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GOODWIN J. 1992. THE ROLE OF MYCORRHIZAL FUNGI IN COMPETITIVE INTERACTIONS AMONG NATIVE BUNCHGRASSES AND ALIEN WEEDS - A REVIEW AND SYNTHESIS. NORTHWEST SCIENCE. 66(4):251-260. Address: GOODWIN J, OREGON STATE UNIV,DEPT RANGELAND RESOURCES,CORVALLIS,OR 97331.
The role of vesicular-arbuscular mycorrhizal (VAM) fungi in competitive interactions among native bunchgrasses and alien weeds is reviewed. Brief reviews of the mycorrhizal symbiosis and rangeland ecology provide a background for discussion of the role of VAM fungi in inter- and intraspecific competition in sagebrush-grass systems. Plants can be significantly better competitors for soil resources when mycorrhizal than when not mycorrhizal, and VAM fungi can alter the outcome of competitive interactions among plants. Additionally, soil resources can be transferred from one plant to another through a network of hyphae produced by VAM fungi; such transport can improve growth in the receiver plant. The response of a plant to the formation of mycorrhizae depends upon the taxa of the plant and fungal symbionts, their phenologies, and on the severity of environmental stress. Native bunchgrasses of the Intermountain West are mychorrhizal. Alien weeds introduced to the sagebrush steppe include both mycorrhizal and nonmycorrhizal species. When a rangeland site is colonized by nonmycorrhizal species, VAM fungi populations decline. In the absence of VAM fungi, mycorrhizal bunchgrasses are at a competitive disadvantage compared to nonmycorrhizal species in capturing limited soil resources. However, the opposite is also true: mycorrhizal species out-compete nonmycorrhizal species when VAM fungi are abundant. One naturalized alien, cheatgrass (Bromus tectorum), is a facultative host of mycorrhizal fungi. Cheatgrass is an effective competitor in the rangeland environment, and its relative competitive ability, compared to native bunchgrasses, is unlikely to be altered by VAM fungi. Though mycorrbizal fungi would enhance the competitive ability of native bunchgrasses, cheatgrass would reap comparable or greater benefit since it also hosts VAM fungi and, due to a rapid rate of growth, acts as a strong sink for soil resources. This is probably one of the factors behind continued, long-term occupancy and/or dominance of rangeland sites by cheatgrass. However, it is not unreasonable to expect greater efficiency among coevolved mutualists compared to a profiteering alien. If assimilates are more efficiently transferred from a native bunchgrass to its progeny, then VAM fungi have an important role to play in the restoration of Intermountain rangelands. Research into the role of mycorrhizal fungi in competitive interspecific interactions, rhizosphere community dynamics, the mycorrhizal dependence of rangeland grasses, and patterns of translocation is needed before definitive answers can be presented.
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