We know that climate change is resulting in increased fire severity and extent in Australia’s temperate Eucalyptus forests. While Eucalyptus forest communities are generally adapted to the presence of fire and in some instances need irregular fires, as the gap between fires become shorter and fires become more severe, there are obvious biodiversity impacts. For instance, forests are already changing – especially the alpine ash and snow gums, which are in a state of decline and even ecological collapse in many parts of the high country.

There is also the question of the how carbon released during these fires adds further fuel to climate change. While the general understanding is that carbon lost to the atmosphere during a fire is drawn down again in subsequent regrowth, is climate driven fire seasons causing more carbon to be lost into the atmosphere, thereby making climate change worse?

New research (Tree mortality and carbon emission as a function of wildfire severity in south-eastern Australian temperate forests, to be published in the journal Science of the Total Environment – a summary is available here) considers these issues and fills some significant gaps in our knowledge about the links between fire and its contribution to climate change.

The 2019–2020 ‘Black Summer’ fires burnt through 6 to 7 million ha of mainly temperate open Eucalyptus forest in south-east Australia, with emission estimates ranging from 97 to 130 tonnes CO2 per hectare.

The researchers sought to develop a sense of the overall contribution of the fires to carbon emissions, and the results are sobering.

We know that climate change is leading to more intense fires, and the news from this research is worrying: ‘High severity fires combusted almost twice as much Carbon from live trees (42 Mg C ha−1) as low/moderate severity fires (25 Mg C ha−1)’.

And the overall contribution of carbon into the atmosphere is enormous:

‘Extrapolating our findings to other tall to medium open Eucalyptus forests across Victoria revealed that 37.33 ± 12.25 Tg C (mean ± s.e.) or 152 ± 50 Mg CO2 ha−1 was lost to the atmosphere from the 0.9 million ha of these productive forests, equating to about 20 % of Australia’s total net annual emissions’.

The report also looked at how fire severity impacted on the structure of forests. In some possibly counter intuitive findings, the researchers noted that at their study sites, higher fire severity resulted in greater overstorey tree mortality but not understorey or loss of dead standing trees than in low/moderate severity fires.

How should we respond?

We know that fuel reduction burning is becoming less useful at reducing the intensity of fires. So a key issue is to do everything we can to keep fire out of the alpine ash and snow gum communities as they recover. This has implications for how we manage fires in the high country and raises the issue of whether we have enough resources to stop fires in bad fire seasons like 2019/20.

And, of course, these findings underscore the need to radically reduce our contribution to further global warming.