Development of vegetation in small created wetlands in southeastern Wisconsin

Abstract

We examined the natural colonization by vascular plants of 11 created wetlands in southeastern Wisconsin. The wetlands studied were small depressional wetlands that were isolated from other wetland sites. Wetlands were sampled over a two-year period, providing samples of wetlands aged one to three years. The development of wetland vegetation in these 11 naturally colonized sites was compared to that in five wetlands to which we introduced 22 species of native wetland plants. We identified 142 species of vascular plants in the naturally colonized wetlands. Of these, 82 (58%) were native, obligate or facultative wetland plants. The diversity and richness of native wetland plants and the proportion of total plant cover that was comprised of native wetland species increased from one-to three-year-old wetlands. The diversity and richness of native wetland species increased with wetland age, wetland size, and with proximity to the nearest established wetland. Distance to the nearest seed source had a particularly strong effect on the number of native wetland species present. Cattail (Typha spp.) accounted for 15% of the cover of native wetland species in one-year-old wetlands; this increased to 55% in three-year-old wetlands. We predict that the naturally colonized wetlands will develop into near monocultures of cattail with a fringe of willows (Salix spp.) and cottonwood (Populus deltoides) at the wetland/upland margin. Wetlands seeded with native wetland species had much higher diversity and richness of native wetland species than unseeded wetlands after two years. Seventeen of the 22 seeded species became established in at least two wetlands after simple introduction of seed to the sites. Cattail cover after two years was lower in seeded sites, both as an absolute cover and as a proportion of native wetland plant cover. Early introduction of a diversity of wetland plants may enhance the long-term diversity of vegetation in created wetlands.