Individual study: A comparison of vegetation and environmental conditions in recently restored and natural prairie pothole wetlands in mid-continental USA
Galatowitsch S.M. & van der Valk A.G. (1996) Vegetation and environmental conditions in recently restored wetlands in the prairie pothole region of the USA. Vegetatio (now Plant Ecology). Plant Ecology, 89-99
In mid-continental USA, over 50% of depressional wetlands (by area) have been converted to agriculture within the last 100 years; wetland loss is considered a major factor in waterfowl population declines and increased river flooding. Thousands of drained wetlands have been restored in the last 10 years in an attempt to reverse these trends. In this study, in order to assess the potential importance of dispersal and environmental conditions as limiting factors for initial plant recolonization, three years after reflooding vegetation composition, water level fluctuations, and various soil properties were measured and compared in 10 restored and 10 natural wetlands.
Study sites: Ten restored wetlands were selected from 62 that were being monitored as part of a general study of restored prairie wetlands. Each had been previously thoroughly drained by tile-drainage and used to grow corn and soybeans for 25 to 75 years. All were restored in 1988 by disrupting tile lines. A nearby natural wetland of similar size and depth was selected for comparison.
Vegetation surveys: A complete list of plant species was made for each wetland during monthly surveys from April to October 1991. During each survey, cover of each species was estimated using a seven-point cover scale: one individual with significant cover; few individuals with insignificant cover; 1-5%; 5-25%; 25-50%; 50-75%; and 75-100%.
Wetland plants were classified into seven guilds based on life history strategy and water depth tolerance:
wet prairie perennials; sedge meadow perennials; emergent perennials; submersed aquatics; free floating
aquatics; mudflat annuals; and woody species.
Environmental characteristics: For of each wetland, a topographic survey was made; water levels were recorded, and water storage volume and water surface area was estimated during five periods from April to October 1991. Soil organic carbon content, soil bulk density, surface water pH, alkalinity, conductivity, and calcium and magnesium concentrations were also recorded.
Vegetation in restored wetlands: Submersed aquatics (9 species found in one or more restored wetlands) most rapidly colonized and had the highest cover; four pondweeds (Potamogeton foliosus, P.nodosus, P.pectinatus, P.zosteriformis), rigid hornwort Ceratophyllum demersum and common bladderwort Utricularia vulgaris dominated the standing water of nearly all of these wetlands. Four emergents (out of 13 species recorded) commonly occurred around margins but after 3 years they only formed dense stands along the water's edge: Polygonum amphibiurn, Scirpus fluviatilis, Scirpus validus and Typha x glauca (an introduced, widely naturalized hybrid). Free floating species (4 species), especially the duckweeds Lemna minor and L.trisulca appeared within 12 months and were generally common. A sparse cover of mudflat annuals developed on bare marginal areas. Four woody species (Salix exigua, Acer negundo, Salix amygdaloides and Populus deltoides) formed scattered thickets on muddy margins of some of the wetlands. Wet prairie (4 species), sedge meadow (13 species), and forbs were scarce.
Vegetation in natural wetlands: In contrast to restored wetlands, areas of standing water were dominated by emergents (20 species), rather than submersed aquatics (4 species) of which none were found in more than four of the natural wetlands, and only U.vulgaris had significant cover. Annuals were represented by only a few individuals, and of nine species of mudflat annuals only two (Polygonumpen sylvanicum and Hordeum jubatum) occurred in more than two of these wetlands. Wet prairie (19 species) and sedge meadow perennials (34 species) were common; 19 species in these two guilds, including Poa pratensis, Phlox pilosa, Lysimachia thyrsiflora, Asclepias incarnata, Calamagrostis canadensis, Carex lanuginose and Stachys palustris, were present in at least six of the 10 natural wetlands. Free floating species were more diverse, the liverworts Ricciocarpus natans and Riccia fluitans, and L.trisulca, were common.
Environmental characteristics: Fluctuations in water storage volume and basin surface area were similar for both restored and natural wetlands. The surface water in restored wetlands had higher pH and lower alkalinity, conductivity, and calcium and magnesium concentrations, and soils had a lower organic carbon content and higher bulk density, than that of natural wetlands.
Conclusions: These results suggest that for submersed aquatics, dispersal of propagules to restored wetlands is rapid and early successional conditions are very suitable for their establishment. For other wetland guilds, e.g. sedges and other wet meadow species, dispersal appears much slower which may pose problem for their re-establishment. Even if dispersal is not limiting, initially low surface organic carbon and high bulk density may prevent the establishment of these species.
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