Contribution of species and functional richness to carbon storage in eucalypt woodland restoration

Biologically diverse forest and woodland restoration can mitigate climate change and biodiversity loss. Understanding the trade-off between carbon storage and biodiversity is important to maximise the value of restoration opportunities. Ecological theory suggests biological diversity contributes positively to carbon storage until the relationship plateaus (i.e., species are added with no effect on carbon storage), and yet in practice, tree monocultures are perceived to sequester more carbon than diverse plantings. Here, we measured the contribution of plant diversity to carbon storage in trees and shrubs ten years after planting for yate (Eucalyptus occidentalis) woodland restoration. We found no evidence for a trade-off between carbon storage and plant diversity. We planted yate with mixes of yate woodland species including native grasses, shrubs, and trees. Ten years after planting, grass establishment was poor, and density of trees and shrubs was patchy. There was some unassisted recruitment of woody species. Diversity was measured as observed species richness and observed functional richness, which incorporated taxonomic diversity, structural diversity, and presence/absence of nitrogen-fixing legumes. Carbon stocks measured were above and belowground tree and shrub biomass, leaf and twig litter biomass, and organic carbon (%) in the topsoil. Across all three carbon stocks diverse yate plantings sequestered as much carbon as yate monocultures. Observed species richness had a weak negative effect on tree and shrub biomass in the best linear mixed model but this effect diminished with the removal of an outlier. Tree density had a positive effect on tree and shrub biomass in models with and without the outlier. There was no effect of diversity on litter biomass and soil organic carbon. Instead, linear mixed models of these stocks showed the influence of soil properties. Litter biomass increased with soil potassium and decreased with soil pH. Soil organic carbon increased with initial soil organic carbon and soil nitrogen. There are potential additional biodiversity benefits of diverse plantings that we did not measure (e.g., soil erosion control, wildlife habitat). We call for more field experiments to explore these potential benefits and to generate the ecological knowledge needed to improve the quality of global forest and woodland restoration efforts.