Restoration progress and flood disturbance at IDOT wetland mitigation sites

As required under Section 404 of the Clean Water Act and the Illinois Interagency Wetland Policy Act of 1989, the Illinois Department of Transportation (IDOT) has established wetlands at mitigation sites throughout Illinois to compensate for impacts to natural wetlands by road construction. One of the mechanisms for meeting regulatory obligations is to mitigate wetland impacts through restoration or creation of wetlands that provide functions similar to natural wetlands under the federal policy goal of "no net loss" of wetland area and function. Progress toward restoring wetland functions is measured through the use of prescribed performance standards issued by state and federal regulatory agencies. In most cases, these performance standards contain measures of quality, health, and structure of wetland plant communities. The majority of extant Illinois wetlands and most IDOT wetland mitigation sites are located in floodplain areas and are exposed to varying degrees of flooding, depending on their location within a given watershed, their position within the floodplain setting, and their climate zone within the state. Among the major challenges in attaining vegetation-based performance standards at IDOT wetland mitigation projects are poor plant-community quality (e.g., low species richness) and mortality of planted trees. Although floodplain wetlands are supported by regular flooding, floods can also produce excessive sediment, cause ice damage, and result in prolonged inundation--leading to mortality in wetland plants thereby interrupting expected trajectories of succession and progress toward achieving mitigation performance standards. In this study, we analyzed data collected by the Illinois Natural History Survey and the Illinois State Geological Survey during past and current site monitoring, to examine the influence that flooding has on ecological mechanisms that lead to variation in the performance levels among a large number of floodplain compensatory mitigation wetlands. The goals of the analyses were to evaluate the influence that flooding has on (1) plant community quality at the landscape scale, and (2) species turnover within plant-community functional groups at wetland mitigation sites. For the first component of the analysis, we used simple linear regression to evaluate the influence of flooding on the levels of ten plant community metrics. Results of linear regression analysis showed significant, inverse relationships between mean annual flood exposure and average levels of three of the ten plant-community metrics: species richness, floristic quality index (FQI), and proportion of perennial species. Also, we used hierarchical mixed models to evaluate the influence of flooding on loss and subsequent gain of species within seven species functional groups. Mixed models showed significant correlation of the loss of species within all species groups with magnitude of flood intensity in the same year, and the gain of non-hydrophytic, annual, and non-native species with magnitude of flood intensity in the preceding year. Results of this study indicate two general tendencies at IDOT wetland mitigation sites: (1) Higher average magnitudes of flooding will lead to decreased species richness, floristic quality, and proportion of perennials in floodplain settings and will significantly decrease the likelihood of attaining performance standards based on these metrics; and (2) higher-magnitude floods will tend to disrupt succession by eliminating the species groups that are associated with higher-quality plant communities and allow more undesirable species to colonize in the year following flood disturbance. These tendencies highlight the importance of considering flood disturbance regimes when evaluating the quality of plant communities in floodplain wetlands.