Nestedness patterns reveal impacts of reduced rainfall on seedling establishment in restored jarrah forest

Directional climate change can potentially cause a nested pattern of species occurrences as species move or go extinct. That is, species-poor communities may become a nested subset of species-rich communities. There is a precedent for understanding these patterns in the context of historical climate change but few researchers have studied these patterns in the context of recent changes to climate. Here we show the value of nestedness analyses for understanding plant community responses to reduced annual rainfall using data on seedling establishment in restored jarrah forest between the years of 1992 and 2010. Specifically, we recorded the annual seedling establishment of species assemblages in plots 15 months after restoration. We tested to what extent jarrah-forest assemblages that established in low (<1000 mm) rainfall years were nested within assemblages that established in years of moderate (1000-1200 mm) and high (>1200 mm) rainfall, and whether assemblages established following lower standard restoration practice were nested subsets of those following higher standard practices. We also tested how both types of nestedness patterns varied among trait groups defined by status (i.e., native or non-native), life-form and seed size. We found high support for species and trait assemblages that established in dry years being a nested subset of assemblages that established in years of moderate rainfall, and consistently low support for nestedness of high in low, and moderate in low, rainfall years. Nestedness patterns associated with restoration practice were as we predicted. Recruitment failure in low rainfall years was the most parsimonious explanation for nestedness patterns associated with rainfall (i.e., selective environmental tolerance). Nestedness patterns associated with restoration practice were explained by differential seed dispersal of species via topsoil and their tolerance of inferior restoration practice. Taken together, we demonstrate the application of the nestedness approach for understanding community responses to climate change in a restoration context. Indeed, generalising species responses to climate change by linking these to ecological processes and traits will help to meet the current global demand for forest restoration. Therefore, we anticipate our findings will interest practitioners working to restore the world's forests under climate change.