Cordgrass

Case study 3: Limited response of cordgrass (Spartina foliosa) to soil amendments in a constructed marsh.

Background information:

In 1985 a 12 hectare plot in the Sweetwater National Wildlife Refuge was created to replace a similar sized plot of land destroyed for construction purposes. At first glance, the wetland appears to be fully functional. It fulfilled the government mandated requirements by establishing wetland vegetation, and consequently resembles a natural wetland. However, the created wetland still did not meet the project's goals. In this case, the wetland was created in order to help provide habitat for the endangered Light-footed Clapper Rail. This wetland was planted with cordgrass, the necessary habitat for Clapper nesting. The cordgrass was planted, and seemed to thrive. Yet, the Clapper did not nest. Superficially, the wetland was successful in its establishment of the cordgrass, but for some reason the Clapper would not use it for nesting. Upon closer examination it was determined that the Clapper would not nest because the cordgrass community planted did not grow tall enough to support nests (Zedler, 1993 in Gibson et al., 1994). Under flooding conditions, the nests of the Clappers would have been washed away. By looking closely at the wetland, it was determined the failure was in the nitrogen (N) depauperate soils. The soils did not contain the necessary N to promote full growth. This was because the base of the wetland was laid on sand substrate, and not on a richer layer of organic matter (Gibson et al., 1994). This is a classic example of hasty restoration projects; while the proximate goal of establishing cordgrass was realized, the ultimate goal of reestablishing the Clapper was unfulfilled.

Observation: Mitigated wetlands (marshes) in San Diego Bay, California, USA, have soils with low organic matter and nitrogen stores (compared to local-natural-sites) and have stunted cordgrass populations.

Hypothesis: Organic and nitrogen amendments would accelerate plant growth and enrich soil nitrogen pools.

Experimental Design: A newly created marsh was used for the experiments. The marsh was:

Sectioned off into four blocks of seven treatment plots.
Treatment plots were 1 x 5 m.
Of the seven treatment plots: 5 had altered nitrogen conditions (via the addition of alfalfa-high nitrogen, straw-low nitrogen, or ammonium nitrate-high inorganic nitrogen), and two were controls (rototilling).
Data was taken on: plot above ground growth, rate of alfalfa and straw decomposition, and soil nitrogen.

Results:

Soil amendments did not significantly affect mean plant height in either 1990 or 1991 (p>0.16).
Amount of plant biomass at each site increased with amount of nitrogen added.
Foliar nitrogen concentrations did not increase over time.
Cordgrass stem densities increased with increasing nitrogen concentrations.
Soil organic matter amendments did not significantly affect soil nitrogen concentrations.

Conclusions:

Aboveground biomass of Spartina foliosa responded to soil amendments in proportion to the amount of nitrogen added
The amount of biomass at the end of the year, was less than half of that found in natural marsh conditions.
In year two, the increase in biomass was predominantly as a function of increased plant height.
Alfalfa and straw litterbags lost most of their biomass in the first 2 weeks.
Organic amendments did not increase soil organic content.
Soil nitrogen amendment experiments did not change soil nitrogen levels significantly.
The plant canopy recovered less than 5% of the nitrogen added.

Implications:

The authors inability to restore wetland nitrogen levels, suggests--and serves as a microcosmic example--of the impossibility (or great difficulty) of ever restoring/creating a wetland that can fully mimic a natural one.