Livestock and kangaroo grazing have little effect on biomass and fuel hazard in semi-arid woodlands

Abstract

Using livestock grazing as a tool to manage biomass and reduce fuel hazard has gained widespread popularity, but examples from across the globe demonstrate that it often yields mixed, context-dependent results. Grazing has potential to deliver practical solutions in systems where grazing reduces not only biomass but also reduces fuel hazard by altering vegetation connectivity or composition. We assessed the extent to which recent rainfall, rabbit and kangaroo grazing and recent and historic livestock grazing alters and accounts for variation in above-ground biomass, biomass composition and fuel hazard ratings across three broad communities in eastern Australia. We used nested linear models to assess biomass in three vertical vegetation strata, that matched the strata assessed in the Overall Fuel Hazard Assessment guide (i.e. litter/surface fuel; groundstorey vegetation/near surface fuel; and midstorey vegetation/elevated fuel) and Ordinal Logistic Regression to assess categorical fuel hazard ratings. Only recent kangaroo grazing reduced groundstorey biomass across all communities. Kangaroo grazing altered litter mass and significantly reduced surface fuel hazard in one community. Recent livestock grazing did not reduce fuel hazard, and despite significantly reducing half of our measures of biomass, these were not practical reductions. For instance, livestock grazing significantly reduced litter mass, however our model predicts that doubling our assessment of livestock grazing intensity only reduces total litter mass by 0.8%, or 8 kg per hectare in landscapes where average litter loads ranged from 3600 to 12,600 kg per hectare. Furthermore, long-term livestock grazing increased shrub biomass and in one community this increased elevated fuel hazard. There were few effects of rabbits. The effects of rainfall on biomass were up to an order of magnitude greater than any effects due to grazing, despite sampling during relatively average rainfall conditions. Our data suggest that management practices that seek to use livestock grazing to reduce biomass or its connectivity in these systems will not achieve practical reductions in biomass and or fuel hazard.