Flood Exposure Affects Long-Term Tree Survival in Compensatory Mitigation Wetlands

Survival of planted trees is commonly used as a performance metric for compensatory mitigation wetlands. However, establishing floodplain forest through planting is difficult due to flood-induced tree mortality. We used multiyear tree census and hydrologic data from 17 compensatory mitigation projects in Illinois, USA, to relate planted and volunteer tree establishment to flood frequency, depth, and duration. Annual survival of planted trees decreased with greater annual maximum flood depth and duration. By the end of official compliance monitoring, sites with greater flood exposure had greater planted tree mortality. We resurveyed 10 sites that were at least 10 years old, and found that long-term tree survival was significantly lower in sites with greater flood exposure. Naturally colonizing trees differed in species composition from planted trees; specifically, wind dispersed species were well-represented among volunteer trees, whereas hard mast species were absent. There was no clear relationship between volunteer tree recruitment and measured flood variables. Across all sites, compliance with tree survival standards was poor, but influenced by hydrologic conditions. Current performance standards for tree survival may be unrealistic in restored wetlands that are exposed to long-duration floods. Regulators and practitioners should seek alternative methods to establish desired floodplain forest structure and function.