Over the past decade many studies have focused on encroachment of woody species into different plant communities in ecosystems around the world. Both the causes and consequences have been well quantified. In contrast, the underlying mechanisms of woody encroachment have not been well studied and certainly not presented in a synthetic fashion. The goal of this OOS is to bring together a collection of presentations focusing on various aspects of why some woody species are successful while others are not, and what specific mechanisms drive their expansion. These studies will include physiology, structure, symbiotic associations, reproductive potential, seed dispersal, etc. and will focus on both natural expansions and the well documented invasion of exotic woody species into grasslands and other communities.