by Rodd Dyer, Department of Business, Industry and
Development (Primary Industries)
From Managing for Healthy Country in the VRD, Tropical
Savannas CRC 2000.
Pastoral management trials
On pastoral lands, controlled fire can be used for a variety of
reasons. It can be used to manage native tree and shrub
populations, maintain pasture condition and quality, reduce fuel to
prevent wildfires, manage grazing distribution, control exotic
weeds, and establish and manage improved pastures. The
effectiveness of fires in each of these roles, however, depends on
a range of factors that can be managed to achieve the most
desirable result. These include the frequency and intensity of
fire, the season in which it burns, the amount of fuel available
and the nature of grazing management both before and after the
burn.
Methods
A project established at Kidman Springs in 1994 investigated
what sort of fire regime is most effective for management of woody
vegetation and pasture condition on both black (cracking clays) and
red soils. Early and late dry season burns were trailled on arid
short grass on red soils and ribbon grass on black soils in two
paddocks with a range of fire frequencies over the five year period
between 1994 and 1998. Frequencies ranged from no burning to
burning up to three times in five years. Specific effects of
burning on pasture response and woody plants were examined.
Effects on trees and shrubs
Tree and shrub species in both vegetation communities are well
adapted to tolerate fire. Survival of plants after burning is very
high. Following burning, plants re-sprout from undamaged buds,
either from branches or from the stem base if aerial branches are
killed (top-killed). Plant mortality following burning for all
species remained below five percent and was not affected by
frequency, intensity or season of burn. Regular fire acts on woody
plants by suppressing growth and reducing plant height. The impact
on trees and shrubs is largely determined by fire intensity and
plant height. A range of factors such as fuel load, continuity and
weather conditions influence fire intensity.
The role of fire in grazing lands is to manipulate plant height
structure, rather than reduce plant density. This can be achieved
by using periodic fire to top-kill taller plants of emerging
populations, thereby reducing canopy height and suppressing woody
plant growth.
When regular fire is removed, the growth of woody plants
continues unhindered. Woody plants one metre in height will take
between seven to 12 years to grow two to three metres. Once plants
exceed two meters in height, rates of top-kill decrease
significantly and only hot to extremely hot fires are able to
achieve sufficient top-kill of aerial branches. Under normal
conditions fuel loads of at least 2000 kg DM(dry matter)/ha are
required for effective fires. Late dry season burns are generally
more intense and effective at controlling woody plants compared to
early dry season fires.
Fire frequency
The most appropriate fire frequency depends on annual rainfall,
as this will determine the rate of fuel accumulation and woody
plant regrowth. Burning frequency should be every:
- two to four years in high rainfall areas;
- three to six years in medium rainfall areas; and
- five to ten years in low rainfall areas.
The most desirable fire frequency will be determined so that
plants do not exceed the height where they become difficult to
control with fire during the interval between burns.
Pasture quality and condition
The use of fire can have significant impact on pasture quality
and condition. Without burning and under low grazing pressures dead
grass material accumulates, reducing pasture vigor and making
pastures unattractive to livestock. Burning pasture in good
condition during the dry season generally has little long-term
impact on pasture production and composition and increases the
availability of nutritious green regrowth to cattle.
Ribbon grass pastures
Ribbon grass pastures on cracking clays are resilient and well
adapted to periodic burning. Under all but the most frequent fire
regimes burning generally results in only small short term
reductions in total yield and cover and has negligible impact on
species composition. Frequent burning (biennial fires) resulted in
significant longer term reductions in yield and increases in
annuals grasses such as Flinders grass. The minimum interval
between burns therefore should be no less than three years when
implementing prescribed burning regimes.
Arid short grass pastures
In contrast, arid short grass communities are sensitive to
grazing and burning. Burning in arid short grass communities
(regardless of fire frequency) may promote the dominance of annual
grasses at the expense of perennials. An increase in the proportion
of bare ground may also occur even during good rainfall years. When
these pastures are in poor condition, the detrimental effects of
burning are exacerbated. In the absence of burning, rapid recovery
of pasture condition is possible when grazing pressure is low or
absent.
Recovery is accelerated during periods of above average
rainfall. Despite increases of trees and shrubs in this vegetation
community, fire should therefore be used with extreme care due to
the potential adverse impacts of burning on species composition and
ground cover. Fire may have a role in pasture management if
pastures are in good condition and if grazing pressure is low.
The sites established for this project have also been used to
examine the impacts of fire on vertebrate and invertebrate animals,
discussed under the 'Grazing, vertebrates and soil type' link above.
