Overstorey-understorey interactions reveal trade-offs for achieving competing land-use goals in jarrah forest restored after bauxite mining: Initial prescription and targets affect restoration success over 32 years

Abstract

Quantitative ‘completion criteria’ agreed between mining companies and regulatory authorities are typically used to assess restoration outcomes. Completion criteria provide the basis for determining whether mining companies have fully met their obligations for site restoration. Relationships among potentially conflicting criteria, particularly around site productivity (e.g., timber production), the return of biodiversity, long-term ecosystem trajectories and other ecosystem services remain poorly understood during the development of mine restoration.For the jarrah forest of southwest Australia, we manipulated site productivity in post bauxite mine restoration by establishing four initial Eucalyptus marginata (jarrah) tree stocking rates in factorial combination with seeding a range of large N2-fixing understorey legume species. We measured jarrah growth rates over the 32-year duration of the experiment, soil-N concentrations at 3 and 32 years-of-age and understorey plant species richness and cover at 15 and 32 years-of-age.Stand basal area over bark (BA) increased with increasing tree stocking rates. However, there was a significant negative effect of increasing stand BA on understorey plant cover and species richness. These negative effects were still significant 32 years after the commencement of the experiment.Seeding large understorey legumes increased total soil-N. However, this did not increase tree growth. Rather, seeding large legumes reduced tree survival and diameter at breast height, and reduced understorey species richness, even though the applied seed mix in the large legume treatment contained 14 more species compared with the control.Our data highlight trade-offs between individual aspects of restoration success, such as tree stocking rates and understorey species richness. Further these results suggest reducing site productivity, by the judicious use of large understorey legumes and establishing low tree stocking rates, may facilitate the return of diverse vegetation communities. Finally, it is important to recognise that when restoration success is viewed through the lens of one-dimensional metrics such as completion criteria, restoration may function as a ‘zero-sum game’ in which a gain for one outcome (e.g., N2-fixation) may result in a loss to one or more other outcomes (e.g., tree growth and species richness).