An assessment of grassland restoration success using species diversity components.
We do not know which aspects of community structure and ecosystem processes are restorable for most ecosystems, yet this information is crucial for achieving successful restoration. We quantified three success criteria for 8- to 10-year-old grassland plantings in large-scale tallgrass prairie restoration (reconstruction) sites in Iowa, USA, relative to three nearby prairie remnant sites. The restoration sites included management of native ungulates and fire, important regulators of diversity and patchiness in intact grasslands. These have not been incorporated simultaneously into previous studies of restoration success. We used the additive partitioning model of diversity, where α is neighbourhood (quadrat) scale diversity, β is accumulation of species diversity across neighbourhoods, and γ is total diversity. We decomposed α into richness and evenness to determine if both were equally restored. The proportion of exotic biomass was similar between the restoration and remnant sites, but the proportion of exotic species and aboveground net primary productivity remained between two and four times higher in the restoration sites. α diversity (Simpson's 1/D) and richness values were exceptionally high in remnant sites, and approximately twice those of the restoration sites. α evenness was similar between the restoration and remnant sites. Distance per se between quadrats was not related to diversity after accumulated quadrat area was taken into account. Therefore, we may be able to use the additive partitioning model of diversity in areas that differ in size, at least at the scale of this study. Contrary to our original predictions, the proportion of β diversity (1-D) was approximately twice as high in the restoration sites than in remnant sites, possibly because patches of individual species were larger in the restoration. Synthesis and applications. We have shown that current restoration methods are unable to restore plant diversity in tallgrass prairie. Grassland restoration will be improved if the number of species that co-exist can be increased. New, local-scale restoration techniques are needed to replicate the high levels of diversity observed in tallgrass prairie remnant sites.