Victoria’s forest management policies need to be urgently reviewed in response to the discovery that logging can contribute to the severity of bushfires in wet forests, like the devastating fires on Black Saturday in February 2009.

Our recent study, based on data from areas that burned on Black Saturday, clearly shows how extensive logging can increase the severity of bushfires in mountain ash forests. We found that the risk of “crown” fires, which burn severely and spread rapidly through the forest canopy, is greatest in mountain ash forests that have been regrowing for about 15 years. Before the 2009 fires, these young trees were established following clearfell logging.

The Victorian government will review its forest management policies in the coming months and we encourage it to consider our findings.

Forests and fires

Mountain ash trees, which can grow higher than 100 metres, are restricted to high-rainfall zones of Victoria: the Central Highlands north and east of Melbourne, the Strzelecki Ranges of South Gippsland, and the Otway Ranges in the state’s southwest.

Some of these forests are up to 500 years old. They can be very complex in their structure, with trees of varying ages that regenerate from seeds after infrequent and sometimes intense fires. The forests have also been a major logging resource since the 1930s.

We studied about 10,000 sites in the Central Highlands mountain ash forest that burned on February 7 2009, spread over two regions 56 kilometres apart. These regions had similar stand ages, topography and burned at the same time of day. This allowed us to look at the variables that influence fire severity.

As expected, most of the severe fire occurred when the fire weather was extreme, both before and after the late afternoon wind change. However, patterns of fire severity during this time were variable. We tested for several environmental factors that could influence these patterns, including slope and aspect, but found that the tree age had by far the greatest influence on fire severity.

The most severe fires, which consume the crowns of the trees, were most prominent in mountain ash trees aged between seven and 36 years of age, with a peak around 15 years. Virtually all trees under seven years of age did not sustain crown fires and crown fires were infrequent (around 10% of forest area) in trees up to 300 years old.

Taylor et al. 2014

Changing risks

Crown fires result in the most severe impact on vegetation and they have greater rates of spread when compared to other fires, such those burning in the understorey. Crown fires often feature large flames extending above the forest canopy and pose the greatest threat to life and property. They are driven by extreme fire weather conditions, combined with high fire intensities and high densities of the crown fuel.

In mountain ash forests, the fine fuel loads approach their maximum levels at around 15-30 years of age. At this stage, trees undergo rapid self-thinning. In the early stages of growth, a mountain ash forest has anywhere between 200,000 and 1,000,000 seedlings per hectare. Competition for light and nutrients is fierce and dominant trees quickly suppress surrounding trees in the early stages of growth.

The suppressed trees die and turn into large amounts of fine, dry material that can act as “flash fuel” in a fire. Where previous logging has taken place, woody debris left by the logging adds to the available fuel. One study found that mountain ash logging leaves behind up to 30% of the forest biomass and this material can remain in logged areas for 50 years.

In older stands, fuel loads remain high, but the risk of crown fire drops because the crown is higher above the ground and the density of the fuel is dispersed. The trees themselves become dispersed and a more moist understorey, including rainforest plants, becomes prominent. These conditions make it more difficult for fire to burn severely.

Logging legacy

Logging is known to increase fire risks in moist forests around the world and our findings show why it’s critical to consider the long-term changes to mountain ash forests resulting from logging. Our findings are largely supported by previous research, which shows that the risk of crown fire decreases with age. Another study also shows a similar trend in its data to our findings, but it downplayed the significance of any relationship.

Adapted from Attiwill et al. 2013 (red arrows added)

Managing the risk

Mountain ash forest dominated by young trees is becoming more widespread. Up to 2009, around 30% had been clearfell logged across the region. Around 35% of mountain ash forest was burnt by the fires. Combined, the fires and the previous logging resulted in young trees occupying around 60% of the mountain ash forest area.

Clearfell logging continues in remaining unburnt areas of mountain ash forest, especially in places like the Royston and Rubicon valleys, northeast of Marysville, which have become a largely continuous chequerboard of young trees.

Google Earth

There are arguments that the amount of logging across Victoria is small and that its influence on overall fire severity is negligible when compared to the overall area impacted by a fire. However, this argument does not hold when looking at mountain ash in isolation. It covers only 15% of the region’s forested area, yet some 60% of the region’s logging operations take place within this forest type.

Remaining unlogged areas are often small and fragmented, mostly forming linear strips alongside streams. The impacts are concentrated and regionalised - now most areas that have mountain ash forest feature extensive logging.

We can’t change the area of young trees resulting from the the Black Saturday fires, but we can determine the other areas of young trees that we establish following logging. Remaining unburnt mountain ash forests are now more important than ever. They not only serve as refuges for wildlife in an extensively burned landscape, but provide areas that carry a much lower risk of crown fire.

Changes are urgently needed to logging policies in these forests. Any future logging must be negligible in its cumulative impact. Given the large area already logged and the huge impact of Black Saturday, an expanded formal reserve system would serve an important role in protecting remaining unburnt areas of mountain ash forest.