Sustainable use and management of natural resources is now a well-accepted principle, but its implementation continues to elude us. This is partly because of the complex and dynamic nature of ecological systems, interdependence among elements, and the impacts that resource management actions have on the functioning of an ecosystem. Livestock grazing is the most extensive landuse in Australia, occurring in over 55% of the continent. In forested landscapes this landuse has been characterised by clearing and/or modification of landscapes through overgrazing and changes to fire regimes, followed by re-growth and increase in density of woody vegetation and associated loss in grazing productivity. Consequently, the Vegetation Management Act 1999, now in place in Queensland, requires the coexistence of conservation of biological diversity and primary production. However, a challenge exists in how to integrate the two outcomes in a sustainable manner. This paper describes a case study application of the Ecosystem Dynamics Simulator (EDS), a forest growth model, as a decision support tool for the integration of sustainable livestock production and conservation of biological diversity at Mt Armour Nature Refuge, in western Queensland. The study was initiated by a request from the landowner for advice on how to manage increasing density of woody vegetation (vegetation thickening) without compromising conservation values. The landholder had expressed concern that the vegetation thickening was impinging on livestock production and was therefore in need of thinning. Thinning of vegetation within this bioregion is regulated by the Regional Vegetation Management Code for Brigalow Belt and New England Tablelands Bioregions, Queensland Government (Code) (DNRM, 2012). Firstly, for thinning to occur, the Code requires a verified crown cover increase or presence of > 250 immature stems in a 0.25 ha area. Secondly, if thinning occurs, all mature stems should be retained, and including 75 or 125 immature stems per 0.25 ha area depending on vegetation type. Our objective was to establish: 1) whether density of immature stems met thinning criteria of the Code and; 2) what thinning intensity and fire regimes were necessary to promote a sustainable ecosystem. In examining thinning intensity, the potential risk of loss of biodiversity based on recommended retention rates was also investigated. Working in collaboration with the landholder, data were collected from the property and the landowner's tree removal preference was also recorded. The EDS was used to project long-term changes in tree species composition and diameter size class structure for the uneven-aged mixed species native forests and to explore management scenarios that included the use of alternative thinning scenarios and fire regimes. The results confirmed: 1) the vegetation on the site was dense enough to be thinned under the current Code and projection indicated thickening is likely to continue if left un-thinned; 2) that the landowner's preferred tree removal intensity was not sustainable and; 3) a thinning intensity based on long-term simulation that considered species composition and stand size structure provided the best compromise outcome. The simulation results also confirmed likely inadequacy in the Code's recommended retention rates for immature stems in the studied vegetation types. Retaining only 75 immature stems (< 20 cm dbh) appeared to be a high risk option because large trees were few, growth rates were low and sapling recruitment was intermittent and vulnerable to drought related mortality. Consequently, retention of at least 140 immature stems would provide a more resilient stand structure and species composition under the dry growth conditions at the study site. This study demonstrated the valuable role that a vegetation growth dynamics simulator can play in elucidating long-term changes in tree species composition and diameter size structure in support of sustainable environmental management.
