We know that climate change is driving longer and more intense fire seasons. We know that fuel reduction can greatly reduce the spread and intensity of wildfire. However, in extreme fire conditions, the value of fuel reduction burning is reduced, and fires will burn through almost anything, regardless of recent fuel reduction treatment an area may have had. We also know that logging will make forests more flammable because of the loss of more humid micro climates and thick growth of the seedlings that will occur after logging. But we also know that older forests are less fire prone, burn less intensely than regrowth forests, and have the ability to slow down fires as they move through the landscape.

This has been highlighted again in research called Propensities of Old Growth, Mature and Regrowth Wet Eucalypt Forest, and Eucalyptus nitens Plantation, to Burn During Wildfire and Suffer Fire-Induced Crown Death by Suyanti Winoto-Lewin, Jennifer C Sanger and James B Kirkpatrick at the University of Tasmania. It highlights the value of older forests in slowing fire. (Available here).

They looked at fire incidence and fire severity in regrowth, mature and old growth wet eucalypt forest, and E. nitens plantation (generally known as shining gum or silvertop), in the Huon Valley, Tasmania after the January–February 2019 wildfire.

They found that:

“Old growth forest and plantations were the least likely to burn. Old growth and mature forest exhibited scorched eucalypt crowns to a much lesser degree than regrowth forests. In a comparison of paired sites, plantation forest was less likely to burn than combined mature and old growth forests, but in all cases of detected ignition the canopy of plantation was scorched.

The lower flammability of older forests, and their importance as an increasing store of carbon, suggests that a cessation of logging outside plantations might have considerable benefits”.

This is the type of forests the research considers:

‘Wet Eucalyptus (predominantly Eucalyptus regnans and Eucalyptus obliqua) dominated forests which have an understorey of broad-leaved shrubs or rainforest trees. The wet forests that are dominated by large eucalypts more than 110 years old have not been substantially disturbed by logging so are here described as ‘old-growth’

Clearfelling and the closely related ‘aggregated-retention’ (in which patches in the clearfelled area are retained) are used to harvest these forests. Clearfelling is followed by high intensity burning and aerial seeding of local eucalypt species. This results in extremely dense regeneration followed by self-thinning. Our study area contains a patchwork of coupes which have undergone clearfell logging at various times in the last 60 years, eucalypt plantations and old growth forest.

Sadly, while old growth forests may slow or ‘self extinguish’ wildfire, their existence is not a guarantee that they will be able to do so in the case of extreme fire danger days. For instance, during the 2009 Black Saturday fire in Victoria, much of the old growth and rainforest in the Central Highlands was badly burnt.

For instance, in the O’Shannassy Catchment in the Central Highlands, the severe fire in 2009 has led to the loss of around two thirds of the Cool Temperate Rainforest previously mapped in the Catchment. However, there is no doubt that the cooler, more humid environment of old growth did slow the fire front in many places, including in the O’Shannassy:

Forest researcher David Lindenmayer says that while the catchment burnt at high severity in 2009, it was less intense than the rest of the landscape, where younger forest dominated. He also says that the rate of recovery is much faster than younger forest, more animals survive the initial fire front, more seeds will germinate and recolonisation is faster than in young/ regrowth forests.

In their conclusion the Uni of TAS researchers say:

‘Our results show a clear relationship between disturbance history and fire incidence and severity under moderate fire weather conditions. Fire severity is related to intensity, which is important for ease of suppression. Thus, the retention of older forests across the landscape may decrease fire risk. Allowing eucalypt forests to mature further than the normal cycles of 40 to 90 years could help reduce fire hazards. This is especially pertinent around built assets and vegetation types vulnerable to fire. Our results are consistent with the ‘landscape trap’ theory’.

There are additional benefits:

‘In addition to providing a valuable damper on fire incidence and severity, old growth Eucalyptus regnans wet forests store the highest density of carbon of any forests in the world. Carbon continues to be captured in the aging old growth forests, rather than reaching an asymptote. Soil carbon in wet eucalypt forest is a substantial store, its magnitude related to the above ground biomass of the forest. The slow release of carbon from the soil as a result of previous conversion from primary forest to production forest, can only be prevented by allowing forest regrowth to continue to old age.

To refine our understanding of the relationship between disturbance history and fire we need to ask: how localised is the effect of patches of more flammable regrowth in a landscape—does a patchwork of forest types make a whole region more susceptible to fire; at what point after clearfell logging does a forest become less flammable; and, is there a way to manage the forest up to this point to reduce fire hazards while maintaining natural values?’