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Browsing by Author "Walker, John F."

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    Diversity and Ecology of Mycorrhizal Fungi Associated With Oak Seedlings in the Appalachian Mountains
    Walker, John F. (Virginia Tech, 2001-12-13)
    Diversity of ectotrophic mycorrhizal (EM) fungi on out-planted seedlings of two oak species (Quercus rubra and Q. prinus) was estimated at two sites in mature mixed forests in the southern Appalachian mountains. Late – stage fungi were well represented. Total richness was 73 types, with 42 types having a frequency of only one. Thelephoroid / tomentelloid, russuloid, and cortinarioid groups were the richest. Dominant fungi included a putative Tuber sp. and Craterellus sp., and Laccaria cf laccata. Diversity was lower at a high elevation chestnut oak dominated site compared to a lower mesic cove – hardwood forest site. There was little evidence for fungal specificity to red oak versus white oak seedlings. We also compared EM fungus distributions on root systems of oak seedlings from samples taken in mid-July and early-September. The majority of EM types occurred only in the mid- or late-summer samples respectively. Dramatic shifts in mycobiont dominance were observed in relation to sample date, including increases in Cortinarius spp. richness, decreases in Thelephoraceae richness, and the disappearance of Amanita spp. types in the late- compared to mid-summer samples. A multi-stage model of seasonal EM dynamics is proposed, with implications for the niche expansion of associated phytobionts. In this model, generalistic mycobionts are most frequent and occur throughout the season. Other more specialized fungi show seasonal specificity. Relationships between species and communities of EM fungi and environmental parameters such as ericoid shrub abundance and edaphic characteristics were also examined. High diversity of EM fungi limited resolution of community level relationships given our sample sizes. Intraspecific variation in EM fungi with regard to microsite characteristics was also undetectable. No association between ericoid shrub dominance (Kalmia latifolia and Rhododendron maximum) and EM fungi was observed. We present a listing of EM fungus types with associated ranges of edaphic parameters and ericoid shrub abundance. The family Sebacinaceae is a basal hymenomycete lineage that includes members of the genera Tremellodendron and Sebacina. We present evidence suggesting the putative mycorrhizal status of two species of Tremellodendron. Tremellodendron appears to form both endophytic associations with achlorophyllous orchids and ectomycorrhizae with species of Quercus, Pinus and Tilia cordata.
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    The Inhibitory Effect of Rhododendron maximum L. (Ericaceae) Thickets on Mycorrhizal Colonization of Canopy Tree Seedlings
    Walker, John F. (Virginia Tech, 1998-04-28)
    Thickets of Rhododendron maximum (Rm) in the southern Appalachians impose severe limitations on the regeneration of hardwood and coniferous seedlings. Interactions between Rm thickets and ectomycorrhizal colonization were examined to explain seedling inhibition. Experimental blocks were established in and out of Rm thickets in a mature, mixed hardwood/conifer forest in Macon County, North Carolina. Planted seedlings of red oak (Quercus rubra) and hemlock (Tsuga canadensis) were harvested at the end of the first and second growing seasons. Litter manipulation had no effect on total mycorrhizal colonization. Mycorrhizal colonization and ramification index (# mycorrhizae cm-1) were depressed and colonization by Cenococcum geophilum increased in blocks with versus without Rm. After the first year, percent colonization of T. canadensis not in Rm thickets (62 %) was three times higher than in Rm thickets (19%), and the ramification index was increased by more than a factor of four (2.83 versus 0.61). Mycorrhizal colonization levels were correlated with root weight and shoot weight in both hemlock and oak seedlings, but did not explain most of the variation observed. Sporocarps of 69 putatively ectomycorrhizal species were collected on the blocks. Species diversity and overall community structure was similar in and out of Rm thickets. Individual species, i.e. Lactarius speciosus and Russula krombholzii, were significant indicators of forest without Rm thickets. Rhododendron maximum thickets probably affect the process of mycorrhization. The reduced level of mycorrhizal capacity under Rm thickets could be a factor in the increased seedling failure in Rm thickets.