Evaluation of Successful Forested Wetland Mitigation in Iowa

Project status

In progress

Start date: 01/27/14
End date: 10/31/17


Principal investigator:

Co-principal investigator:

About the research

Wetlands are transitional areas between aquatic and terrestrial systems that predominantly support the growth of hydrophytic plants, contain undrained hydric soils, and/or hold water at least semi-permanently (Cowardin et al., 1979). These important landscape features contribute to nutrient retention and cycling, sediment capture, flood attenuation, recharge for groundwater, wildlife habitat, and recreational opportunities (Mitsch, et al., 2009). Concerns over widespread loss of wetlands across the nation (due to agricultural expansion, growth of urban areas, and road construction) led to federal legislation to minimize wetland disturbance and require mitigation to address damage or replace loss of wetlands (Section 404 of the Clean Water Act), most recently addressed in the 2008 Final Rule on Compensatory Mitigation for Losses of Aquatic Resources (33 CFR Parts 325 and 332; 40 CFR Part 230). As part of this legislation, compensatory mitigation is allowed in cases of unavoidable wetland damage or loss as per requirements outlined in permits granted on a case-by-case basis. Historically, Iowa's landscape contained significant wetland areas, with many poorly drained closed depressions in glaciated upland areas, and extensive riparian wetlands in floodplains of major rivers and streams throughout the state (Thompson 1992). Following settlement, it is estimated that approximately 89% of these wetlands were drained or lost due to channelization (USDA, 1995). Remaining natural wetlands that occur in Iowa include emergent wetlands and fens (dominated by herbaceous vegetation), forested wetlands (containing significant woody vegetation), and wet meadows (dominated by sedges and grasses). Forested wetlands, the focus of this proposal, are areas dominated by shrub and tree species that occur either as small upland systems fed by rainwater that have not been affected by artificial drainage, or larger bottomland forest systems associated with rivers (or reaches of them) that were not channelized. Common tree and shrub taxa that occur in forested wetlands include species of willows, ash, and elm, as well as eastern cottonwood, hackberry, boxelder, silver maple, river birch, swamp white oak, American sycamore, bur oak, and species of dogwood, as well as chokecherry, elderberry, and wahoo (Herring, 2012; NRCS,2005). Activities to compensate for loss of wetlands in Iowa have been ongoing for close to 30 years. With respect to a sample of compensatory wetlands, there is evidence that overall acreage of wetlands impacted has been matched, and that ecological functions provided by mitigation wetlands are similar to those of reference wetlands (VanDeWalle et al., 2007). However, efforts to establish forested wetlands in particular have met with less success. Restoration of woody plants has been hampered by variable quality of planting stock, severe damage from wildlife, and variability in precipitation (causing long-term inundation and/or drought) during the establishment period. Because of impacts to forested wetlands that had not been mitigated nationwide, the new Final Rule stipulates greater requirements for compensatory area, an increase in length of the monitoring period, and new performance standards for species, planting density, survival rates, and tree growth over time. Thus, there is a need for a general framework and recommendations for the selection, design, construction, and future assessment of forested wetland mitigation sites in Iowa, based on the literature documenting prior research and close examination of the current status of previous mitigation projects in the state. Information generated by this research will support development of future mitigation projects that fully address new regulatory requirements for the establishment of forested wetlands, and provide guidelines for evaluating their performance over time.


Sponsor(s): Iowa Department of Transportation