Mycorrhizae and fine root dynamics of Centaurea maculosa and native bunchgrasses in western Montana

Centaurea maculosa is an invasive exotic pest plant of intermountain grassl ands of the western United States. We investigated whether there were diffe rences in fine root and mycorrhizal development between the exotic forb C. maculosa and two native grasses, since these are factors known to influence competitive ability of plants. We measured colonization by arbuscular myco rrhizal (AM) fungi of the native bunch grasses Pseudoroegnaria spicata and Festuca idahoensis, and C. inaculosa, and used root periscopes to investiga te fine root patterns of P. spicata and C. maculosa. AM fungal colonization of the exotic was similar to F: idahoensis in 1995 (43.6% vs. 44.7%), and was higher than I? spicata in 1995 and 1996 (43.6% vs. 26.9%, and 55.4% vs. 15.6%). Colonization of the grasses was not affected by the presence of C. maculosa. Fine roots of C. maculosa developed earlier, with a higher propo rtion of deep roots than P. spicata. Thirty nine per cent of C. maculosa ro ots, but only 25% of P. spicata roots, were deeper than 30 cm. Early and de ep root development and extensive mycorrhizal colonization in the field may contribute to competitive dominance of C. maculosa.