Abstract Subcanopy shrubs and perennial herbs inhibit recruitment of canopy trees in forests around the world. Although this phenomenon is widespread, and can have significant effects on community dynamics, the mechanisms of inhibition are not well understood. In the southern Appalachian region, Rhododendron maximum inhibits the recruitment of canopy trees in forests of northern red oak (Quercus rubra). We have shown, in previous research, that processes occurring before canopy tree seed germination are not responsible for this inhibition. Therefore, post-germination processes, such as competition for resources are most important. In this study we show that the presence of a thicket of R. maximum in the understory reduced the availability of light by 80%, the frequency and duration of sunflecks by 96%, the availability of water by 20% and the availability of several soil nutrients (particularly cations) by variable amounts. Moreover, the survival of Q. rubra seedlings in the understory over 3 y was signif...