Long-term vegetation development of restored prairie pothole wetlands

Although wetland restoration has been a key part of U.S. environmental policy for 20 years (i.e., "no net loss"), there is little long-term data on restorations to guide planning and assessment. Understanding how restored wetland communities deviate from natural conditions, and how long those deviations persist, can provide important insights into the mechanisms of recovery and improve restoration practice. This study reports the results from a 19-year survey of 37 restored prairie pothole wetlands in northern Iowa, southern Minnesota, and southeastern South Dakota. Complete floristic surveys were performed for each of the wetlands in 1989, 1990, 1991, 2000, and 2007. The accumulation of wetland species across all sites was greatest during the first 12 years after reflooding (14.4 species/year), after which the rate declined to 1.6 species/year. Proximity to natural wetlands and a semi-permanent water regime favored species accumulations during the first 12 years, but changes since then are primarily linked to water regime. Semi-permanent wetlands have experienced fewer major gains and losses in species richness, whereas temporary and seasonal wetlands have been less stable. From 2000 to 2007, extinctions exceeded colonizations in all wetlands, resulting in a convergence of beta diversity. Although 77% of the species considered common to natural wetlands in the region established in these restorations, 70% of those considered infrequent have not. The likelihood that these restorations will eventually support many additional species appears low, given the presence of barriers to recovery, especially the dominance of invasive perennials (e.g., Phalaris arundinacea and Typha angustifolia/x glauca) on all sites and the low colonization efficiency of wet prairie, sedge meadow, and woody perennial species. Management, such as active revegetation of these low efficiency species guilds, particularly sedge meadow and wet prairie perennials, and invasive species control is needed to ensure that restored prairie wetlands support the region's biodiversity. The important barriers to the recovery of prairie pothole restoration: isolation, infrequent flooding, and invasive species, are all factors that do not self-correct over time and need to be addressed during planning by establishing sound practices for initial implementation and long-term vegetation management.