Over the last century, many grasslands worldwide have transitioned from a graminoid to atree/shrub-dominated state in a short period of time, a phenomenon referred to as woody encroachment.Positive feedbacks and bi-stability are thought to be important drivers of woody encroachment, but there islittle empirical evidence to suggest that positive feedbacks accelerate the woody encroachment of mesicgrasslands. In mesic tallgrass prairie, shrub establishment does not directly facilitate seedlingestablishment. Yet, shrub establishment may facilitate the clonal spread of existing shrubs into nearbypatches, because clonal reproduction might circumvent barriers that typically limit woody seedlings. Ourresults show that when Cornus drummondii (the predominate encroacher of mesic tallgrass prairie) extendsrhizomatous stems into open grasslands, these stems use the same deep soil water sources as maturestems—thereby avoiding competition with grasses and gaining access to a reliable water source. Inaddition, herbaceous fuel concentrations are lower at the shrub/grass interface than in open grasslands,reducing the potential impacts of subsequent grassland fires. We propose that the release from resourceand fire limitation results in a positive feedback loop as clonal stems are able to extend into surroundingpatches, circumvent demographic barriers, mature, and spread by developing their own clonal stems.Long-term data on site (26 years) corroborates this interpretation: the size of deep-rooted clonal shrubspecies has increased 16-fold and their cover has increased from 0 to 27%, whereas the cover of shallowrooted species (both clonal and non-clonal) has only increased marginally. Together, these results suggestthat (1) positive feedbacks can facilitate mesic woody encroachment and (2) bi-stability exists in mesictallgrass prairie.