MYCORRHIZAE INDIRECTLY ENHANCE COMPETITIVE EFFECTS OF AN INVASIVE FORB ON A NATIVE BUNCHGRASS

Abstract

Mycorrhizae are important mediators of plant competition, but little is known about the role of mycorrhizae in the intense competitive effects that exotic plants can have on native species. In the greenhouse, we tested the effect of arbuscular mycorrhizal (AM) fungi on interspecific competition between Cerztaur-ea maculosu and Fest~ccu idahoensis, on intraspecific competition between individuals of both species, and the growth of C. rnac~closawith either inorganic or organic phosphorus. Mycorrhizae had no direct effect on either species, but mycorrhizae increased C. maculosa's negative effect on F. iduhoe~zsis. When competing with C. rnac-ulosa, nonmycorrhizal F. idahoensis were 171% larger than they were when mycorrhizae were present. In a second experiment, C. maculosa grown with larger F. idahoe~zsis were 66% larger, in the presence of AM fungi, than when AM fungi were absent. Centaurea maculosu biomass was not affected by AM fungi, in either phosphorus treatment, in the absence of F. idahoensis. Root: shoot ratios differed between phosphorus treatments, but this difference seemed to be a result of slower growth in the organic phosphorus treatment. Our results were unusual in that the direct effects of my- corrhizae on both species were weak, but the indirect effect of AM fungi on the interactions between C. muc~tlosa and F. idahoerzsis was strong. Our results suggest that AM fungi strongly enhance the ability of C. ~naculosa to invade native grasslands of western North America. Key ~.ord.~: nrb~isclllcir rrrycoi-rhieal (AM) ,fuizgi; Centaurea maculosa; conzperition; exotic plants; Festuca idahoensis; interi7lor~tanr, gra.sslunris; iiz~,a.sion; mycorrhizae. ~~~~~~~~i~~~ between pairs of speciesare often me- diated by other species (Kareiva 1994, Miller 1994). Well documented indirect interactions include those be- tween sea otters, kelp and sea urchins 19781, starfish, molluscs and algae (Paine 1966),parasitic and autotrophic plants (Pennings and Callaway 1996), plants and soil microflora, (van der Putten et al, 1993. Bever 1994) and plants and mycorrhizal fungi (Grime