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Established plants may facilitate the regeneration of closely related seedlings if they increase populations of mutualistic symbionts that would otherwise be limiting. In this study we examined the influence of ectomycorrhizal and vesicular-arbuscular mycorrhizal (VAM) trees on Quercus rubra seedlings to determine how trees influence mycorrhizal infection, nutrient uptake, and growth of seedlings. In two related experiments, we planted Q. rubra acorns adjacent to stump sprouts of Q. montana (=Near-Quercus) and Acer rubrum (=Near-Acer), and, in the second experiment, near Quercus spp. stumps that had not resprouted (=Near-Dead-Quercus). Congeneric Quercus were used to prevent root grafting; using stump sprouts minimized aboveground differences between treatments by limiting canopy size. In both experiments, Near-Quercus seedlings were infected by ectomycorrhizal fungi to a significantly greater extent than Near-Acer or Near-Dead-Quercus seedlings. Near-Quercus seedlings were also infected by a different and more diverse community of ectomycorrhizal fungi. Cenococcum geophilum, the only ectomycorrhizal fungus more abundant on Near-Acer seedlings than on Near-Quercus seedlings, appears to be relatively ineffective at increasing seedling nutrient uptake. Near-Quercus seedlings had greater concentrations and contents of N and P than other seedlings in both experiments. In the first experiment, Near-Quercus seedlings had greater growth than Near-Acer seedlings, although it was not clear if this represented beneficial influences of Q. montana, or an undetermined negative influence of A. rubrum. No significant growth responses were found in the second experiment; severe drought may have prevented the expression of growth potential. The results demonstrate indirect facilitation of seedlings by established congeneric trees through increased seedling ectomycorrhizal infection.
Dickie et al. (Fri,) studied this question.