An Invitation to Debate

by Dr David Ward
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I have read that in parts of India, when they want to start a new rice field, they traditionally go to an old one, and make balls of mud from the soil therein. These they put in the sun to dry. They then level the new field, build the banks around it, flood it, and fling in the mudballs. In former days, this behaviour might well have puzzled onlooking sahibs and memsahibs, but the Indians are no fools. The mudballs inoculate the new field with cyanobacteria from the old, so enabling nitrogen fixation, and a good crop.


Similar to rice, cycads from around the world depend on nitrogen-fixing cyanobacteria (Nostoc) in their roots. Nostoc flourishes best after fire, in part due to the alkalinity of ash. Our local cycad, djiridji (Macrozamia riedlii), is no exception. The benefits of increased nitrogen, and other nutrients, after fire, is obvious for three to four years. This was well described in a fine paper by several local scientists (Grove et al. 1980).

Together with lightning, we can regard the djiridji, and short lived nitrogen fixing legumes (Fabaceae), as the ‘engine room' of the jarrah forest. Coondli (Allocasuarina spp) also fix nitrogen, but they are more localized in groves. Since nitrogen is essential to all life, without the nitrogen fixed in the ‘engine room', the jarrah forest would eventually collapse, as would all the wildlife within it. Interestingly, the bacterium Rhizobium in legume roots also fixes nitrogen vigorously for only three to four years after fire (Hansen et al. 1988). The legumes then senesce, and await the next fire.

A local scientist, Dr Tony Start, has described Aborigines, in the Kimberleys, burning as often as a fire will carry, so maintaining a mosaic of different aged bush. Early Europeans, and current Nyoongar Elders, have described past traditional burning in the south-west in similar terms. For example, most jarrah forest will just carry a mild, patchy fire every three to four years. Like the Indian villagers with their mudballs, Aborigines knew, and still know, things that some academics don't. There is undeniable evidence, from fire marks on old grasstree stems, that most of the jarrah forest was, at least up to the First World War, burnt patchily every three to four years, regular as clockwork (Ward & Van Didden 1998). This would have maintained better nitrogen fixation, and hence better forest health, than current blanket fire intervals of fifteen or twenty years. The sickness of jarrah forest shrubs in long unburnt areas is rather obvious. Leaves are small and discoloured and flowers are few.

Given the tendency for decay to lead to acidification, which inhibits Nostoc, does fire exclusion for decades make sense? Some assume that long fire free periods are needed to build up seed banks, but is this assumption true? From my observations, it is false. Some seeds last a long time, but others actually decline after only a few years, due to ant and weevil predation, and mould attack. Also, due to nutrient lock-up in unburnt dead matter, seeds may look fine on the outside, but turn out to be empty husks.

The claim that weeds follow fire is true, but is only half the truth. Long unburnt areas can be severely infested with weeds, including the notorious Bridal Creeper. Under long fire exclusion, some native plants become weeds. Examples are Parrot Bush, Rock Sheoak, and Bull Banksia, which smother other native plants. Bull Banksia is a notorious host for the mould Phytopthora cinnamomi, which kills jarrah.

I have long noted problems with the health of marri trees. They are bleeding, and their bark rotting. They are immune to Phytopthora cinnamomi, so are they starved of nitrogen, or possibly zinc? Are they succumbing to fungi which thrive in long unburnt areas? Are those who urge long fire exclusion, on the mantra that ‘burning destroys biodiversity', actually themselves potentially guilty of large scale forest destruction, not to mention exposing human lives and property to uncontrollable summer fires in heavy fuel?

Others may have other ideas about the interaction between fire and the nitrogen supply of the jarrah forest. Medieval universities engaged in vigorous disputes, and, more recently, some philosophers have recommended disputes as a road to truth. Articles such as this can play a role in building roads to truth, without the stiffness of a refereed journal. I am genuinely interested in the arguments of others, provided they are not based on re-cycled dogma.

Grove, T.S., O'Connell, A.M. & Malajczuk, N. (1980) Effects of fire on the growth, nutrient content and rate of nitrogen fixation of the cycad Macrozamia riedlii. Australian Journal of Botany 28: 271-81.
Hansen, A.P., Stonemen, G. & Bell, D.T. (1988) Potential inputs of nitrogen by seeder legumes to the jarrah forest ecosystem. Australian Forestry. 51(4): 26-231.
Ward, D. & Van Didden, G. (1998) Re-constructing the fire history of the jarrah forest of south-western Australia. Report to Environment Australia.