Grassland regions key to reduce global carbon cost of fire

Grassland regions key to reduce global carbon cost of fire

Fires cause widespread damage and CO2 emissions. But vegetation on these landscapes tend to recover over time, meaning that over decades, little or no carbon is directly lost through fires. However, recent scientific advances suggest that in order to understand if global fires actually represent a net carbon source or sink, one must consider indirect carbon flows that largely occur after a fire event.   

On the carbon ‘loss’ side of the fire balance sheet, increases in fire frequency that lead to tree death, less average vegetation cover and various forms of erosion, can suck carbon out of an ecosystem in the long term. On the positive end of the balance sheet, fire charring can cause the structural transformation of fire-affected plant molecules into a form known as pyrogenic carbon that is extremely difficult for microorganisms to decompose, at a rate tens to hundreds of times slower than regular soil organic matter. To this extent, fires can facilitate landscape carbon retention, mitigating losses.  

A study just published in Nature Geoscience, a team led by Simon Bowring and colleagues from LGENS, LSCE and univ. of East Anglia explored which of these indirect fire carbon gains and losses might outweigh the other, by simulating them using one of the land surface models employed in UN climate change forecasting. They found that fire carbon losses unambiguously outweighed gains in forest fires, and that in general, carbon loss increased the less adapted a type of vegetation is to fire.    

By contrast, in grassland and savannahs systems, which represent the lion’s share of the world’s fire-affected land (~80%), they found that fires could potentially result in quite small losses -or even gains -of carbon at both landscape and global level. This was particularly true of the tropical grasses, which have evolved over time to deal with fires brought on by intense wet-dry seasonality and heat extremes, resulting in many species holding the majority of their biomass belowground. This protects them from fire and enables extremely rapid recovery of their aboveground weight after a fire has passed. In a sense, burning begets burning. One upshot of this and their findings is that these systems produce the most carbon-saving pyrogenic material and burn at a higher temperature on average than forests. The higher the fire temperature, the more this charred material is transformed and the less amenable it is to decomposition to the atmosphere. Thus, grassland-savannahs may produce the most pyrogenic carbon of the greatest average longevity, without which the global carbon balance of fires would be decidedly negative -important in a world where wildfires of increasing frequency and severity are widely predicted.  But grasslands are rarely considered in nature-based CO2 mitigation schemes, having been historically targeted for conversion to crop or rangeland.   

The researchers found, however, that « the mechanisms through which pyrogenic carbon decomposition occurs aren’t sufficiently well understood to represent them mathematically, meaning we can’t give a clear answer to whether on balance fires at the global scale are at a positive, negative or neutral balance with the atmosphere on a timescale of decades. What is clear is that they are very likely to be close to neutral », said Abiven (Ecole normale supérieure PSL). He added, « and if we assume they are, the average lifespan of this charred pyrogenic matter on Earth is around 5000 years, well over a hundred times that of your average soil material. We urgently need increased efforts towards understanding the mechanisms underlying its movement to the atmosphere. As important, and independent of the previous consideration, the positioning of wild grasslands in the climate perspective likely requires substantial reassessment. » 

But, he emphasised that their findings should not be taken out of context. « This study should not be read as: « fires are ‘good’ for the climate. » What we are saying is that the rather surprising outcome of this study is that at the global scale, the evolution of vegetation with fire, which depends on a number of things such as atmospheric oxygen levels, may have resulted in processes of material transformation which ensure that the net carbon balance of fires on the planet is roughly at zero, which would be quite remarkable. This is what our future research will be looking into. »  

En savoir plus
Pyrogenic carbon decomposition critical to resolving fire’s role in the Earth system
Bowring, S.P.K., Jones, M.W., Ciais, P. et al.  
Nat. Geosci. 15135–142 (2022)
DOI: 10.1038/s41561-021-00892-0

À lire aussi
Sur le site de l’INSU : Les prairies sont essentielles pour réduire l’impact des incendies sur le bilan global du carbone