Does grazing exclusion in Australia's rangelands affect biomass and debris carbon stocks?

Abstract

Australia is dominated by 6 million km2 of rangelands that contribute significantly to the livestock industry and carbon (C) market; yet, few studies have quantified the influence of grazing management on woody biomass and C stocks across these rangelands. Here, we quantified stand-level C stocks (Mg C ha−1) in live aboveground biomass (AGB), live belowground biomass (BGB), standing dead mass, coarse woody debris (CWD) and litter of rangeland vegetation at 46 long-term (average 40 year) grazing trials comprising grazed control plots paired with exclosures protected from grazers. Four major woody vegetation types were considered, namely, those dominated by Eucalyptus species, Acacia aneura, other Acacia species, and other species. Most C stocks (except litter C) were highest in Eucalyptus-dominated vegetation and lowest in the ‘other species’ types. There was high variability and few significant grazing effects on C stocks. There was a trend of higher AGB-C, BGB-C, woody basal area (m2 ha−1) and litter C with grazing exclusion at sites with mean annual precipitation of >300 mm, but the magnitude of increase was highly variable. The impact of the type of grazer was highly variable, although there was a trend of higher woody biomass C where rabbits and domestic livestock were excluded than with exclusion of domestic livestock only. The relative effects of grazing declined as the woody basal area of grazed control plots increased. Our results showed that although grazing exclusion can facilitate C stock accumulation in woody vegetation, it is difficult to predict when and where this may occur. To enable greater clarity, future studies may need to collect additional information, including data on soil water availability and current and historical grazing intensity.