Hydrological restoration and invasive plant control facilitate native plant community recovery in a heavily urbanized Great Lakes coastal wetland complex
Widespread agriculture and urbanization have altered the hydrology and water quality of many Great Lakes coastal wetland systems, resulting in greater volumes of surface water run-off with higher concentrations of chloride, nitrate, phosphorous, and suspended sediment, relative to presettlement conditions. Given these changes, the floristic quality of these wetlands has been altered from historic conditions and invasive Typha spp. and Phragmites australis occur in greater numbers and achieve higher densities than in the past. In an attempt to facilitate natural regeneration of wet prairie and sedge meadow communities, we undertook an effort to restore the hydrology and control invasive species in portions of a 5,000 acre coastal wetland complex in the heavily urbanized Chicago-Milwaukee corridor. A Before-After Control-Impact framework was used to assess the efficacy of treatments as well as the native plant community response using univariate indicators of floristic quality and diversity, coupled with multivariate indicators of vegetation composition. Differences in floristic similarity between surveys were assessed using PERMANOVA and Bray-Curtis distance as a measure of dissimilarity. Vegetation monitoring was conducted for 1 year before and 4 years after restoration to quantify ecological changes. Simultaneous monitoring was conducted in a nearby wetlands that served as an experimental control.
Restoration activities significantly reduced surface water and groundwater levels as much as 0.45m in wetlands adjacent to restoration treatments, and perhaps to lesser extent at more distant wetlands, though inter-annual variability made more definitive results difficult to assess. Invasive plant control required multiple treatments, but after five years, invasive species abundance and density were reduced over 90% & 95%, respectively (p < 0.001). Before restoration, plant composition of the treatment wetlands was significantly different from the adjacent control wetlands (F= 17.07, p < 0.001). After four growing seasons, vegetation of the restored wetlands had changed from its pre-restoration condition (F = 5.31, p < 0.05) and was dominated by native annuals and short-lived perennials. Although still not similar to the control wetlands (F = 9.41, p < 0.01), native species abundance, diversity and floristic quality increased and composition moved toward the control wetlands in multivariate species space, suggesting a convergence toward typical wet prairie and sedge meadow vegetation. Plots from 12 year old restoration that were part of a pilot study were much closer in species composition to control wetlands, suggesting that full recovery of sedge meadow and wet prairie communities may take decades.