Walling and Zabinski determined the effects of simulated herbivory on the growth and nutrient content of spotted knapweed, Idaho fescue and bluebunch wheatgrass plants, with or without arbuscular mycorrhizae. All plants that were clipped or had arbuscular mycorrhizae were smaller than plants not exposed to these treatments, suggesting that arbuscular mycorrhizae limited plant production instead of enhancing it. Clipping enhanced N and P concentrations and arbuscular mycorrhizae enhanced plant N concentrations. However, since biomass of these plants was lower, the total plant N and P contents were actually decreased by these treatments. The results of this study suggest that arbuscular mycorrhizae negatively affected the three plant species used in this study because the costs (reduced plant size) of maintaining these organisms outweighed the apparent benefits.
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