Tree Establishment in Response to Hydrology at IDOT Wetland Mitigation Sites

The Illinois Department of Transportation (IDOT) has compensated for unavoidable impacts to wetlands in transportation project corridors by restoring and creating wetlands throughout Illinois. As part of the IDOT Wetlands Program, monitoring of performance measures is conducted by the Illinois Natural History Survey (INHS) and the Illinois State Geological Survey (ISGS). The goals of this research were to determine the effect of flood events on the establishment of planted and naturally recruiting trees in IDOT mitigation wetlands and to make specific recommendations for tree planting and the establishment of mitigation performance standards. The authors compiled and analyzed existing data from INHS and ISGS monitoring reports and conducted additional field surveys to determine long‐term planted tree survival and assess natural tree recruitment. Based on the compilation of data from INHS wetland monitoring reports, the authors determined that the number of planted trees alive at mitigation wetlands by the end of mitigation monitoring was, on average, 57% the number of trees planted originally. The authors revisited ten older mitigation wetlands in 2014 and recounted surviving planted trees, and found that survival rate continued to decline beyond site monitoring periods. Tree mortality was clearly related to site exposure to flood disturbance during individual years, through the end of site monitoring, and beyond site monitoring periods. Depth and duration of inundation were more important than flood frequency in determining tree survival. Natural colonization greatly exceeded planting in terms of both stem density and basal area. However, natural colonization was not clearly related to site flood exposure. In sites that are exposed to long‐duration or deep flooding, planted tree survival is likely to be low regardless of species planted or the degree of on‐site management of plantings. The authors recommend that realistically attainable mitigation performance standards be developed on a site‐specific basis, considering the likely hydrologic regime of the site. In some situations, faster‐growing species provide tangible environmental benefits. In addition, natural tree colonization can supplement planting, even in sites where planted hard mast species are unlikely to persist.