Date of Award
Master of Science
Exotic plant species pose a great risk to restoration success in post-agricultural bottomlands, but little information exists on their dynamics during early succession of actively restored sites. Compositional trends of exotic plants may be similar to those published for natives in other systems, with an early peak in herbaceous richness followed by a decline as woody species establish. I established 16 sites in an 18-year chronosequence (1991-2008) of restored forests, with an additional four mature sites for comparison, within the Cypress Creek NWR, Illinois. Within each site, I identified all vascular plant species and quantified soil texture, total soil C, total soil N, and canopy openness at three strata (1.5m, 1.25m, & 0.75m). Trends in exotic assemblages were significantly correlated with canopy openness at all strata (all p < 0.0001). Richness of exotic herbaceous species and native herbaceous species were related to stand age consistent with a non-linear Weibull regression model (R2 = 0.543, p = 0.005; R2 = 0.483, p = 0.013, respectively). Average percent herbaceous species cover also showed a similar reduction in overall abundance for both native and exotic plants but followed an exponential decay model (R2 = 0.3777, p = 0.0039; R2 = 0.3003, p = 0.0124, respectively). Woody native richness over time conformed to a logistic model (R2 = 0.404, p = 0.012). Woody exotic plants exhibited no discernible relationship with stand age, although they were in sites of all ages. My results indicate that herbaceous exotic species exhibit successional trends similar to natives and therefore may not pose a lasting threat to restoration projects in these floodplain forests. In contrast, woody exotic species can establish earlier or later in succession, persist under closed canopy conditions, and may pose a lasting threat. Thus, bottomland restorations and mature forests are quite vulnerable to exotic plants even after canopy closure.
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