There is debate in ecology about the relationship between diversity and ecosystem functions. While functional diversity indices have been adopted in addition to simple species richness, biodiversity's effect on ecosystem functions is still largely unclear, even for well-studied functions such as primary productivity. This is especially true for woody systems, where long generation times make from-scratch experiments across large spatial scales impractical. Ecological restoration projects have long been recognised for their potential value as experiments to test fundamental questions in ecology, but maturing forest restoration projects are rarely used to examine diversity-ecosystem function relationships. Because these projects vary in the number and types of species planted, they provide an opportunity to study maturing woody communities with different diversities across large spatial scales. I will discuss recent research on diversity-productivity relationships in Australian forest plantings using inventory data collected from wide rainfall, age, productivity and diversity gradients. Using these data, we assessed relationships between plot-level productivity and species richness, and between productivity and a set of functional diversity indices, across Australia and within smaller, environmentally similar, groups.
Overall, we found that diversity was a poor predictor of productivity across our study sites, regardless of the diversity index used, or whether forests are assessed within individual climate regions or across the entire continent. There were patterns associated with individual functional traits, but evidence that functional traits or diversity are useful predictors of forest productivity was much weaker than expected based on work in other systems and climates. On a positive note, the lack of any clear relationship with diversity suggests that including more species in these plantings, so long as they have viable ecological strategies, does not come at the cost of reduced productivity. This bodes well for achieving dual biodiversity and carbon benefits from ecological restoration in woody systems.