RESEARCH ARTICLE Succession of Exotic and Native Species Assemblages within Restored Floodplain Forests: A Test of the Parallel Dynamics Hypothesis Craig R. McLane, 1,2,3 Loretta L. Battaglia, 1 David J. Gibson, 1 and John W. Groninger 4 Abstract Exotic plants pose a threat to restoration success in post- agricultural bottomlands, but little information exists on their dynamics during succession of actively restored sites. We hypothesized that exotic assemblages would establish during succession and that their compositional trends dur- ing succession time would mirror those published for native species in other systems, with an early peak in herbaceous richness followed by a decline as woody species establish. In the summer of 2008, we sampled 16 sites across an 18-year chronosequence of restored forests, with an addi- tional four mature forest stands for comparison, within the Cypress Creek NWR, Illinois, U.S.A. We identified all vascular plant species and quantified canopy openness at three canopy strata, and soil texture and chemistry. Trends in exotic assemblages were significantly correlated with canopy openness at all strata. Richness of exotic and native herbaceous species was related to stand age and consistent with a Weibull regression model. Native and exotic herba- ceous cover followed an exponential decay model. Woody native richness over time conformed to a logistic model; woody exotics exhibited no relationship with stand age and were present in sites of all ages. Our results indicate that although their rates of decline differ, herbaceous exotics and natives exhibit similar successional dynamics; there- fore, herbaceous exotics may not pose a lasting threat to restoration success in reforested floodplains. Woody exotics can establish across a range of successional stages and per- sist under closed canopy conditions. Bottomland restora- tions are vulnerable to the invasion and expansion of exotic plant species even after canopy closure. Key words: bottomland hardwoods, invasive species, restoration, succession. Introduction Second only to land use changes and associated habitat loss, invasive non-native species are the biggest threat to the survival of native species (Vitousek et al. 1997) and hence, biodiversity. Once established, non-native invasives can oppor- tunistically colonize other areas and facilitate invasions of other non-natives (Simberloff & Von Holle 1999), thereby dis- placing species with similar resource requirements, reducing habitat quality for species dependent on those native species for cover and/or food, and in some cases leading to an “inva- sional meltdown” (O’Dowd et al. 2003). Effects of a non- native species may be minimal at first, but then rapidly cascade as the population grows and impacts other species and ecosys- tem processes (Vitousek et al. 1987). The success of exotic plants has been attributed to many underlying factors such 1 Department of Plant Biology and Center for Ecology, Southern Illinois University, Carbondale, IL 62901-6509, U.S.A. 2 Address correspondence to C. R. McLane, email crmclane@pbsj.com 3 Present address: PBS&J Corporation, PO Box 710, Yankton, SD 57078, U.S.A. 4 Department of Forestry and Center for Ecology, Southern Illinois University, Carbondale, IL 62901, U.S.A.  2011 Society for Ecological Restoration International doi: 10.1111/j.1526-100X.2010.00763.x as altered nutrient cycling (Vitousek et al. 1987), allelopathy (Bais et al. 2003), and biotic interactions (White et al. 2006). In their introduced range, exotic species may colonize habi- tat where they often have no natural enemies, rapidly expand their range, become invasive (Keane & Crawley 2002; Wolfe 2002), and affect native species composition. Alternatively, exotic species may display a pattern we refer to as the parallel dynamics hypothesis. This hypothesis sug- gests that succession of exotic species assemblages may simply resemble that of native species; in this case, exotic species can be expected to decline with forest stand age and may not cre- ate a lasting threat (Rejm´ anek 1989; Compagnoni & Halpern 2009; Matthews et al. 2009). Exotic species assemblages can appear and disappear during succession (Meiners 2007a) with changes in subcanopy light availability. The changing light environment may exclude shade-intolerant, herbaceous species (Meiners 2007a, but see Von Holle 2004). The changing light environment may not prevent persistence and proliferation of shade-tolerant, woody exotic plants; these species can occur at low and high levels and establish very soon after field aban- donment or restoration (Allen 1997). In some ecosystems where the threat of exotic species is recognized and economic interests for control exist, 202 Restoration Ecology Vol. 20, No. 2, pp. 202–210 MARCH 2012