Vegetation changes over time in Prairie Pothole Region reference wetlands.

Periodic assessments of reference condition wetlands are needed to determine changes over time; however, they are rarely conducted. The vegetation from past assessments, 1998 to 2004, was compared to 2016 assessments of 12 reference wetlands in the Missouri Coteau sub-ecoregion of the Prairie Pothole Region using nonmetric multidimensional scaling and permutational multivariate analysis of variance. Analyses indicated the vegetation in the 2016 assessments trended away from the abundance of native highly conservative species as found during the 1998 to 2004 assessments. Instead, the 2016 plant communities trended towards lower abundance of the same native conservative species and higher abundance of non-native species. Both the average coefficient of conservatism values and floristic quality index values significantly declined, supporting the interpretation that reference wetlands were moving towards plant communities with lower abundance of highly conservative species. The assumption that reference wetlands in the Prairie Pothole Region will change little over time is challenged by these findings. Vegetation in refence wetlands within the Prairie Pothole Region is no longer resembles past monitoring and is trending towards a distinct vegetation composition. Future management will need to consider the potential of reference wetlands' vegetation composition moving away from a historic baseline and how this may impact future wetland assessment, especially when vegetation is compared to reference conditions.