Suppression of ectomycorrhizae on canopy tree seedlings in Rhododendron maximum L-(Ericaceae) thickets in the southern Appalachians

Thickets of Rhododendron maximum(Ericaceae) (Rm) in the southern Appalachia ns severely limit regeneration of hardwood and coniferous seedlings. Experi mental blocks were established in and out of Rm thickets in a mature, mixed hardwood/conifer forest in Macon County, N.C. Litter and organic layer sub strates were removed, composited and redistributed among plots within the b locks (except for control plots). Seedlings of northern red oak (Quercus ru bra) and eastern hemlock (Tsuga canadensis) were planted in the plots and h arvested at the end of the first and second growing seasons. Litter manipul ation had no effect on total mycorrhizal colonization, but the distribution of Cenococcum geophilum mycorrhizae was altered. After the first year, per cent mycorrhizal colonization of hemlocks not in Rm thickets (62%) was at l east three times higher than in Rm thickets (19%), and the ramification ind ex (no. of mycorrhizae cm(-1)) had increased by more than a factor of four (2.83 versus 0.61). In addition, colonization of 1-year-old hemlocks by C. geophilum was significantly higher within blocks with (10.4%) than without (4.6%) Rm. Differences in mycorrhizal colonization, ramification indices an d colonization by C. geophilum were absent or less pronounced on 2-year-old hemlocks and 1- and 2-year-old oak seedlings. The biomasses of first year oak roots and shoots and second year shoots were 50% less in Rm thickets. B iomasses of first year hemlock roots and second year shoots were also reduc ed. Mycorrhizal parameters were correlated with some growth parameters only for hemlock seedlings, but did not explain most of the variation observed.