Defoliation effects on arbuscular mycorrhizae and plant growth of two native bunchgrasses and an invasive forb

Centaurea maculosa, an invasive forb in western North American grasslands, dominates more than 4 million ha of rangeland in the Rocky Mountain region. Our research examined the effects of herbivory and arbuscular mycorrhizae (AM) on the growth of C. maculosa, and two native grasses, Festuca idahoensis and Pseudoroegneria spicata. Plants were grown for 11 weeks, with or without AM, prior to simulated herbivory. We removed 75% of aboveground biomass from half of the plants, and harvested all plants 4 weeks later. All species increased their growth rate after clipping, but clipped plants were smaller than unclipped plants. The difference in biomass between clipped and unclipped plants was smaller for C. maculosa (clipped:unclipped = 0.62) than for the native grasses (0.42 and 0.52). Plants with AM were smaller than non-AM plants across all species, with the greatest reduction in F. idahoensis (77% versus 18% in C. maculosa and P. spicata). Levels of AM colonization were minimal in F. idahoensis (<1% of root length colonized), intermediate in P. spicata (13% colonization) and highest in C. maculosa (20%). Extra radical hyphae (ERH) lengths were highest for C. maculosa, and equal between the two grasses. Because of the lower biomass, plant tissue N concentrations were higher in AM plants, but P concentrations increased in AM plants for only C. maculosa and P. spicata. The superior competitive ability of C. maculosa may be the result of greater compensatory growth after herbivory and higher amounts of ERH, both of which result in greater resource capture.