Negative Associations among VA-Mycorrhizal Fungi and Some Ectomycorrhizal Fungi Inhabiting the Same Root System

Earlier surveys of Populus deltoides Bartr. and Salix nigra x ? had suggested displacement of VA-mycorrhizal fungi by ectomycorrhizal fungi from the middle of soil moisture gradients. Succession from VAto ectomycorrhizal associations had also been observed previously in several plant species. Data were therefore analyzed for evidence of negative interactions among VAand ectomycorrhizal fungi inhabiting the same root system. Fine roots (< 2 mm diameter) were sampled from natural populations of cottonwood growing in the eastern USA, and from two field-infected cottonwood clones grown under uniform soil conditions in North Carolina. Percentages of non-mycorrhizal, VA-mycorrhizal, ectomycorrhizal, and dually colonized root lengths were determined for terminal (most distal branches of the fine roots) and non-terminal roots in each sampling unit. Dual mycorrhizae (VAand ectomycorrhizal fungi in the same cross section) in terminal roots of almost all Populus sampling units were significantly lower than expected from a random assortment model. Negative association with VA-mycorrhizal fungi varied among ectomycorrhizal morpho-types. The degree of negative association among mycorrhizal fungi was determined using a new statistical test based on linear regression. High levels of ectomycorrhizal colonization were related to depression of VA-mycorrhizal colonization in terminal as compared with non-terminal roots within the same sampling unit. Although these data are not conclusive, they suggest that ectomycorrhizal fungi might displace VA-mycorrhizal fungi from roots of VA-/ectomycorrhizal hosts.