Individual study: Native wet prairie and sedge meadow species fail to effectively recolonize restored prairie pothole wetlands 12 years after reflooding in Iowa, Minnesota and South Dakota, mid-continental USA
Mulhouse J.M. & Galatowitsch S.M. (2003) Revegetation of pothole prairie wetlands in the mid-continental US: twelve years post-reflooding. Plant Ecology, 169, 143-159
In mid-continental USA, a survey of 64 recently restored prairie pothole wetlands in 1991, three years after initial reflooding, found that aquatic species had recolonized well whereas sedge meadow and wet prairie species were scare or absent. In 2000, 41 of these wetland restoration sites that had not been significantly altered since the 1991 survey were revisited and their floras reassessed to see if the vegetation was returning to desired native wet prairie and sedge meadow communities.
Survey area: The 41 reflooded wetlands surveyed are located in northern Iowa, southern Minnesota and southeastern South Dakota, mid-continental USA.
Vegetation surveys: In the initial 1989–1991 survey, a plant list was compiled for each wetland and species cover was estimated by zone i.e. open water, mudflat and vegetated margin at high water line (Galatowitsch & van der Valk 1995). Surveys undertaken in July and August 2000, followed these methods to enable comparison; estimates were made for open water, emergent, wet meadow and buffer zones (i.e. area between meadow and cultivated land) for each site, as appropriate. Species covers were estimated within each zone on a seven-point scale: r - one individual with insignificant cover; + - few individuals with insignificant cover; 1-4% cover; 5-24%; 25-49%; 50-74%; 75-100%.
Since the 1989-1991 surveys, species richness had increased in all 41 wetlands, but the rate of accumulation varied widely. Native wet prairie and sedge meadow species were disappointingly present only at very low abundance and large areas (characteristic of natural prairie potholes) did not appear to be establishing. In contrast, invasive perennials were common: canary reed-grass Phalaris arundinacea was present on every site with cover of around 75–100% in the marginal zones. Other invasives, e.g. creeping thistle Cirsium arvense and reedmace Typha angustifolia/glauca, had expanded greatly on many sites.
Variations in species richness and composition can be attributed to flooding frequency, basin size and degree of isolation from natural wetlands. Basins not flooded in summer for at least seven of 12 years were among the most depauperate; even if frequently flooded they lacked diversity if small (< 1.5 ha) or isolated from seed sources. Numerous species found in natural prairie pothole wetlands were notably absent or infrequent, e.g. blue-joint grass Calamagrostis canadensis, hairy sedge Carex lacustris and tufted loosestrife Lysimachia thrysiflora.
Conclusions: The authors consider that given the dominance of invasive perennials and absence of many characteristic native species, it appears that without seeding, planting and control of invasives and maintenance of appropriate flooding regimes, wetland restorations of this type are unlikely to succeed in developing natural vegetation communities.
Galatowitsch S.M. & van der Valk A.G. (1995) Natural revegetation during restoration of wetlands in the southern prairie pothole region of North America. In: Wheeler B.D., Shaw S.C., Fojt W.J. & Robertson R.A. (eds), Restoration of Temperate Wetlands. John Wiley and Sons, Chichester, UK, pp. 129-141.
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