Two-year extension OK for savannah burning: study
Dr Dionne Walsh using a drip torch to light an experimental burn at the Kidman Springs fire experiment. Image: NT DAF.
SPELLING northern savannah country post-burn could extend the intervals between controlled burns from four years to six, according to the Australia’s only long-term study into grazing land savannah burning.
A combination of burning and post-burn spelling promoted palatable grasses and managed the tree-grass balance, according to the 30-year study at Victoria River Research Station 400km south of Darwin.
The study, driven by NT’s Department of Agriculture and Fisheries, compared the impact of fire on vegetation with varying fire frequency and season of fire.
The team tested the effects on woody cover with early burns (June) versus late burns (October) at either two, four and six-year intervals. The experiment was conducted on both a eucalypt woodland and on a grassland.
Dr Robyn Cowley
Lead researcher and senior rangeland scientist Dr Robyn Cowley said provided fuel loads were suitable for effective fires, a six-yearly, late-season fire might be sufficient to the manage the tree-grass balance on the woodland.
“Early dry season fires had little impact on woody plants in the woodland. However, on the grassland, early six-yearly fires were just as effective as late fires,” Dr Cowley said.
Since 2013, the research team has wet-season spelled the fire-experiment plots for around six months every two years. They reported that the spelling period effectively reduces the average stocking rate by one-quarter, compared to a site continuously grazed at the same stocking rate.
Spelling after fire is recommended to promote pasture recovery post-fire.
Dr Cowley said: “Before we spelled post-fire, the burnt areas were sometimes heavily grazed and we were seeing a worrying decline in the preferred pasture species in the grassland.”
Ongoing fire management of woody growth has a cumulative effect, and rainfall variation also affects woody plant density, she said.
“In the lead-up to the study there had been multiple decades of average-to-dry weather, but during the study there was higher rainfall than normal, and the woody cover just increased and increased.”
“Before we started spelling post-fire in 2013, the six-yearly and early-burnt plots tended to have more woody cover and less fuel loads, driving less-effective fires, and we concluded that fires need to be four-yearly to manage woody cover. However, as long as fuel loads are well-managed, six-yearly fires are likely to be effective,” she said.
Drone footage of controlled burns in October 2019 at the Kidman Springs grassland site. The Late 6 yearly burnt site is burning in this shot. Image: NT DAF.
Dr Cowley said the experiment was initially established to study how controlled burns affected vegetation structure and productivity.
“The semi-arid tropical savannas of northern Australia have evolved with fire. However, the presence of fire in the landscape has changed significantly with the reduction of traditional Aboriginal burning and increased control of wildfires. The incidence of fire is now greatly reduced on most land used for grazing by livestock,” she said.
Fluctuating rainfall and drought affected woody cover but the grass understorey growth proved resilient, with perennial grasses relatively unaffected. The study has revealed greater grassland diversity post-fire.
“Without fire, native woody encroachment into the productive grasslands can change the structure from a grassland to an open woodland and may negatively impact carrying capacity for livestock,” Dr Cowley said.
Other researchers involved in the study were Dr Rodd Dyer and Dr Mark Hearnden.
