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Breadfruit, which is thickening up in large areas of the northern
Gulf of Carpentaria in Queensland
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In the left of this photo a heavy stand of breadfruit grows above
the grass of the paddock. This part of the paddock was left as an
unburnt control. The photo below shows the burnt part of the
paddock, and the successful reduction of breadfruit
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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.
Photos: Jim Kernot
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The grasslands and open woodlands of savanna
grazing lands are undergoing woody plant thickening says Jim
Kernot. Here he reports on a project that has been trialling
burning regimes—in the northern Gulf of
Carpentaria—that aim to help manage vegetation change.
Collaboration with
landholders | Results to date | Fire as a management tool | Recommendations
If graziers were asked to name the greatest threat to the
long-term viability of the north Queensland beef industry, few
would identify woody plant thickening and shrub encroachment as the
most important. I would argue that no other factor poses as great a
risk and the existing problem is becoming worse every year.
Firstly, just what is the problem? The grasslands and open
woodlands that support our grazing industries are undergoing woody
plant thickening. The thickening consists of trees, shrubs and
re-growth and involves both native and introduced species. The
result is that our grasslands are becoming woodlands and our open
woodlands are becoming closed forests. This in turn results in a
significant reduction in the amount of available pasture due to the
competition provided by the woody species. Less available pasture
translates into reduced carrying capacity, reduced weight gains and
therefore a significant decline in animal production and
profitability.
Examples abound across tropical Queensland: the invasion of
Mitchell grass plains by gidgee, boree, blackwood and in the north
by prickly acacia. Gutta percha is invading the black soils of the
Gulf, and breadfruit tree in the Georgetown district. Currant bush,
wattles, eucalypts, rubber vine, the list is endless.
The Gulf Fire project is being conducted under the umbrella of
the Tropical Savannas CRC and is a collaborative effort between the
Queensland Department of Primary Industries, CSIRO, Meat and
Livestock Australia, the Northern Gulf Resource Management Group,
Rural Fires and landholders. The project began in 2002, and is
testing the use of fire to manage woody species in the Gulf
Savanna. Two main species were targeted by the study: breadfruit
(Gardenia vilhelmii) and gutta percha (Excoecaria
parvifolia) as well as yellow-woods (Terminalia spp.),
eucalypts and acacias. Breadfruit is a small straggly tree that
grows to about 7 m high and is found on red earths, granites and
gravelly soils. Gutta percha is a straggly shrub growing to 6 m
high and preferring heavier (clay) soils. The two species tend not
to occur together.
Unlike some other Australian rangelands, including other regions
of tropical savannas and woodlands, fire is quite broadly accepted
by the pastoral community of the Gulf savannas as a useful
management tool for woody plant control.
The project team has had no difficulty locating sites that
landholders are prepared to burn as part of the program. This has
made it relatively easy to develop a network of study sites through
which the scientific and pastoral communities can together broaden
their experience of fire in the Gulf savannas. The selected
sites—five core sites and 10 satellite sites—are all
located on working cattle properties, and are spread widely across
the northern Gulf savannas from east of Mt Surprise to north of
Normanton. Together they represent considerable diversity of soils,
vegetation, climate, grazing and fire history and general land
condition.
Below-average rains in wet seasons from 2002 to 2004 slowed down
the progress of the project, with only low fuel loads available on
a number of core and satellite sites. Fires were therefore delayed
in an attempt to build up at least 1500 kg/ha dry matter on these
sites. In some cases, low rainfall precluded prescribed burning or
has meant that fire intensities were very low. On grazed lands
rainfall and grazing inevitably interact as principal determinants
of fuel loads, but generally dry conditions can place pressure on
pastoral managers to graze paddocks that were intended for spelling
as a precursor to burning.
Early indications were that storm season burning, after
spelling, can halt or reverse woody thickening trends. It was
apparent even from the few species being addressed in this project,
that there were meaningful differences between species. After two
burns at one core site, there was a 58% death rate for breadfruit;
however, following single fires, the level of mortality was always
less than 20%. Death rates for gutta percha reached 45% at another
core site two years after a single burn.
These levels of mortality are considerably 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 suggests there is value in examining the effects
of multiple fires, and the consequences of even single fires
for the structure of populations of woody species.
While the total number of some less susceptible woody species
may not be greatly reduced by fire, their average height will be
significantly reduced. This will change the competitive balance
between woody species and grasses and lead to enhanced quality and
quantity of native pastures.
Fire can be a useful tool to manage vegetation, and of
particular species within it, even if species targeted for control
do not experience high mortality following a single fire. This is
because the goals of vegetation management may be achieved 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 management objectives (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 considering 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 was 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 environmental variables
determining them.
Recommendations concerning the use of fire should be made on the
basis of an understanding of the species’ long-term dynamics.
However, 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.
Graziers must identify and appreciate the cause of their
thickening problem. In most cases sustainable long term grazing
management is fundamental to addressing the situation. Fire is a
tool that can be used to manipulate woody components of Gulf
savanna vegetation—however, species will respond differently
from one another. Gutta percha is relatively susceptible to fire
and breadfruit is relatively resilient to fire. Landholders need to
be aware of these differences, and 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.
Prescribed, targeted burning needs to be planned well in
advance. There will always be factors such as rainfall that are
beyond our control, 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 involve
spelling and appropriate grazing management.
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 landholders develop grazing management strategies that
facilitate appropriate fire regimes.
Long-term fuel dynamics in relation to grazing and rainfall also
need to be examined. 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.
As the work at the core sites progresses, it will be possible to
document rates of recovery from fire, at least for the main target
species, breadfruit and gutta percha.
This information will indicate which fire frequencies are likely
to be helpful in managing species that sprout after fire. A full
picture of post-fire recruitment should also emerge.
