In the left of this photo dense stands of breadfruit grow above the grass of the paddock. This part of the paddock was left as an unburnt control.

Looking into the trial paddock site after it had received three burns in 2000, August 2002 and October 2003. Before the burns, this paddock had similar levels of breadfruit to the unburnt paddocks.

What are the important findings emerging from the information collected so far?

As far as the responses of individual species are concerned, it is apparent even from the few species that are being addressed in this project, that there are meaningful differences between species. For both of the two main targeted species single fires resulted in less than 50% mortality. For breadfruit, the level of mortality from single fires was always less than 20%. These levels of mortality are considerable lower than those obtained for species such as mesquite and rubber vine, species for which fire is recommended as a key element of a control package.

However, the work does suggest there is value in examining (i) the effects of multiple fires; (ii) the consequences of even single fires for the structure of populations of woody species.

There is only one site at which we have data regarding shrub responses to more than one fire. This was burned the year before the project commenced and again in the first year of the project (August 2002). The apparent mortality level of breadfruit at this site was over 50%, considerably greater than the <20% recorded for other sites. This result is analogous to the situation with mallee eucalypts which are generally perceived as being resilient to fire and yet succumb to repeated autumn fires (Noble and Grice 2002).

Useful tool

Fire can be a useful tool for the management of vegetation, and of particular species within it, even if species being targeted for control do not experience high mortality following a single fire. This is because the goals of vegetation management may be achievable even if the densities of the target species do not change. This is the case, for example, in southern African savannas where top-kill of shrubs meets the objectives of management (Trollope 1996). In such cases, repeated burning can be used to suppress woody species without necessarily reducing their density. This may be appropriate for the northern Gulf savannas, for species such as breadfruit that appear relatively resilient to single fires.

A key factor in any consideration of fire to manipulate the composition of vegetation is the effects of fire on recruitment regimes. Fire may stimulate germination by breaking seed dormancy and promote seedling growth by removing competition and increasing soil nutrient levels. There has be no indication from this work that fire promotes establishment of either breadfruit or gutta percha. On the other hand, a major gap in knowledge about these species concerns temporal patterns of recruitment and the environmental variables determining them. One might expect, on the basis of parallels with other members of the genus, that fire does stimulate germination of the Acacia spp. of the Gulf savannas, but the current work does not address this question directly.

Interpretation of short-term responses to fire, and recommendations concerning the use of fire, should be made on the basis of an understanding of the species’ long-term dynamics. The trees and shrubs of the Gulf savannas have not been subject to long-term studies of their population biology and one is left to rely on knowledge of comparable species from other areas and anecdotal information from people with long-term experience in the region.

Unlike in some other Australian rangelands, including other regions of tropical savannas and woodlands, fire is apparently quite broadly accepted by the pastoral community of the Gulf savannas as a useful management tool for woody plant control. This has made it relatively easy to develop a network of test or demonstration sites through which the scientific and pastoral communities can together broaden their experience of fire in the Gulf savannas. This project will add systematically collected information to this body of knowledge though it is of rather limited scope in terms of time-frame and plant species covered. The project is on schedule to meet its objectives, the constraints of fuel availability due to dry seasons notwithstanding.

Recommendations and ideas for further work

Working with landholders to ensure they understand the cause of timber thickening and, in particular, acceptance that poor grazing management has been a major factor. Long- term sustainable grazing management must be encouraged to prevent rehabilitated areas from re-thickening.

Fire can be used to manipulate the woody component of Gulf savanna vegetation. Species will respond differently from one another. Gutta percha is relatively susceptible to fire and breadfruit is relatively resilient to fire. Land-holders should be made aware of these differences. They should also be realistic in their expectations of fire as a management tool. In particular, single fires must not be portrayed as providing a one-off solution to a vegetation management problem.

Land-holders should plan targeted prescribed burning well in advance. There will always be factors such as rainfall that are beyond the control of land-holders but improved grazing management is likely to broaden the window of opportunity for using fire. Further work should be undertaken to determine how pasture recovery can most effectively be encouraged after fire. Again, this will require appropriate grazing management.

Encourage discussion of the relationship between fire and grazing. Capacity to exploit fire as a tool for managing populations of woody species will be limited when heavy grazing reduces fuel loads below critical levels. The fires in this study were generally reliant on relatively light fuel loads. Ideally, some of the sites in this study should be burned under heavier fuels. It is important that land-holders develop grazing management strategies that facilitate appropriate fire regimes. This should be a key part of the communication plan of this project.

Undertake work to examine long-term fuel dynamics in relation to grazing and rainfall. This could be done using a combination of modelling of herbage dynamics (e.g. using existing software) and field work to provide any necessary base-line data. This would allow prediction of fire frequency and intensity and so provide information of fire regimes that are possible. It would also indicate the grazing regimes that would be required under different climatic conditions in order to facilitate prescribed burning.

Develop and implement research to examine the long-term population biology of key shrub and tree species. One emphasis should be on temporal patterns of recruitment. Some short-cuts to an understanding of long-term population dynamics may be possible. Possible approaches include examination of already-available sequences of remotely-sensed data and examination of population structures in conjunction with measured growth and mortality rates.

As the work at the core sites progresses, it will be possible to document rates of recovery from top-kill, at least for the main target species, breadfruit and gutta percha. This information will indicate which fire frequencies are likely to be most appropriate for helping manage species that sprout after fire. A full picture of post-fire recruitment should also emerge.

Medium- to long-term predictions of climate (rainfall) would be helpful in identifying times when either large-scale recruitment of native woody species is likely and what are the best times for burning. For example, it would be useful to predict the occurrence of seasons when rapid recovery of pastures post-fire can be expected.