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Loss of biodiversity can impair the functioning of ecosystems. That is the broad consensus after 20 years of research, encompassing the diversity of plants and other taxa in a range of ecosystems. Thus, current global threats to plant diversity may also threaten the functioning of ecosystems and the ecosystem services that they provide to human societies. But, a recent consensus paper concluded that we need to better understand biodiversity-function relationships in the context of real-world change. A research frontier was laid out with two main areas of focus: the mechanisms for biodiversity-function relationships, and the real-world applications of biodiversity-functioning for ecosystem management. I present four studies that span this frontier. Firstly, I show that existing methods underestimate the impact of diversity loss on the remaining plant community. I tested the accuracy of a method for measuring the strength of competition in natural communities. Competition is one of the ecological processes that structure plant communities, so is key to understanding the causes and consequences of diversity loss. I found that this method consistently underestimates the strength of competition, which undermines our ability to infer how strongly plant communities are structured by competition. Methods need to be improved to progress our ability to predict the impacts of diversity loss. Secondly, I show how better retention of resources can lead more diverse communities to function as well as, or better than depauperate communities. I developed a mechanistic model of competition for a single resource that is lost over time, to explain how diverse communities could increase ecosystem functioning more than less diverse communities. I found that transgressive overyielding, where species mixtures outperform the best monoculture, was possible but rare. Mixtures commonly performed as well as the best monoculture. This model offers explanations for results found in long-term experiments. Thirdly, I show how using higher levels of tree diversity could improve the effectiveness of re-afforestation schemes restoring tropical forest structure and functioning. I analysed the survival and growth of seedlings during the first decade of a field-scale long-term experiment in selectively logged tropical forest. I found that species varied independently in their survival, growth and responses to environmental variation. The number of surviving trees, and their stem area, might be less variable throughout this complex landscape if more diverse mixtures of species are planted. This could improve the effectiveness of restoration of these degraded forest ecosystems. Finally, I show that organic agriculture supports higher levels of biodiversity with the potential for associated ecological benefits. I assessed how organic and conventional farms relatively impact biodiversity. Agriculture is the largest global land use and therefore greatly impacts biodiversity, which may undermine ecosystem functioning and even the provisioning services that farming depends upon. Thus, there is great need to understand realistic impacts of agriculture on biodiversity, to understand its consequences for real-world ecosystems and potential implications for food security. I found that biodiversity is reduced by a third overall on conventional farms relative to organic farms. Plant diversity is reduced by almost three quarters. There is a clear local ecological benefit to organic farming. Future work should ascertain how this realistic level of diversity loss will impact the functioning of ecosystems. I conclude by discussing this research in the context of the mechanisms and applications of plant biodiversity effects on ecosystem functioning. Finally, I describe some emerging topics and methods that will aid future progress in this field.