Interactions between invasive plants and soil ecosystem: Positive feedbacks and their potential to persist

Plants indirectly affect their neighbors in many ways, but one of the most important is by altering the biotic, physical, and chemical characteristics of soils (Hobbie 1992; Angers and Caron 1998; Berendse 1998; Binkley and Giardina 1998; Northrup et a/. 1998; Schlesinger and Pilmanis 1998; Van Breemen 1993; Wardle et al. 1998; Chen and Stark 2000; Eaton and Farrell 2004). These general effects have been understood for decades, but only recently have experiments demonstrated that complex interactions between plants and soil microbial communities can have strong effects on plant populations (Bever et al. 199 7; Clay and Van der Putten 1999; Packer and Clay 2000), interactions among plant species (West 1996), and the organization of plant communities (Grime et al. 1987; Van der Putten et al. 1993; Bever 1994; Van der Putten 1997; van der Heijden et al. 1998; Hooper et al. 2000; Klironomos 2002). Soil communities alter competitive outcomes among plants through their pathogenic effects (Van der Putten and Peters 199 7), by favoring obligate mycorrhizal species over non-mycorrhizal or facultative mycorrhizal species (Hetrick et al. 1989; Hartnett et al, 1993), and by transferring resources and fixed carbon between species (Chiarello et al, 1982; Francis and Read 1984; Grime et al. 1987; Moora andZobel 1996; Watkins et al. 1996; Simardet al. 1997; Marler et al. 1999; but see Robinson and Fitter 1999). By altering the biotic and abiotic characteristics of soils, plants can drive positive or negative feedbacks (Box 1), and these feedbacks can profoundly affect plant populations and communities.