Conservation Maven

Kawai Nui Marsh, sunset. Image credit, Jason Turse.A new study compares natural wetlands in Hawai‘i against those that have been restored and created. While all three wetland types had similar vegetation characteristics, natural wetlands had very different soil properties. These results indicate that wetlands created and restored as compensatory mitigation under the Clean Water Act may not adequately replace the function of natural wetlands lost to development. 

The study is also important because it represents the first assessment of costal lowland wetlands in Hawai‘i. Invasive species dominated the vegetation cover in natural, created, and restored wetlands. According to the researchers:

"Our findings suggest that the vegetation of costal low wetlands in Hawai‘i needs more intensive management and invasive species control. These results also suggest that it may be difficult to use vegetation to locate ‘‘reference’’ sites for costal lowland wetlands in Hawai‘i due to the pervasive nature of invasive species regardless of wetland status. From a management perspective, however, there are a few sites (e.g., Lawai Kai, Nu‘u, Kamilo Point) that have mostly native vegetation and could be considered the least-impacted."

From a global perspective, this research adds to a growing list of studies showing that restored and created wetlands do not replace soil properties of natural wetlands. This study found that natural wetland had lower bulk density and pH and higher soil organic matter, silt, total nitrogen and total carbon versus created and restored wetlands.

The researchers list the likely explanations for the differences in soil properties:

1) The use of heavy machinery in wetland creation and restoration leads to compaction of soils, resulting in higher bulk densities.

2) Higher bulk density and lower soil organic may be an artifact of excavation into subsurface soil horizons that are compacted and have lower organic matter content.

3) Organic matter accumulation is a function of time, established vegetation, and hydrology. Organic matter accumulation is favored in natural wetlands due to the inhibition of decomposition caused by the long-term anaerobic conditions typical of natural wetlands.

4) The low pH and high total carbon and total nitrogen values observed in natural wetlands were likely the result of saturated soils, low oxygen levels, and subsequent inhibition of organic matter decomposition.

The study offers the following recommendations for wetland mitigation:

1) An effective method for reducing soil compaction of created and restored wetlands is to use a chisel plow to mechanically rip both the topsoil and subsoil layers, prior to planting.

2) Amendments such as compost, mulch, or other organic material have proven to be effective methods for increasing soil moisture.

3) Due to the greater similarities between the soil properties of natural and restored wetlands, it will likely take longer for created wetlands to develop functions characteristic of natural wetlands. Therefore, given the option between restoration and creation, restoration should be the preferred mitigation option in Hawai‘i.

--Reviewed by Rob Goldstein

Bantilan-Smith, M., Bruland, G., MacKenzie, R., Henry, A., & Ryder, C. (2009). A Comparison of the Vegetation and Soils of Natural, Restored, and Created Coastal Lowland Wetlands in Hawai‘i Wetlands, 29 (3), 1023-1035 DOI: 10.1672/08-127.1