Blue Gums Proving Firestoppers this Summer

Three recent bushfire events have triggered this article, exploring the startling, ‘damping down’ affects that blue gum plantations can have on fire behaviour.  In late January 2006, a period of very hot weather combined with lightning storms caused bushfires on Kangaroo Island in South Australia, in the Grampians in western Victoria and in Gippsland.  While all three fires burnt into blue gum plantations, each fire caused very little damage to the plantation trees.

Since the rapid expansion of the hardwood plantation sector about a decade ago, it has been quite remarkable that there have been very few blue gum plantation losses to wildfire.  This article briefly highlights personal observations or investigations of 28 fires in blue gum plantations on former cleared agricultural land that have occurred since 1994.  These fires were in six plantation regions across three states (WA, SA and Victoria) and include most of the blue gum plantation fires in Australia and all of the serious fires affecting blue gums.

Starting with the facts from 28 case study fires;

-          In the 12 years since the first reported blue gum plantation fire in Australia (Parker 1994), the worst area loss was on 9 January 2005 in western Victoria.  On a day of Extreme forest Fire Danger, that fire burnt 350ha of 6 and 7 year old plantations (Crowe & Sheldon 2005).  But a very hot fire in high fuel load native forest changed to a fire of significantly reduced intensity when it entered the blue gum plantation.

-          The next largest area loss was in Gippsland in February 2003.  About 250ha of a low quality 10 year old plantation was burnt.  Despite high grass loads in the plantation, fire behaviour was mild until it exited the blue gums and entered a pine plantation, at which stage fire behaviour became extreme with multiple crown fires, very high flame heights and severe destruction in the pines.

-          Plantation area losses of greater than 100ha occurred in only three of the 28 fires.  In those cases, losses in other vegetation types exceeded thousands of hectares.

-          Area losses in more than 85% of the fires observed were less than 30ha. In 10% of cases, this was due to small scale plantations caught in the path of large wildfires. In 45% of cases the mild fire behaviours in the blue gum plantations was a contributing factor to the fires being brought under control.  In several of the case fires, the fire went out in the blue gum plantations, without any human intervention, due to lack of fuel.

-          About 75% of the fires occurred on days when the Forest Fire Danger was Very High or Extreme.  Under such conditions, plantation losses would normally be expected to be severe (Vercoe 2003).

Observed Behaviour

Drawing on actual fire behaviour in the 28 blue gum fire case studies, and from discussions with fire fighters who observed actual fire behaviour in the plantations in several of the more serious fire losses, it can be concluded that where blue gums have been established on formerly cleared agricultural land, fire behaviour is different to that in grassland, pine plantation and native vegetation fuel types.  Fire behaviour is likely to be more severe where there is a large amount of grassy fuel understory.

There is a difference in the fire spread into a young blue gum plantation, depending on row direction relative for  to the fire travel.  It is likely rows perpendicular to the fire direction provide a better barrier to fire spread than rows parallel to fire direction. Where rows are in the same direction as the fire, grassy fuel between the rows may enable a fire to carry for short distances.

………... In older plantations there is a gap between ground fuel and the crowns, reducing the chances of crown fires and leading to similar low intensity fire behaviour, such as in well thinned pine plantations.

It is known that a number of fires have been stopped by fire breaks and have not entered the blue gums.  Plantation managers in several regions have also mentioned that lightning strike fires have started within blue gums, but due to lack of ground fuel, have not developed.  These fires are not included in the 28 case study fires.

Despite the low area losses to date, fire authorities should not become complacent when dealing with blue gum fires.  When drawing conclusions for case studies there is a need to recognize fire experience in blue gum plantations is still at a relative early stage, and there is still not a good balance of data from spring, summer and autumn fires.  There is still potential for much higher losses than have been experienced, particularly in older blue gum plantations with leaf litter and dry branches on the ground.  Furthermore, the 28 case study fires were all established on formerly cleared agricultural land.  Where plantations have been established on former native forest sites, understorey fuel levels can be more like those in a native forest (Gould, McCaw and Cheney 2001).

Depending on harvesting techniques, woody ground fuel loads may be high in second rotation plantations, giving rise to more severe fire behaviour (Wetttenhall 2003).

In summary, where blue gums have been established on formerly cleared agricultural land, fires in plantations will be less intense than in pine plantations where the grassy or shrubby understorey has been almost non-existent.  If dry grass or woody shrubs are present under plantations, it is quite likely fire behaviour will be more severe. Second rotation plantations also have a higher potential rate of fire spread because of the high quantity of wood debris.

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