Long-term recovery of carbon stocks in Acacia aneura (mulga) woodland following thinning

Abstract

Management of woody vegetation in grazed woodland systems has important implications for livestock production, carbon (C) sequestration and biodiversity. Many of Australia’s C sequestration projects are in grazed woodlands dominated by the fodder tree Acacia aneura (mulga), requiring management of competing priorities, particularly livestock production and C stocks. Partial clearing or thinning of mulga regrowth can be used to increase pasture (and therefore livestock) production. This paper will focus on the impact of thinning mulga woodlands on the recovery of woody C stocks. Three experimental sites in Queensland were established in the 1960s to study the impact of thinning of mulga woodlands. From an initial density of 1900-5600 trees ha-1, three replicated thinning treatments were implemented at each site: 40, 160 and 640 trees ha-1. We remeasured the sites after 50-60 years and estimated woody biomass, litter and coarse woody debris C stocks. Over this time, all treatments demonstrated additional pastoral productivity relative to uncleared woodlands (Bray et al. this conference). After 50-60 years, woody biomass and debris C stocks were similar among the three thinning treatments. Although there were no significant differences in C stock in live or total aboveground biomass, standing dead C stock was significantly lower in the 40 trees ha-1 treatment. Our findings indicate that mulga woodlands have the capacity to recover over 50-60 years and reach similar C stocks after thinning to low stand densities. Thinning of mulga should be considered in the context of regulatory guidelines and whole property management to balance multiple values. Further work is warranted to explore the potential of ecological thinning in mulga regrowth as a tool by which land managers could optimise their production-carbon-biodiversity goals.