1237 Int. J. Plant Sci. 162(6):1237–1245. 2001.  2001 by The University of Chicago. All rights reserved. 1058-5893/2001/16206-0005$03.00 EFFECTS OF ENDOPHYTE INFECTION IN TALL FESCUE (FESTUCA ARUNDINACEA: POACEAE) ON COMMUNITY DIVERSITY Greg Spyreas, 1, * , † David J. Gibson,† and Beth A. Middleton† *Illinois Natural History Survey, 607 E. Peabody Drive, Champaign, Illinois 61820-6917, U.S.A.; and †Department of Plant Biology, Southern Illinois University, Carbondale, Illinois 62901-6509, U.S.A. Recent studies have suggested that the presence of endophytes in tall fescue can lead to decreased species richness in the associated plant community. To assess the generality of this hypothesis, a field study tested the effects of endophyte infection on a 3-yr-old successional field dominated by Festuca arundinacea. The potential importance of endophyte infection relative to other environmental factors was tested by including two ad- ditional treatments: the effects of soil fertility and mowing. Contrary to previous studies, a positive relationship was found between endophyte infection frequency and diversity ( , , , ). A 2 N p 23 F p 5.23 R p 0.19 P ! 0.03 strong interaction was found between the mowing treatment and endophyte infection frequency in predicting diversity ( , , , ), where the maximum species richness was present in 2 N p 22 F p 36.1 R p 0.84 P ! 0.0001 plots that were both mowed and highly endophyte infected. The relationship between endophytes and diversity varied through the successional continuum (the mowing treatments) but was generally positive. The soil in mowed plots was drier than in unmowed plots ( , , ). We suggest that heavy mowing t p 2.1 df p 28 P ! 0.05 decreases soil moisture levels enough to reduce the interspecific competitive ability of infected F. arundinacea, thereby promoting local diversity. Endophyte presence is important, but the previously reported negative relationship between endophyte infection and community diversity is probably overly simplistic in complex ecological settings. Keywords: community diversity, cool season grasses, C 3 , endophyte, Festuca arundinacea, tall fescue. Introduction An endophyte is a parasitic to mutualistic internal fungal resident of a host plant that is asymptomatic for some portion of the life cycle of the fungus (Carroll 1988; Clay 1988; Petrini 1996). Endophytic fungi in grasses occur throughout the aboveground parts and have more fungal biomass than endophytes of other plants. Many grass endophytes are ma- ternally transmitted via the seed; these are an extremely host- specific and specialized subset of perennial, systemic endo- phytes (Clay 1988, 1992; Bills 1996; Petrini 1996). The majority of these belong to the asexual genus Neotyphodium (sexually Ascomycota; Clavicipitaceae) (White 1987; Glenn et al. 1996) and infect the Pooideae grasses that dominate tem- perate areas (Clay 1990a, 1997b). Thus, it is likely that Neo- typhodium and the Pooideae are coevolutionarily linked (Clay 1990b). Despite the knowledge of systemic fungal infections, the eco- logical significance of endophytes was not realized until the mid-1970s, when the connection between the presence of fungi in forage and various livestock poisonings was made (Bacon et al. 1977). Consequently, the majority of grass endophyte studies have dealt with the two widely planted forage grasses, Festuca arundinacea Shreber. (tall fescue) and Lolium perenne L. (perennial ryegrass) and their predominantly mutualistic endophytes (Morgan-Jones and Gams 1982; Siegel et al. 1987; 1 Author for correspondence; e-mail spyreas@inhs.uiuc.edu. Manuscript received March 2001; revised manuscript received July 2001. Bacon and Siegel 1988; Siegel 1993; Clay 1998). Studies of F. arundinacea are of particular importance since, while native to Europe, it is widely planted in temperate regions of the world, providing a mosaic of endophyte-infected and en- dophyte-free populations (Gibson and Newman 2001; Spyreas et al. 2001). Benefits to the fungi include nutrition, long-term protection, and improved dissemination (by seeds) (Siegel et al. 1987). Reported host benefits include increased resistance to both grazing and insect herbivory, decreased nematode predation, antimicrobial and antifungal properties, both heat and drought stress tolerance, and increased overall vigor (e.g., tiller pro- duction, biomass, height, seed crop, seed germination; Clay 1987, 1990a, 1997b; Cheplick et al. 1989; Petroski et al. 1990; Guo et al. 1992; Siegel 1993). Costs to the plant, though poorly understood, are thought to be insignificant (Hill 1994; Bacon and Hill 1996). Previous Population and Community-Scale Studies Greenhouse studies have indicated increased vegetative vigor in infected versus uninfected individuals of the same species, with and without herbivory (Clay 1997b). Festuca arundinacea has almost exclusively been the species studied although field observations suggest that increased vigor with endophytic in- fection also occurs in other grass species (Clay 1997b). The primary exception reported to this pattern may be a slight advantage to uninfected F. arundinacea and Lolium perenne