The impacts of fuel reduction burns on soil greenhouse gas exchange

Researchers using TERN’s national ecosystem monitoring infrastructure have, for the first time, found that repeated fuel reduction burns in temperate forests have little long-term impact on soil greenhouse gas exchange. The new findings fill an important information gap and provide new science to the ongoing debate surrounding prescribed burning targets in Australia.

Researchers using TERN’s national ecosystem monitoring infrastructure have found that repeated fuel reduction burns in temperate forests, like this one in southeast Australia, have little long-term impact on soil greenhouse gas exchange (photo courtesy of Geoff Carey, ANU)
The OzFlux tower at the Victorian Dry Eucalypt Forest Supersite, just one component of the comprehensive research infrastructure at this site

In response to recent catastrophic bushfires in Victoria and the recommendations of the Bushfire Royal Commission, the state government recently more than tripled the amount of forested land for prescribed burning each year – from around 100,000 ha per year to around 380,000 ha per year. This increase will lead to more frequent burning in any given area as compared to the historic forest fire management over the last 30–40 years.

Recent studies—including one published in the journal Ecological Applications and highlighted in The Conversation and Decision Point—indicate that increases in prescribed burning may be detrimental to ecosystem biodiversity, but, until now, we had very little information on how such increases will impact long-term forest soil greenhouse gas exchange – a very important component of the carbon cycle.  In order to form a complete picture of the impacts of planned burning increases in Australia we must consider all aspects of ecosystem functioning.

Forest soils in temperate climates, like that of south-eastern Australia, are the main terrestrial sink for atmospheric methane (CH4), thanks to bacteria that use the methane as their source of carbon and energy. Temperate forest ecosystems also sequester, store and release carbon and are therefore of major importance to the global carbon cycle. In fact, movements of soil carbon dioxide (CO2) are the second largest after gross primary productivity – the carbon fixed during plant photosynthesis.

To investigate how increased forest fire frequency will affect the uptake of methane and the movement of carbon dioxide, a team of researches undertook a series of studies at one of TERN’s monitoring SuperSites in Victoria. Researchers from the University of Melbourne used the infrastructure at TERN’s Victorian Dry Eucalypt SuperSite, part of the Australian SuperSite Network, including the OzFlux tower at the site. One of those researchers was Prof Stefan Arndt.

“Depending on their intensity, fuel reduction burns have the potential to alter some, if not all, of the factors regulating soil atmosphere exchanges of methane and carbon dioxide,” says Stefan. “A shift in forest burning policy therefore raises concerns that frequent burning of forest ecosystems may lead to changes in the forest soil source and/or sink strength for greenhouse gases.”

“We used sites in the Wombat Forest that received different burn frequencies in the last 30 years: burns every 3 years, every 10 years and no burns, to study the long-term impact of planned burns on carbon based greenhouse gas fluxes from the forest,” says Stefan.

“Our study showed that the long-term impact of planned burning is minimal on these processes. We found small increases in soil CO2 emissions in the burnt areas compared to those un-burnt. But, in contrast, the soils were a constant sink for methane and methane uptake was not influenced by fire treatments.”

“So, while there is evidence that planned burning can reduce carbon stocks like forest biomass or coarse wood debris in this forest type, it may not alter biogeochemical processes in the long-term, indicating the resilience of the system,” concludes Stefan.

The new findings fill a knowledge gap and provide new, much needed science to the ongoing debate surrounding prescribed burning targets in Australia. Despite this study and the one mentioned above, Stefan says that we still know very little about the ecological consequences of increased planned burning and more research is needed.

“It’s re-assuring that we did not detect any changes in methane uptake of the forest soils, but we should remember that methane uptake is a small component of the carbon cycle,” says Stefan.

“Burns will release large amounts of methane from the burnt biomass and we have evidence that frequent planned burning reduces carbon stocks. Also, we don’t know what impacts the burns will have on carbon sequestration. And, to add to these unknowns, we have very little data on the impacts of planned burning on plant or animal biodiversity.”

“So there is a lot we don’t know, but at least it’s a positive sign that some aspects of biogeochemical cycles are not impacted by fire, probably because the forest systems have a large amount of resilience to fire disturbance due to long term adaptations to this disturbance.”

TERN’s research infrastructure will continue to play an important role to address these open questions. The Victorian Department of Environment Land Water and Planning is planning a fuel reduction burn this spring in Wombat State Forest—about 100 kms west of Melborune—around TERN’s Wombat OzFlux tower. This will provide researchers with a fantastic opportunity to study the immediate impacts of planned burning on carbon sequestration, the water and energy budgets of the forest, as well as greenhouse gas emissions.

Hence, long-term scientific studies, like this one that utilizes TERN’s research infrastructure, provide us with the knowledge on the ecological consequences of forest management options that reduce fire risks and mitigate the effects of severe fire events. However, such studies can only be achieved if there is long-term resourcing and provision of research infrastructure – something that is clearly in the national interest.

Published in TERN newsletter June 2015

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