Commandeering of arbuscular mycorrhizal fungi by Juniperus virginiana: Unseen consequences of woody encroachment in the tallgrass prairie

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Aisha V. Sams, Eric B. Duell and Gail W.T. Wilson, Natural Resource Ecology and Management, Oklahoma State University, Stillwater, OK

Background/Question/Methods
A current threat to grasslands worldwide is expansion of woody species. Due to fire exclusion, J. virginiana has expanded into native grasslands across central North America, with cascading effects throughout the terrestrial ecosystem, including potential alterations in soil C storage. Many ecosystem-level functions, including soil C-storage, are strongly influenced by arbuscular mycorrhizal (AM) fungi; mycorrhizal abundance has been shown to be tightly correlated with soil C-storage in tallgrass prairie ecosystems. However, AM fungal abundance following encroachment of J. virginiana into native grasslands has not been determined. The contrast between d13C signatures of J. virginiana with the d13C signatures of native prairie grasses allowed us to directly assess the source of host-C supplied to AM fungi associated with J. virginiana and surrounding vegetation. We collected AM fungal production with 50-µm mesh ingrowth bags placed at the base of mature J. virginiana trees, at the dripline, and ~3 meters from the dripline in areas dominated by C4 grasses to determine d13C signatures of associated AM fungi. Following one growing season, AM hyphae, soil, and host roots were collected (0-5 cm). At each location, intra- and inter-radical AM fungal abundance and d13C signatures of soil, roots, and associated AM hyphae were determined.
Results/Conclusions
Intra- and inter-radical fungal abundance associated with J. virginiana, including soils and roots at the dripline, were greater than that associated with adjacent C4 grasses, although C4 grasses have been well documented as obligate mycotrophs in this ecosystem. Prairie vegetation (predominantly C4 grasses) and C3 juniper trees have strongly contrasting C isotopic signatures due to discrimination against 13C in C3 and C4 photosynthetic pathways (C4 d13C = ~ -12%; C3 = ~ -270/00). In our study, δ13C of the soils were -23, -16.5, and -14.80/00 at the base of the tree, dripline, and in adjacent C4 dominated vegetation, respectively, reflecting strong influence from the dominant C3 or C4­ vegetation. However, δ13C of extra-radical AM fungal hyphae was -24.4, -23.9, and -17.7 at the base of the tree, dripline, and in adjacent C4 dominated vegetation, respectively. Our data suggest AM fungi at the dripline are receiving host-C more strongly biased towards J. virginiana, although vegetation at the dripline was predominately composed of C4 grasses. If J. virginiana is able to commandeer AM fungal associations, this facilitation may be one potential mechanism for success as these encroaching species expand into native grasslands.