- transition from invasive species control to native species promotion establishment - 131 Applied Vegetation Science 11: 131-138, 2008 doi: 10.3170/2007-7-18335, published online 7 December 2007 © IAVS; Opulus Press Uppsala. Abstract Question: Does the seed density of invasive species affect establishment by native species in a bare ground context (following invasive species control efforts), and is it possible to promote transition to a native species dominated state by manipulating sowing density of the native community? Location: Experimental wetland basin in Chanhassen, Min- nesota, USA. Methods: A mesocosm experiment investigated the infuence of Phalaris arundinacea (invasive species) propagule pressure on establishment of native wet meadow species in the context of a newly restored wetland. Mesocosms were sown with P. arundinacea (0, 10, 50, 100, or 500 seeds/m 2 ) and a mix of native species (3000 or 15000 seeds/m 2 ). Results: When planted at densities > 100 seeds/m 2 , P. arun- dinacea increased suppression of native species. Also, high native seed density suppressed P. arundinacea biomass produc- tion. This effect was more pronounced when P. arundinacea seed density was high (> 100 seeds/m 2 ), but high native seed density (15000 seeds/m 2 ) did not suppress recruitment of P. arundinacea from seed. Conclusions: The transition from post-control bare ground (a common result of efforts to control invasive species) to native species establishment depends on both native spe- cies and invader seed density. These results suggest that a threshold of P. arundinacea propagule pressure exists, beyond which transition to a native community is less likely without management intervention. P. arundinacea can establish in the presence of a newly developing native plant community, even at very low densities of P. arundinacea seed. Invader control (following initial site clearing efforts) is essential to native species establishment. Keywords: Colonization; Phalaris arundinacea; Propagule pressure; Restoration; Revegetation; Sedge meadow; Seed bank; Threshold dynamics; Wetland. Nomenclature: Gleason & Cronquist (1991). The transition from invasive species control to native species promotion and its dependence on seed density thresholds Reinhardt Adams, Carrie * & Galatowitsch, Susan M. University of Minnesota, Department of Horticultural Science, 305 Alderman Hall, 1970 Folwell Avenue, St. Paul, MN 55108, USA; * Corresponding author; present address University of Florida, Environmental Horticulture Department, PO Box 110675, Gainesville, FL 32611-0675, USA; Tel. +1 352392183; Fax +1 3523921413; E-mail creinhardt@ifas.uf.edu; Introduction Dominance by invasive species prevents the estab- lishment of target native communities and is a common barrier to achieving restoration objectives (Kusler & Kentula 1990; McKinstry & Anderson 1994; Galatow- itsch et al. 1999a). Thus, invasive species control is often the focus of early stages of restoration. Large-scale invasive species clearing programs are increasingly com- mon, e.g. removal of Tamarix spp. from foodplains in the southwestern US (Taylor & McDaniel 1998), mass eradication of alien woody species from riparian areas in South Africa (van Wilgen et al. 1998), and biocontrol of Lythrum salicaria throughout the northern US (Blossey et al. 2001)]. Large-scale restoration efforts often do not plan for active revegetation. They assume that once the barrier of the invasive species is removed, revegetation will occur through natural recolonization (Galatowitsch et al. 1999a). For example, natural recolonization is sug- gested as the best strategy for revegetation in wetlands following site preparation (Mitsch et al. 1998). However, several studies have shown that failure to plant, results in depauperate communities because of barriers to dis- persal and establishment of native species (Lewis 1982; Forbes 1999; Mulhouse & Galatowitsch 2003). Research in California grasslands suggests native establishment following invader removal is complicated by native propagule availability and environmental modifcation by the invader (Seabloom et al. 2003a, 2003b). Despite the widespread application of clearing programs and the debate on active revegetation, little is known about how the transition from post control bare ground to the establishment of native species occurs (Kellogg & Bridgham 2002), save that initial dynamics during this establishment phase are expected to have effects on the ultimate plant community composition (Weiner 1990). Studies of vegetation development fol- lowing Pteridium aquilinum control in British heathlands have shown that the direction and rate of vegetation community establishment can be highly variable (Marrs & Lowday 1992; LeDuc et al. 2000), and that vegetation