Elsevier

Ecological Engineering

Volume 35, Issue 3, 4 March 2009, Pages 351-368
Ecological Engineering

Restoration of vegetation communities of created depressional marshes in Ohio and Colorado (USA): The importance of initial effort for mitigation success

https://doi.org/10.1016/j.ecoleng.2008.09.018Get rights and content

Abstract

Many studies have attempted to assess the ability of created wetlands to replace the ecological structure and functions of natural wetlands over short time periods (<5 years). Few studies have repeatedly monitored vegetative community development of created depressional wetlands over longer time frames or assessed the return on the level of initial restoration efforts. Here, the vegetation communities of 17 created freshwater marshes in two different geographic regions of the U.S., Ohio and Colorado, ranging from 5 to 19 years old, were monitored over multiple years and compared to natural reference sites. Findings suggest that created marshes in Ohio achieved floristic equivalency with natural reference sites for measures of plant species richness, number of native plant species, number of hydrophytes, and percent plant cover within a decade. Yet, created marshes in Ohio contained double the amount of non-native plant species observed in natural reference sites. In Colorado, created marshes were less successful, failing to achieve floristic equivalency for plant species richness, number of native plant species, and number and percent hydrophytes given more than a decade of restoration. Soil chemistry data suggest that although created marshes achieve certain hydric soil characteristics, they were significantly lower in organic matter, cation exchange capacity, and extractable phosphorus than natural wetlands. Equivalency for soil chemistry will require longer time periods (>14 years). Data suggest that created marshes that seem to be approaching floristic equivalency in early years following construction may level off or even dramatically decline over longer time periods (10–20 years) for certain floristic indicators. Restoration trajectories for Ohio created marshes with strong initial restoration efforts predict floristic equivalency in a median of 14 years compared to 24 years for sites with weak initial efforts. Created marshes with strong initial restoration efforts displayed significantly greater plant species richness, number of native plant species, and number of hydrophytes than sites with low initial efforts, indicating the importance of planting, soil transport and/or contouring in establishing a wetland's restoration trajectory.

Introduction

Numerous recent studies have focused on the ability of created wetlands to restore the ecological functions of natural sites in the process of wetland mitigation (Niswander and Mitsch, 1995, Mitsch and Wilson, 1996, Zedler, 1996, Bystrom, 1998, Malakoff, 1998, Mitsch et al., 1998, Nadis, 1999, Spieles, 2005, Spieles et al., 2006, Fink and Mitsch, 2007, Fennessy et al., 2008, Matthews and Endress, 2008). Some studies have extended analyses to forecast trajectories of the recovery of restored wetland structure and functions over time to monitor progress towards equivalency with natural sites (Kentula et al., 1993, Simenstad and Thom, 1996, Zedler and Callaway, 1999, Morgan and Short, 2002). Most studies analyzed created mitigation wetlands over a short time horizon (<5 years) or used created or restored wetland projects of varied ages to project ecological functional and structural restoration (Niswander and Mitsch, 1995, Gray et al., 2002, Matthews and Endress, 2008). However, relatively few studies have returned to the field to monitor created wetlands multiple times over longer time frames or assess the return on the level of initial restoration efforts.

Atkinson and colleagues (2005) indicate that few studies have documented the vegetation of wetlands more than 10 years after construction. Zedler (2000) stated that the long-term development of restored wetland ecosystems is “poorly recorded” due to the regulatory context of wetland restoration. Zedler highlights that stakes are high in the mitigation process where there exists a need to achieve compliance with permit conditions and specific standards in a short time frame, usually five years (Zedler, 2000, Matthews and Endress, 2008). Kusler and Kentula (1990) have indicated that revegetation of created wetlands over short time periods is no guarantee that the wetland will continue to function over longer time frames and that for hundreds of created and restored inland freshwater marshes almost no long-term monitoring has occurred (Campbell et al., 2002). Recent studies have begun to address this need for long-term monitoring of created freshwater wetlands such as assessments of created sites in Pennsylvania ranging up to 18 years (Campbell et al., 2002), 12-year prairie pothole studies throughout South Dakota, Iowa and Minnesota (Mulhouse and Galatowitsch, 2003), 20-year created depressional wetlands in Virginia (Atkinson et al., 2005) and an ongoing assessment of created riverine wetlands (>10 years) in Ohio (Mitsch et al., 2005). Generally however, our understanding of vegetation community development in created depressional wetlands is lacking (Atkinson et al., 2005).

In this study we address three main questions pertaining to wetland mitigation; how do the vegetation communities of created depressional wetlands compare to natural reference sites given at least 5 years and in certain cases up to 19 years of development? How quickly are the vegetation communities of created depressional wetlands restored to the levels observed for natural reference sites? Finally, how does the level of initial restoration effort affect progress towards floristic equivalency of created depressional wetlands compared to natural sites?

Section snippets

Study sites and methodology

In this study, 17 mitigation wetlands were assessed, comprised of 8 freshwater depressional emergent marshes in Ohio and 9 freshwater depressional emergent marshes in a wetland complex in Colorado (see Appendix A). This allowed us to compare trends in wetland development in different geographic areas of the United States. In Colorado, 4 marshes were established in 1985 (referred to as the 85-site), 4 marshes were created in 1990 (90-site) and a single site was established in 1999 (99-site).

Floristics

Created sites displayed significantly greater numbers of non-native plant species in Ohio (p = 0.01) and Colorado sites (p = 0.004) within the first eight years of restoration (Table 1). In Ohio, the mean percent hydrophytes in the created sites in the first four years of development were significantly lower than the natural reference sites (p = 0.0001) as were percent native species (p = 0.01), which was the converse of the high number of non-native plant species. No significant differences were

Discussion

Field data indicate mixed results when assessing the ability of created depressional marshes to restore vegetation communities equivalent to those found in natural wetlands. Results of this study reiterate the point that ‘success’ of a created wetland project may be contingent upon selection of specific wetland structural and functional indicators and project performance goals (Ehrenfeld, 2000, Kentula, 2000, Matthews and Endress, 2008). Created marshes in Ohio were relatively successful in

Acknowledgements

Completion of this research was made possible in part by funding from the Ohio State Environmental Policy Initiative and through a cooperative grant with Murray State University funded by the National Science Foundation as a Research Opportunity Award (DEB 0109436 to Dr. Howard Whiteman) in the NSF Research at Undergraduate Institutions (RUI) program. Special thanks to Colleen Moriarty and Howard Whiteman for the opportunity to collaborate with Hawaii Pacific University and Murray State

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