Fire and vegetation management in pasture lands of the Victoria River District, Northern Territory

Abstract

Anecdotal and scientific evidence suggests that tree and shrub thickening isnoccurring throughout savanna woodlands in the Victoria River District (VRD) of thenNorthern Territory. A likely contributing factor is thought to be changes to firenregimes, primarily a reduction in the incidence, extent and intensity of burning,nresulting from both direct and indirect impacts of grazing and pastoral land-use. Anprogram of long-term fire research was established in late 1993 on Victoria RivernResearch Station (Kidman Springs). This work investigated the impact of firenfrequency, fire season, fuel loads and fire intensity on woody plants and pasturencondition in arid short grass (ASG) and ribbon-blue grass (RBG) communities innthe VRD.nWoody plant height and pasture community, but not fuel load or burning season,nsignificantly influenced woody plant mortality. Mortality rates following burningnaveraged 2% in RBG and 5% in ASG. Smaller plants had higher mortality rates.nMortality rates for Lysiphyllum cunninghamii (bauhinia) and Carissa lanceolatan(conkerberry) were 3% and 5% respectively, and higher compared to 1% fornTerminalia volucris (rosewood). Despite low plant mortality, fire induced top killncan be used to manipulate canopy cover and plant height in both pasturencommunities. Top kill occurs following the death of aerial branches and is relatednto plant height and factors that influence fire intensity. Intense fires have the mostnimpact on top kill and plant structure. Although top kill was greater following latedrynburns, season of burn had no impact on average canopy cover. Increasingnburning frequency did result in significant reductions in canopy cover for bothnpasture types. Once woody plants exceed 2.0 -2.5 meters in height they becomenmore difficult to control with fire. Implementing a prescribed bum every 5-6 yearsnwill maintain woody plants below this critical height.nFuel loads of at least 2000 kg DM/ha, fuel cover of at least 60%, as well asnappropriate fire weather and fuel curing state are required to cause significantnchanges to woody plant structure. Grazing pressure on each pasture type needsnto be managed prior to burning to ensure suitable fuel conditions.Without burning, native woody plant growth/regrowth in both RBG and ASGnpasture communities continues rapidly. Results indicated that woody plant growthnand recovery in the absence of fire continues at the rate of between 10-20 cm pernyear for all species less than 200 cm. Obvious recovery of woody plant height wasnobserved in plots unburnt for 5 years, but no increase in plant density wasndetected. n