In the mid-1980's, thousands of wetlands in the mid-continental Unites States were restored by interrupting drainage lines; revegetation of these systems, often cropped for decades and positioned in a predominantly agricultural landscape, relied solely on natural recolonization. A study of 64 of these wetlands determined that by 1991, three years after initial reflooding, aquatic species had efficiently recolonized whereas sedge meadow and wet prairie species had not. In 2000, 41 of these restorations that had not been significantly altered or returned to cultivation were revisited and their floras characterized by cover within distinct zones. While species richness increased on every site, the rate of accumulation varied widely. Furthermore, species that had colonized since 1991, including a variety of native wet prairie and sedge meadow species, were detected only at very low abundance. In contrast, Phalaris arundinacea L., an invasive perennial, was now present on every site, often at covers approaching 75-100% in the zones in which it occurred. Other invasive perennials, including Cirsium arvense (L.) Scop. and Typha angustifolia L./glauca Godr., had expanded significantly on many sites. The overall dominance of invasive perennials has resulted in basins that are becoming more similar over time. However, present variations in species richness and composition can be attributed to flooding frequency, and, potentially, basin size and isolation from nearby natural wetlands, as shown by TWINSPAN and graphical analysis. Basins that have not been flooded at midsummer for at least seven of 12 years are among the most depauperate in the study. Yet even frequently flooded basins lack diversity if they are small (
