Evaluation of Successful Forested Wetland Mitigation in Iowa

Project Details







Iowa Department of Transportation

Principal Investigator
Jan Thompson
Co-Principal Investigator
Timothy W. Stewart

About the research

Wetlands contribute to important landscape-level functions such as nutrient retention and cycling, sediment capture, flood attenuation, groundwater recharge, wildlife habitat, and recreational opportunities. Historic losses of wetlands due to land use conversion led to federal regulations requiring mitigation for unavoidable adverse impacts to them. Road development projects are among activities that cause unavoidable impacts to wetlands, and agencies such as the Iowa Department of Transportation (Iowa DOT) have expended considerable effort and funding to meet regulatory requirements.

This study evaluated tree seedling performance and site characteristics on 25 forested wetland restoration sites in Iowa, where Root Production Method (RPM) stock, bare root (BR) seedlings, and/or balled-and-burlapped (B&B) stock were planted. Initially, researchers assessed 2,533 seedlings representing 22 species, including 1,994 BR and 539 RPM trees. Overall, BR seedlings had higher mean survival rates (91%) than RPM stock (74%). Trees protected by shelters had lower survival rates. Seedling survival was also lower on sites associated with higher order streams and relatively fine surface soil textures. A more detailed assessment of 1,050 of the seedlings indicated considerable variation in performance across species, but on average BR seedlings had greater survival, height, crown depth, and root collar diameter than RPM seedlings. The study also compared stem densities for these plantings with naturally regenerating stands and reference forest areas near Iowa DOT project sites. Average stem densities for RPM stock were considerably lower than the averages for BR seedlings and naturally regenerating stands. An experimental site assessment indicated BR and RPM seedlings were similar to each other in size and morphology, and both were significantly smaller than B&B stock. Stem densities on this site were also low for RPM and B&B stock compared to naturally regenerating stands.

Overall, the researchers recommend selection of sites near lower order streams with surface soil clay contents less than 25%, and they suggest a strategy of planting BR seedlings at high densities without tree shelters to achieve more consistent success, specifications that clearly identify contractor responsibility for removal of any foreign material from seedlings at planting, and greater attention to site maintenance to promote seedling success. Finally, more realistic benchmarks for compliance should be set on a site-specific basis and tied to features of the surrounding landscape.