Does Rhododendron maximum L. (Ericaceae) Reduce the Availibility of Resources Above and Belowground for Canopy Tree Seedlings?
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 significantly reduced (by about 40%) in the presence of a R. maximum thicket compared with forest without a thicket. Seedling survival was positively associated with light availability, but the slope and intercept of that relationship was different in forest witb or without R. maximum. Therefore, belowground processes are involved in reduced seedling survival under the R. maximum thicket. The resources most associated with survival of Q. rubra seedlings were water and light. Although many soil nutrients were significantly lower in forest with R. maximum than in forest without R. maximum no individual nutrient was a significant covariate with Q. rubra survivorship.Our data indicate that competition for resources both above- and belowground is an important mechanism for inhibition of canopy tree recruitment by R. maximum. Light is important to seedling survival, but is not the only important factor. Water availability and the ability to accumulate soil nutrients are equally or more important than light to survival of canopy tree seedlings in the presence of a subcanopy thicket of R. maximum.