From observations to evidence about effects of mixed-species stands

A critical understanding of the scientific evidence that we have about the effects of tree species diversity on ecosystem properties and processes is required to guide practical forest management as well as future research. However, current understanding is limited by the lack of an appropriate framework for evaluating the reported evidence. In this chapter we outline how research on mixed-species forests may fit into concepts of ecosystem hierarchy and how previous studies may be ranked regarding their level of evidence. We introduce the most important hypotheses and theories underpinning research on the relationship between tree diversity and ecosystem functioning and illustrate how these may be tested by analyses of forest inventories, experiments, and exploratory research platforms or a combination of these.