Effects of Rhododendron maximum thickets on tree seed dispersal, seedling morphology, and survivorship

In the southern Appalachian forests, the regeneration of canopy trees is severely inhibited by Rhododendron maximum L., an evergreen understory shrub producing dense thickets. While light availability is a major cause, other factors may also contribute to the absence of tree seedlings under R. maximum. We examined the effects of R. maximum on several life history stages of tree species, including seed dispersal, seed bank germination, seedling growth, and survivorship. We found no significant effect of R. maximum on seed reaching the forest floor for Acer rubrum, Liriodendron tulipifera, Quercus rubra, Quercus prinus, Carya spp., and Nyssa sylvatica. This indicates that either seed output of maternal trees rooted within the thicket were unaffected by R. maximum or seed dispersal from surrounding areas into thickets compensated for a lower seed production of canopy trees rooted in the thickets. Germination of tree seeds (A. rubrum, L. tulipifera, Q. rubra, and Betula lenta) from the seed bank also was not reduced by leaves and substrates within the thickets. Seedling mortality of all species (Q. rubra, Prunus serotina, and Tsuga canadensis) planted in our experimental plots was up to fivefold higher in thickets of R. maximum compared with those outside the thickets. The order of mortality under the R. maximum thickets,, was consistent with the shade tolerance Prunus 1 Quercus 1 Tsuga ranking of these species. Loss of Tsuga seedlings was attributed to burial by litter rather than shade. Surviving seedlings of Quercus and Prunus in R. maximum thickets were taller than those outside the thickets, but the seedlings in R. maximum thickets produced significantly fewer leaves, smaller total leaf area, leaf mass, and stem mass. Leaf N (%) was significantly higher in Quercus seedlings in R. maximum thickets compared with seedlings outside the thickets. Moreover, no difference was found in leaf N (%) between forest types for Prunus and Tsuga, indicating that seedlings in R. maximum thickets were not N limited. Rather, light limitation, herbivory, and litter fall contributed to the lack of tree regeneration under R. maximum thickets.