Effects ofRhododendron maximumThickets on Tree Seed Dispersal, Seedling Morphology, and Survivorship

Abstract

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 JR. 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 hi the thickets. Germination of tree seeds (A. rubrum, L. tulipifera, Q. rubra, and Eetula lento] 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, Prunus > Quercus > Tsuga, was consistent with the shade tolerance 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 hi 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 hi 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.

Light environment under Rhododendron maximum thickets and estimated carbon gain of regenerating forest tree seedlings

“…Therefore, it is clear that seedlings of both species had a greater chance of experiencing irradiance near or below the light compensation point for photosynthesis in F + R sites than in F sites. Consistent with the observed carbon gain at these light levels, mortality of P. serotina seedlings in Coweeta ( > 90%) was much greater than Q. rubra (50%, Lei et al 2002) over 2 years. While our findings of the greater shade tolerance of Q. rubra over P. serotina agree with Farrar (1995), Canham et al (1994) found higher sapling mortality over 5 years in Q. rubra (81%) than P. serotina (31 %).…”

“…Three, tree seedlings were unable to achieve increased shade acclimation under the dimmer R. maximum cover, resulting in greater carbon deficit. Our data showed that most tree seedlings will not persist for more than 2-3 years in the shade of the thickets (Lei et al 2002 Seedlings in these brighter F + R locations should persist for longer periods of time. However, the absence of older tree seedlings in these brighter patches under R. maximum thickets implicates other contributing factors, such as below-ground competition and interference with mycorrhizal association Nilsen et al 2001), are further restricting tree seedling survival under the thickets.…”

Tree Seedling Establishment Under the Native Shrub, Asimina triloba

“…Shrub understories decrease seedling density, growth and survival (Beckage et al, 2000;Nilsen et al, 2001;Collier et al, 2002;Lei et al, 2002;Fagan and Peart, 2003;Gorchov and Trisel, 2003;Lambers and Clark, 2003). This effect has been found with invasive shrubs outside their native range, such as Lonicera maackii and Rhamnus frangula, as well as shrubs in their native range such as Rhododendron maximum, Buxus sempervirens and Juniperus communis.…”

“…Shrub canopies limit resources required for seedling establishment and survival. Shrubs reduce light levels on the forest floor (Beckage et al, 2000;Nilsen et al, 2001;Lei et al, 2002) and decrease soil moisture and nutrient levels (Putz and Canham, 1992;Nilsen et al, 2001). Litter accumulation (Beckage et al, 2000) and increased seedling predation by small mammals under shrubs (Rousset and Lepart, 2000) also create difficulties for seedling establishment.…”

“…This effect has been found with invasive shrubs outside their native range, such as Lonicera maackii and Rhamnus frangula, as well as shrubs in their native range such as Rhododendron maximum, Buxus sempervirens and Juniperus communis. In the Southern Appalachians, where the effect of a R. maximum understory has been extensively studied, seedling densities for at least six canopy tree species were decreased under that species: Acer pennsylvanicum, Acer rubrum, Fraxinus americana, Quercus rubra, Tsuga canadensis, and Prunus serotina (Beckage et al, 2000, Nilsen et al, 2001Lei et al, 2002;Lambers and Clark, 2003). An understory of L. maackii decreased both the number of species and seedling density (Collier et al, 2002), while removing the shoots of L. maackii increased the survival of four native tree species that were transplanted into plots with the shrub (Gorchov and Trisel, 2003).…”