Case study 1: A long term assessment of wetland restoration efforts comparing created and natural wetlands.





Restoration Decisions: (and theory)


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State of the Science

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Created by:

Abby Rokosch
Jessen Book
Siobhan Fennessy

Description of initial experiment: Comparison of created and natural freshwater emergent wetlands in Connecticut, USA (Confer and Niering, 1992).

  • Five created palustrine/emergent wetlands were compared with five natural wetlands of approximately the same size and type. Each natural wetland was within five miles of the created wetland.
  • No wetland plant species were planted in the created wetlands.
  • Hydrologic data (amount of open water, and water depth fluctuations), soil data (color, texture, and depth to saturation), and vegetation data (cover and frequency) were collected at both the natural and created sites.


  • Created wetlands had more open water and a greater water depth than natural wetlands, (p < 0.0001).
  • The majority of the created wetlands, all but one, showed less water depth fluctuation than in the natural wetlands, (p < 0.05- 0.01).
  • Mean water depth of created wetlands was greater than in the natural wetlands, (p<0.05).
  • The soils in created wetlands and natural wetlands were significantly different. In the created wetlands, the soil was composed of reddish brown mineral soil absent or rare of mottles. In the natural wetlands, the soil was composed of black organic soil with the presence of mottles. (Mottles indicate hydric soils.)
  • Total emergent plant cover was greater at natural sites (97%) versus the created sites (71%).
  • Species richness was greater at the created sites (22-45) than at the natural sites (20-39).
  • Changes of species composition at the created sites shifted over a period of 1 year while the species composition remained relatively steady in the natural wetland sites. Typha angustifolia and Phragmites australis increased in the created wetland sites.
  • No overall increase in invasive species was seen in the natural wetland sites.

Conclusions/ Future Research:

  • The water level fluctuation difference between the created and natural sites may be due to the position of the wetlands within the watershed. The majority of the natural wetlands were associated with streams while the created wetlands received their water from highway runoff.
  • The absent of hydric soils in the created wetlands occurred because the created sites had only been under hydric conditions for a few years.
  • Species richness was greater at the created sites, but this is often found in the early stages of vegetation development where pioneer species rapidly colonize an area. Long term monitoring of vegetation composition is necessary to determine if plant communities can successfully avoid invasive species.
  • Their study showed that it is possible to create wetlands that can possess some of the functions associated with natural wetlands such as flood storage, sediment accretion, and presence of limited wildlife. However, long term monitoring of these created wetlands is needed to be able to measure the actual success of these wetlands. Until they prove to be self-sustainable wetland ecosystems, they cannot be described as successful restoration efforts. The following study was done in order to assess long term effects of restoration attempts.....

Description of Experiment- Vegetation change in created emergent wetlands (1988-1996) in Connecticut (USA) :

  • Five out of four previously studied created wetlands were reevaluated for changes in hydrology, water quality, and vegetation.
  • Hydrology was assessed by measuring its water depth, percent open water, and water quality. Water quality measurements included, total phosphorus, total nitrogen, chlorophyll-a, sodium chloride, magnesium, specific conductivity, alkalinity, and pH.
  • Transects were used to identify plant species present in the wetlands.
  • Wildlife observations were made during each visit to a wetland site. Frequency of each species was recorded as rare, occasional, frequent, and common.


  • Mean water levels in the created sites decreased from 15 cm in 1988 to -1 cm in 1999. The water levels of the natural sites increased from 4 to 12 cm.
  • The monthly water level change for created sites decreased from -15 cm to 35 cm of standing water in 1988 to ????
  • Open water at three of the created sites was completely gone in 1996 compared to open water cover of 4-39% in 1988. At the other created wetland site, open water decreased from 70% in 1988 to 15% in 1996. Open water at the natural sites only decreased 10-12% from 1988 and 1996.
  • Insert Figure 2????
  • Base cation and anion levels were typically higher at the created wetland sites, with the exception of total phosphorus.
  • Total emergent plant cover was greater at the natural sites (112%) versus the created sites (96%).
  • The mean increase in total number of species recorded in 1996 was similar in both created (8%) and natural wetland sites (7%).
  • The range in total number of species present was greater at the created site in 1988 (22-45) compared to the natural site (27-39) in 1988. But in 1996, the range of species present in the created site was only 25-33 while in the natural site, the range of species present was 27-46.
  • Invasive species, Typha sp., Phragmites Australis, and Lythrum salicaria increased in the created sites from 1988 (<1%) to 1996 (18%). In the natural sites these invasive species increased from 1988 ( <1%) to 1996 (6%).
  • Shifts in species composition were observed in the created sites from Typha sp. dominated transects to Phragmites sp. dominated transects. T. latifolia has decreased from 16%-5% while T. angustifolia has increased from 2%-10%. Phragmites that was once absent in 1988, dominates the transects with 29% cover.
  • Greater number of species of birds and animals were observed in the created wetlands versus the natural wetlands in both 1988 and 1996. The created wetland with through-flow-hydrology had the highest number of species, 1988 (13) and 1996 (14).


  • The declining water levels in the created wetland sites show the difficulty of achieving hydrological equivalence.
  • Water quality measurements reflect the importance of the surrounding landscape on the wetland ecosystem. The created wetland sites were affected by highway runoff which probably increased the amount of nitrogen and salt present in the created wetland versus the natural wetlands.
  • Increased salt levels have been shown to allow for the invasion of Phragmites or T. angustifolia.
  • The species composition shift from Typha sp. to Phragmites can be associated with the increased amount of nutrients into the wetland from highway runoff. Other studies have shown that Phragmites can out compete Typha sp. as a result of nutrient enrichment.
  • Though species richness increased in the created wetland sites, the invasion of Phragmites and Lythrum salicaria (recognized as a major competitor in emergent wetlands) could be a potential problem in the future.
  • Created wetlands are providing valuable habitats for wildlife.
  • Functional equivalence has not been established between the natural sites and the created sites but some wetland functions have been restored successfully, such as sediment accretion, flood control, and wild life habitat.

Future Research:

  • Upland buffers to the wetland should part of the highway management plan in order to alleviate some of the effects of increased nutrient loads and invasion of invasive plant species.
  • In order to limit invasive species such as Phragmites, the initial wetland design should involve a two tiered system with adjustable water flow. The interception of roadside contaminants would occur in the first basin before moving water into the second basin.