Suppression of ectomycorrhizae on canopy tree seedlings in Rhododendron maximum (Eriqceae) thickets in the southern Appalachians
Thickets of Rhododendron maximum (Ericaceae) (Rm) is the southern Appalachians severely limit regeneration of hardwood and coniferous seedlings. Experimental 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 substrates were removed, cornposited and redistributed among plots within the blocks (except for control plots). Seedliugs of northern red oak (Quercus rubra) and eastern hemlock (Tsuga canadensis) were planted in the plots and harvested at the end of the first and second growing seasous. Litter manipulation had no effect on total mycorrhizal colonization, but the distribution of Cenococcum geophilum mycorrhizae was altered. After the first year, percent mycorrh.izal colonization of hemlocks not in Rm thickets (62%) was at least three times higher than in Rm thickets (19%), and the ramification index (no. of mycorrhizae cm-1) had increased by more than a factor of four (2.83 versus 0.61). In addition, colonization of l-year-old hemlocks by C. geophilum was significautly higher within blocks with (10.4%) than without (4.6%) Rm. Differences in mycorrhizal colonization, ramification indices and 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. Biomasses of first year hemlock roots and second year shoots were, also reduced Mycorrhizal parameters were correlated with some growth parameters only for hemlock seedlings, but did not explain most of the variation observed.