Inhibition of seedling survival under Rhododendron maximum (Ericaceae): Could allelopathy be a cause?

In the southern Appalachian mountains a subcanopy species, Rhododendron max imum, inhibits the establishment and survival of canopy tree seedlings. One of the mechanisms by which seedlings could be inhibited is an allelopathic effect of decomposing litter or leachate from the canopy of R. maximum (R. m.) on seed germination, root elongation, or mycorrhizal colonization. The potential for allelopathy by R.m. was tested with two bioassay species (let tuce and cress), with seeds from four native tree species, and with three e ctomycorrhizal fungi. Inhibitory influences of throughfall, fresh litter, a nd decomposed litter (organic layer) from forest with R.m. (+R.m. sites) we re compared to similar extractions made from forest without R.m. (-R.m. sit es). Throughfall and leachates of the organic layer from both +R.m. and -R. m. sites stimulated germination of the bioassay species above that of the d istilled water control, to a similar extent. There was an inhibitory effect of leachates of litter from +R.m. sites on seed germination and root elong ation rate of both bioassay species compared with that of litter from -R.m. sites. Native tree seed stratified in forest floor material from both fore st types had a slightly higher seed germination rate compared with the cont rol. A 2-yr study of seed germination and seedling mortality of two tree sp ecies, Quercus rubra and Prunus serotina, in field plots showed no signific ant influence of litter or organic layer from either forest type. Incorpora ting R.m. leaf material into the growth medium in vitro depressed growth of one ectomycorrhizal species but did not affect two other species. Leaf mat erial from other deciduous tree species depressed ectomycorrhizal growth to a similar or greater extent as leaf material from R.m. In conclusion, R.m. litter can have an allelopathic effect on seed germination and root elonga tion of bioassay species as well as some ectomycorrhizal species. However, this allelopathic affect is not manifest in field sites and is not likely t o be an important cause for the inhibition of seedling survival within thic kets of R.m.