OIKOS 100: 362–372, 2003Mycorrhizal species identity affects plant community structure andinvasion: a microcosm studyElizabeth D. Stampe and Curtis C. DaehlerStampe, E. D. and Daehler, C. C. 2003. Mycorrhizal species identity affects plantcommunity structure and invasion: a microcosm study. – Oikos 100: 362–372.Previous studies have shown that arbuscular mycorrhizal fungi (AMF) can mediateplant interactions, thereby affecting plant community structure. Little is known,however, about whether the presence of different AMF species leads to differences inplant community structure or invasion success by introduced species. To investigatethe effects of AMF species on community structure and invasion, we created replicatemicrocosms containing soil inoculated with one of three different AMF species(Glomus spurcum Pfeiffer, Walker & Bloss, Scutellospora erythropa (Koske & Walker)Walker & Sanders, or Scutellospora 6errucosa (Koske & Walker) Walker & Sanders)or a mixture of all three AMF species. Seeds of seven naturally co-occurring plantspecies (Ageratum conyzoides L., Cyperus compressus L., Chamaecrista nictitans (L.),Crotalaria incana L., Hyptis pectinata (L.) Poit., Sida rhombifolia L., Melinis repens(Willd.) Zizka) in Hawai‘i were sown equally into these microcosms, which wereplaced on outdoor benches. Plant community development was monitored over aseason. Mid-way through the experiment, an invader (Bidens pilosa L.) was added tothe established communities to determine whether mycorrhizal species identity af-fected invasion success. Final aboveground and belowground phytomass were used toassess plant community differences among treatments. Although the identity of thedominant plant species (Melinis repens) remained the same in all treatments, commu-nity dominance, community productivity, plant species richness, Shannon index ofdiversity, and invasion success all varied with AMF species identity. Invasion successwas not inversely related to species richness or diversity. Instead, increased richness,diversity, and invasion success all appeared to be related to decreased dominance byM. repens in the presence of certain AMF species. These results indicate that thecomposition of the AMF community belowground can influence the structure of theplant community aboveground, and may play a role in facilitating or repellinginvasion.E. D. Stampe and C. C. Daehler, Botany Dept, Uni6. Hawai‘i at Manoa,3190MaileWay, Honolulu, HI96822, USA([email protected])Arbuscular mycorrhizal fungi (AMF) are symbioticfungi that take up resources, primarily phosphorus,from the soil, and supply them to host plants in ex-change for photosynthate (Sylvia 1998). AMF can alsoconfer increased nitrogen uptake, drought and heavymetal tolerance, and pathogen resistance to their hostplants (Ames et al. 1983, Busse and Ellis 1985, New-sham et al. 1995, Weissenhorn et al. 1995). Ninety-fivepercent of the world’s plant species belong to familiesthat are typically mycorrhizal (Smith and Read 1997).AMF can have strong effects on plant growth (Fitter1989, Allen 1991). These effects can be positive oroccasionally negative, and the degree of these effectsdiffers among plant species (Buwalda and Goh 1982,Plenchette et al. 1983, Allen et al. 1989, Wilson andHartnett 1998). By affecting different plant species indifferent ways, AMF can mediate interactions betweenplants. Many studies have found that AMF can affectcompetition between two plant species by differentiallybenefiting the weaker competitor, allowing the weakerAccepted 4 August 2002Copyright © OIKOS 2003ISSN 0030-1299OIKOS 100:2 (2003)362competitor to persist, or even reversing the outcome ofcompetition (Hall 1978, Allen and Allen 1984, Hetricket al. 1988). This mediation of plant-plant interactionshas implications for plant community structure anddynamics; AMF have the potential to affect plantspecies richness, diversity and productivity (Grime et al.1987, van der Heijden et al. 1998b, Hartnett andWilson 1999, Klironomos et al. 2000).AMF may also influence whether a community canbe invaded by new plant species. If AMF effects varyamong plant species, both in type and degree, thesevarying effects in turn may mediate the competitiveinteractions between the native and invasive plant spe-cies. Although several studies have examined the role ofAMF in succession (Allen and Allen 1984, 1988, Gangeet al. 1990, 1993), few studies have explicitly addressedeffects of AMF on invasion by non-native species(Hoffman and Mitchell 1986, Halvorson and Koske1987, Goodwin 1992). In competition experiments be-tween a native bunchgrass and an invasive weed, Mar-ler et al. (1999) found that the presence of AMFconferred a strong competitive advantage on the in-vader even though AMF had little effect on either plantspecies when grown alone. Emergent effects like these,which could not have been predicted from the single-species inoculation trials, illustrate the need for furtherexperiments on AMF mediation of plant-plantinteractions.Although AMF are not generally characterized asbeing host-specific, AMF species can display prefer-ences for certain host species (Johnson et al. 1992,Bever et al. 1996), and the effects of AMF species ontheir hosts can vary (Jensen 1984, Streitwolf-Engel et al.1997). van der Heijden et al. (1998a) created microcosmcommunities composed of plants from European cal-careous grasslands and inoculated them with each offour different species of AMF. The abovegroundbiomass of each plant species varied depending onwhich AMF species was in the soil. van der Heijden etal. (1998b) also created microcosm plant communitiesusing North American old-field species, in which AMFspecies diversity varied from two to 14 AMF species.Plant community diversity and ecosystem productivityincreased with increasing AMF species diversity, re-gardless of which AMF species were present.It is likely that AMF species identity and diversityplay important roles in structuring other plant commu-nity types as well, but studies from a variety of systemsare necessary to determine the general importance ofAMF species identity. This study utilized co-occurringspecies from a disturbed, low elevation, mesic habitat insubtropical Hawai‘i to examine whether AMF speciesidentity or diversity could affect community structureand invasion success. We inoculated soil in replicatedmicrocosms with one of three different species of AMF,with a combination of all three, or with no AMF. Wemonitored plant