Non‐native grass alters growth of native tree species via leaf and soil microbes

Abstract

Summary Non‐native plants can alter both native microbe communities and ecosystem processes. These changes may indirectly affect native plant communities. We tested whether an invasive plant influences other plant species through above‐ and below‐ground microbial associations. These factors have received less attention than direct competition for resources in research on plant invasions. Previous work in the mid‐western US revealed that the above‐ground fungal endophyte (Neotyphodium coenophialum) in non‐native tall fescue grass (Lolium arundinaceum) reduced establishment of native tree species, slowing plant succession from grassland to forest relative to communities lacking the symbiont. Here, we examined potential soil‐mediated mechanisms underlying these negative impacts on trees. We challenged nine tree species with soil conditioned by tall fescue grass either with (E+) or without (E–) the symbiotic endophyte. We collected field soil from replicated, experimental E+ and E– field plots, sterilized half to eliminate soil microbes, then conditioned the soil in the greenhouse with either E+ or E– tall fescue. Individual tree seeds received one of five soil treatments: E– live, E– sterile, E+ live, E+ sterile, or no tall fescue conditioning (control). Tree survival and biomass were assessed after approximately 80 days. Soil conditioning by E+ tall fescue reduced the biomass of three tree species (Elaeagnus umbellata, Fraxinus pennsylvanica, Platanus occidentalis) and the survival of P. occidentalis relative to conditioning by E– tall fescue. However, decreased biomass occurred only in live soil, suggesting indirect effects of the endophyte via altered soil microbes. In addition, two species (F. pennsylvanica, Prunus serotina) grew larger in both unconditioned soils and E– live soil relative to all other soil treatments, and two species (Ailanthus altissima, P. occidentalis) performed poorly in all soil treatments relative to unconditioned soil. Finally, three species did not significantly respond to any soil treatment (Acer negundo, Quercus palustris, Cercis canadensis). Synthesis. Our results show that negative impacts of a non‐native grass on native trees can depend on both above‐ and below‐ground microbial associations. Because tree species responded divergently to soil conditioning, the endophyte symbiosis has the potential to alter plant succession and the future composition of forest communities.