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 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 regime, when evaluating the

quality of plant communities in floodplain wetlands.

Additionally, planted-tree counts and mapping were conducted to provide a baseline for future tree-survival studies at

three sites. Potential future studies would track survivorship of each tree species in response to flood disturbance.