The Impact of Mycorrhizal Networks on Quercus rubra Seedling Recruitment

Abstract

Main Objectives – Mycorrhizal fungi are ubiquitous plant mutualists and can be classified into two types: ecto- (EMF) - dominant in temperate forests and arbuscular (AMF) - globally dominant. Both types form mycorrhizal networks (MN), consisting of fungal hyphae that connect plants of the same and different species. The degree to which the MN of adult trees facilitate or inhibit other plants, specifically seedlings, is unclear. This study examines how the MN associated with different species of adult trees affect mycorrhizal colonization, growth, survival, and root fungal community of Quercus rubra seedlings, an EMF tree species. Methods – Seedlings were planted under four adult tree species of varying mycorrhizal types: Q. rubra (EMF), Q. velutina (EMF), Carya glabra (EMF), and Acer saccharum (AMF). Before field transplant, two thirds of the seedlings were grown within micromesh bags that allow hyphae to pass while blocking roots. Seedlings were separated into treatment groups: no bag control (C), bagged control (BC), and disturbed (D). C and BC groups were transplanted and allowed to grow undisturbed. Seedlings in the D group had their connection to the MN disrupted every 2-3 days. Seedlings were collected at the end of the growing season and survival and biomass were recorded. A subset of the seedlings was examined for EMF colonization, and all colonized tips were collected. Collected tips had fungal DNA extracted, amplified, and sequenced to determine the EMF community present on the roots of the seedlings. Results – Seedlings in the D treatment were colonized by a different suite of mycorrhizae than the two control groups, complicating interpretation of the effect of the MN on seedlings performance. Q. rubra seedlings benefited from MN connection when grown under con-specific and hetero-specific trees; but the effect of EMF colonization when connected to the MN was negative under C. glabra. Furthermore, under A. saccharum, seedlings benefited more from the EMF community that colonized the roots when they were disconnected, than from the EMF community when connected to the MN. Conclusions – The findings in this study underline the importance of MNs in the recruitment of Q. rubra, a common canopy tree in the temperate forests of eastern North America. The effects are highly variable, ranging from facilitation to inhibition, and vary on the species and mycorrhizal type associated with neighboring canopy trees. Overall, this study highlights the importance of the MN in structuring temperate forest ecosystems.