robust measurements are needed to quantify the role and amount of Sphagnum and other plant species in the process of peat formation.

In summary, the review concludes that there is no robust data to support the claim that prescribed heather burning reduces Sphagnum abundance and peat formation, and in fact, several datasets reject it.

The claim is undermined by unsubstantiated assumptions about the unique role of Sphagnum and by direct evidence showing that burning can benefit Sphagnum cover by managing heather dominance.

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