Effects of water regime and competition on the establishment of a native sedge in restored wetlands

1. Restoration of depressional wetlands in mid-continental North America typically involves reflooding formerly drained basins. There have been several thousand restorations in this manner over the past 10 years. Deliberate revegetation is rarely attempted as it is assumed that native vegetation will eventually establish naturally. Carex (sedges), which occur commonly on the periphery of native wetlands and in adjacent saturated meadows, do not readily re-establish in reflooded basins. It is unlikely that Carex will reappear without deliberate reintroduction. Criteria for introduction and establishment of native sedges do not currently exist because environmental constraints are not well understood. 2. In this study we investigated the environmental constraints that might limit the establishment of the native wetland sedge, Carex lacustris, in reflooded wetland basins. Immature sedges were planted in three experimental wetlands with different water-level fluctuation regimes (i.e. falling, rising, static) along an elevational gradient, to determine the effects on survival and stand development of changing water level, water depth, competition from naturally recruited vegetation, and planting density. 3. The response of immature sedges to water-level fluctuation in the first growing season did not indicate the outcome of mature stand development by the end of three growing seasons. Whereas initial survival and growth were lowest in the falling water basin, stem density, height and biomass were greatest under this type of water-level fluctuation by the third growing season. 4. Interspecific competition was most intense at the upper elevations, particularly in the rising water basin. Although sedge growth was limited by increasing water depth in the absence of competition, growth was uniformly low across all elevations in the presence of competition. Competition reduced sedge growth to differing degrees in the rising vs. falling and static water basins 5. Results suggest that C. lacustris can produce dense stands under a primarily annual weed community within two to three growing seasons, but that Phalaris arundinacea (reed canary grass) can preclude successful establishment of: C, lacustris. 6. Control of water-level conditions and competition is most crucial during the first growing season to ensure appropriate conditions for successful stand establishment. The potential for growth as well as mortality is greatest during this first year. 7. Competition can be a significant constraint on the successful establishment of C. lacustris and should be considered explicitly when developing a restoration plan.