Restored wetlands are greatly influenced by hydrology and non-native plant invasion

Unpredictable changes in species composition remain a critical problem for restoration practitioners. Determining both what influences plant species composition and how species composition is changing in restored wetlands is important for better understanding restored wetlands and subsequently improving the wetland restoration process. To address this problem, we surveyed 66 100-m² plots in 22 wetlands in Illinois, USA, that were restored between 1991 and 2010 and analyzed data on herbaceous plant and canopy cover collected at each plot in 2015 and 2020. We performed canonical correspondence analysis to relate hydrological variables, soil variables, canopy cover, time since restoration, and latitude to herbaceous species composition. We also performed non-metric multidimensional scaling to assess changes in species composition between 2015 and 2020, and then used a generalized random effect linear model to determine which predictor variables influenced plot-specific change between 2015 and 2020. Variables related to hydrology and fertility primarily explained variation in species composition. Northern sites were clustered together, which was likely due to homogenization caused by high abundance of Phalaris arundinacea. In contrast, forested plots had greater dissimilarity between 2015 and 2020, either a result of ongoing successional turnover or relatively high turnover compared to open P. arundinacea-dominated plots. It appears that extreme flooding in some plots reduced P. arundinacea cover and facilitated the establishment of more hydrophytic vegetation. Our results confirm the role of hydrology in determining plant species composition in restored wetlands, and highlight problems related to invasive species that can occur after restoration monitoring has ceased.