Research Letter 2.3

From “CFFZ” to “FMZ”

Crown fires burn the entire fuel array of forests; as such they are truly “forest fires”. They are the spectacular fires of TV documentaries and news bulletins. They are a major problem for fire fighters, not only because of the intensity of the fires and the immensity of the flames, but because the lofted burning brands sailing overhead start spot fires downwind of current fire lines. Burning brands - and their lesser ‘cousins’ - burning embers - may shower down on houses and gardens and set them alight. If fires could be kept below the forest canopy then they would be easier to put out and be less likely to cause damage. Therefore, there is considerable impetus to prevent crown fires especially in areas where there are significant economic or social assets. In pine plantations, the asset may be within it or external to it – like the suburbs. In Canberra, a crown-fire-free-zone (CFFZ) concept was introduced into pine plantations adjacent to one of the suburbs. This Letter looks at the origin of the concept.

The concept of the CCFZ began in South Australian exotic pine plantations[1]. Because of the difficulty in declaring anything “free” in relation to fire, the zone of treatment is now called the fuel-modified zone (FMZ). CCFZ gave way to FMZ. Currently there are about 1000 ha of FMZ in South Australia (SA).

In SA in the 1960s ‘protection strips’ 20 to 50m wide were created in plantation edges by pruning branches to 2.5m height. The pruned branches were burned in winter. In the 1970s economic efficiencies were introduced such that prunings were not burnt. Near the town of Furner, 20 km north of Millicent, there were stands of pines planted in 1969 and 1970 that became involved in the Ash Wednesday fires of 1983. A compartment-width strip of the 1970 plantations had been pruned but the 1969 plantations had not been pruned. Crown fires burned through the unpruned and unthinned 1969 plantations leaving very little unburnt foliage while in the pruned but unthinned 1970 stands there were even areas of green crown left among scorched trees. Thus, the fire passed through the 1970 plantation but at reduced intensity. This demonstrated that the pruned strip was effective in reducing fire intensity even under the conditions of Ash Wednesday.

The task force set up after the 1983 SA fires took this observation and built upon it. In short, they established FMZs of 200m wide (including roads, tracks or treeless areas) -– approximately 100m wide in the plantation on the expected upwind edges (north and west) and 50m in the plantation on the downwind edges. The trees in the FMZ were pruned to 6m height thereby creating the potential to attract premium prices for logs. Pruned materials and thinnings were crushed as much as possible but no burning in these zones took place. There are many stages to the process of establishment of FMZs as stands are replaced. FMZs divide the plantations into blocks of between 2000-3000 ha.

ForestrySA considers that FMZs reduce the range of intensity in unplanned fires, increase the time available for successful initial attack and provide areas where fire crews can operate and travel safely during a fire.

The success of the SA ‘experiment’ in the fires of ’83 attracted ACT Forests so much that they set up an FMZ adjacent to the suburb of Duffy. There, high pruning and thinning were carried out but, unlike the South Australian practices, they incorporated fuel reduction of litter by periodic prescribed burning. The result can be seen today.

Under what conditions will the FMZ work? A brief response follows.

The scientific concepts date back to the work of Van Wagner (1977) in Canadian conifer forests. The general idea was that a crown fire can begin when the flames from the litter are long enough to reach the base of the conifer crown. If you increase the distance from the ground to the base of the crown by pruning there is less chance that the flames will reach it and set it alight. However, by pruning, the surface fuels are enhanced and the chances of longer flames - and crown fire - are increased, all else being equal. If the prunings are compressed or have decomposed or have been burnt under mild weather, the potential flame lengths are reduced and the chances of crown fire lessened.

“Crown fires” come in three types (Van Wagner 1977). First, there is the “passive” type where individual crowns are burnt because the trees are scattered and the crown fire is dependent on the fire in the litter beneath for its combustion. There is the “active” crown fire in which the fire in the litter and the crown both affect overall rate of spread. The third type is the “independent” crown fire which, as its name suggests can spread independently of the fire in the litter below. “An active crown fire should be most likely in forests that have:

  1. ground fuel that permits development of a substantial surface fire [i.e. in litter],
  2. a crown base moderately high above ground, and
  3. a fairly continuous crown layer of moderate to high bulk density [i.e. dense canopy] and low to normal foliar moisture content” (Van Wagner 1977).

For an effective FMZ areas treated will be larger rather than smaller, high-pruned rather than not-pruned, litter-fuel reduced rather than litter-fuel enhanced (by prunings), and have canopies separated by tree thinning rather than being continuous. Having said all that, burning material may travel long distances so there is no guarantee that a FMZ will eliminate the problem crown fires can create - and this is why “Crown Fire Free Zones” are now called “Fuel Management Zones”.

Literature

Van Wagner, C.E. (1977). Conditions for the start and spread of crown fire. Can. J. For. Res. 7, 23-34.

A. Malcolm Gill
21 November 2001


[1] I am indebted to Tony Fearnside (formerly of ACT Forests) and Dennis Page and John Pratt (ForestrySA) for the history of the concept in the ACT and SA respectively.