Since the early 1990s large areas of previously cleared agricultural land in higher rainfall regions of southern Australia have been afforested with plantations of Tasmanian bluegum (Eucalyptus globulus). Bluegum plantations have been primarily established for short rotation pulpwood production, although opportunities for growing sawlog plantations on longer rotations are also being explored. To date, unplanned fires have inflicted only limited losses on the bluegum plantation estate with individual fires mostly less than 50 hectares. This can be attributed in part to implementation of effective fire planning guidelines and provision of fire suppression resources by the plantation industry. A further significant factor limiting wildfire losses has been the relatively sparse fuels typical of young plantations in the first few years after establishment. In a number of instances, bluegum plantations less than seven years old have contributed to a reduction in the spread and intensity of wildfires burning under conditions of very high or extreme fire danger.
Around noon on 23 March 2005 a fire ignited adjacent to the South West Highway 11 km north of Manjimup in the south-west of Western Australia. Rainfall had been substantially below average during the previous three months resulting in very dry forest fuels and fully cured grass in paddocks. Forest fire danger at Manjimup was very high by 9am and extreme between noon and 2pm, with a peak McArthur Forest Fire Danger Index of 56. The fire developed rapidly under a gusty NNE wind in heavily grazed pasture with scattered remnant eucalypt trees and then burnt into a six-year-old bluegum plantation comprised of trees that were 16-20 metres tall and 13-15 centimetres in diameter, planted at a density of 800 per hectare. The plantation had been grazed by sheep since 2001 and carried a surface fuel layer of dry grass, bluegum leaf litter and twigs with occasional patches of bracken.
Within 50 metres of entering the bluegum plantation the fire began to crown extensively with flames extending up to twice the height of the trees. Crowning continued as the fire spread from the north-east to the south-west corner of the plantation, uninterrupted by a 10 metre wide powerline easement and a gravel road. The fire subsequently breached a perimeter firebreak on the southern edge of the plantation and continued spreading through remnant eucalypt forest on private property towards the small rural settlement of Palgarup. Fire intensities in native eucalypt forest were very high and sufficient to cause complete crown scorch, widespread defoliation, and complete bark charring of trees up to 30 metres tall. Firefighters from the Department of Environment and Conservation and volunteer bushfire brigades, supported by two Dromader waterbombing aircraft (2500 litre capacity), contained the fire during the later part of the afternoon in open pasture fuels as weather conditions moderated.
Of the total area of 117 hectares of bluegum plantation about 20 percent (25 hectares) was completely defoliated by crown fire with a further 70 percent fully crown scorched. About 10 percent of the plantation area was burnt at low fire intensity insufficient to cause crown damage.
Factors contributing to the severe fire behaviour include very low relative humidity (<10 per cent) and extremely dry fuels. The very low relative humidity during the afternoon can be attributed to mixing down of dry air from aloft under conditions of strong convective heating. Research by Dr Graham Mills from the Bureau of Meteorology Research Centre undertaken as part of Bushfire CRC Program A has shown that entrainment of dry upper air is a feature of some major bushfires in southern Australia.
This case study clearly illustrates that bluegum plantations six years and older may carry sufficient fuel to support high intensity crown fire when fire danger is extreme and all fine fuel is available to burn. Under these conditions the difficulty of suppressing a fire in a bluegum plantation will be much greater than in grazed pasture. From a plantation industry perspective the probability of serious fire losses being incurred is likely to increase as the average age of the plantation estate increases. In rural districts where new plantations are being established fire managers need to be aware that fire risk can change in the space of a few years as plantations mature.
By Bushfire CRC Project Leader Lachlan McCaw, of the Department of Environment and Conservation, WA
(This article first appeared in the Winter 2008 issue of Fire Australia magazine)