| Summary: | Invasion of native rangelands in the western United States has serious ecological and economic effects. Understanding the mechanisms behind invasion of Centaurea maculosa (spotted knapweed) is necessary to effectively manage this species. Arbuscular mycorrhizae (AM), which are a type of plant fungal symbiosis, are ubiquitous in grasslands. My research explores the role of AM for increasing the competitive ability of C. maculosa. A greenhouse experiment tested the effects of AM fungi and neighbor species growth of C. maculosa, Festuca idahoensis (Idaho fescue) and Pseudoroegneria spicata (bluebunch wheatgrass). A mesh barrier permeable to AM hyphae allowed comparison of species interactions by either roots and/or AM hyphae in pots without a barrier or by AM hyphae alone in pots with a barrier. Centaurea maculosa plants had high AM colonization levels within roots and ERH (extraradical hyphae), and may have increased AM colonization of neighboring plants. I found no evidence, however, that ERH affected competition, as C. maculosa neighbors had the greatest effect on native grass neighbors there was the potential for root contact. Additionally, plants grown with AM fungi were always smaller than non-mycorrhizal plants. In the second experiment, I investigated growth response after herbivory (simulated by clipping), with different neighboring species and AM fungi. Centaurea maculosa, F. idahoensis and P. spicata were grown in the greenhouse with a C. maculosa neighbor, with or without AM fungi, and with one of three clipping treatments (no clipping, focal plant clipped or neighbor plant clipped to remove 75% of aboveground biomass). Compensatory growth was dependent on AM fungi and neighbor species. Centaurea maculosa compensated for herbivory only when grown with a conspecific and with AM fungi, or with a F. idahoensis neighbor and no AM fungi. Clipping decreased AM colonization in F. idahoensis only, and colonization also decreased in F. idahoensis when C. maculosa neighbors were clipped. This research suggests that AM fungal effects vary between species in the grassland system, and is important for determining plant species response to herbivory. I also find that high levels of herbivory may reduce C. maculosa biomass enough to be a method for weed control, but neighbor species is important to determining plant response to herbivory.
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