Research Letter #4

Irradiated radiata pine

Fire has an ambiguous image. On the one hand, 'wild' fire is seen as an enemy rushing across the landscape destroying homes and other valuable objects in its path. On the other hand, 'domesticated' fire is seen as a valuable commodity in the home (hearth) and in the landscape. In the wilderness 'wild' fire may be seen as a good thing by some people.

In the fire season just past we saw examples of fires in the wrong place. There were fires in valuable pine plantations near Chisholm and at Mt Macdonald. Many pines came to grief before they had reached their prime. While fire is the enemy in such cases, it can also be used among pines as a management friend, ridding areas of wildlings ('volunteer' pines) growing outside plantation areas. In both of these cases the players are the same - pines and fires - but the value judgements of their interaction are diametrically opposed. Maybe it is obvious, although it is sometimes overlooked, that fire is seen to be 'good' or 'bad' depending on whether it enhances, or detracts from, the achievement of the aims of management of the area.

Pinus radiata is an interesting example in relation to fires because even mature trees of this species are readily killed by fire. A rule of thumb is that if the tree crowns are fully scorched (all the canopy is killed by fire) then the tree is likely to be dead. The reason for this is to be found in the locations of buds on the tree.

Trees of P.radiata have basically two sorts of buds. There are those on the ends of branches and many more embedded within clusters of needles. I haven't done the experiment on this species but I guess that the buds embedded in the needle clusters can be induced to grow out to form a shoot rather than remain dormant if all buds at the ends of branches are removed. There are no buds hidden in, or under, the bark, and there are none in the root system in this species. To kill the tree then, all a fire has to do is kill the buds at the ends of the branches and in the needle clusters. This point is reached, approximately, when the needles adjacent to the buds have been killed (as shown by needle browning, 'scorch'). When scorch height - the height to which browning occurs as a result of the fire - is equal to or greater than the height of the pine tree, it is dead.

Scorch height can be predicted according to the intensity of the fire. Burrows et al. (1989) found that a management fire of only 200 kW m-1 would scorch 'wildling' crowns to about 8m tall and kill most plants up to 10m tall.

Killing the trees may not 'kill' the species. If the trees have cones they can distribute seed after the fire and, through germination and growth, maintain the presence of the species on the site. Another fire is needed if these regenerating wildlings are to be eliminated. If the plants took 10 years to form cones and had then reached 10m in height, a timely fire in year 9 - of the same intensity mentioned earlier - would eliminate the population, and the species, from the area.

This case study is a good example of the effects of a fire regime on a species. We needed to know about the plant's characteristics, the intensities of the fires and the intervals between the fires if we were going to predict whether or not wildlings of Pinus radiata would persist on the site. For ecosystems we might need to know about the effects of peat fires and fires at various times of year, as well as the effects of surface fires of various intensities and between-fire intervals, if we were to predict the effects of fires. Thus a complete 'fire regime' would consider categories of the components of regimes - fire type, fire intensity, season of fire and between-fire intervals.

In grasping for a value judgement on a fire - a 'good' fire or a 'bad' fire - we need to be conscious of what the fire does and what the aims of management for the fire- affected area are. In environmental matters, the effects of the individual fire need to be seen in the light of the fire regime, not the heat of the current event.

Further reading:
  1. Burrows, N.D., Woods, Y.C., Ward, B.G. and Robinson, A.D. (1989). Prescribing low intensity fire to kill wildlings in Pinus radiata plantations in Western Australia. Australian Forestry 52, 45-52.
  2. Gill, A.M. and Williams, J.E. (1996). Fire regimes and biodiversity: the effects of fragmentation of southeastern Australian eucalypt forests by urbanisation, agriculture and pine plantations. Forest Ecology and Management 85, 261-278.

Malcolm Gill
Division of Plant Industry, CSIRO
11 June 1998