Twenty-Five Years of Ecosystem Development of Constructed Spartina alterniflora (Loisel) Marshes

Abstract

Wetland creation and restoration are frequently used to replace ecological functions and values lost when natural wetlands are degraded or destroyed. On many sites, restoration of ecological attributes such as secondary production, habitat/species diversity, and wetland soil characteristics do not occur within the first decade, and no long-term studies exist to document the length of time required to achieve complete restoration of wetland dependent functions and values. Characteristics of community structure (macrophyte aboveground biomass, macro-organic matter [MOM], benthic invertebrates) and ecosystem processes (soil development, organic C, N, and P accumulation) of two constructed Spartina alterniflora (Loisel) marshes (established 1971 and 1974) and paired natural S. alterniflora marshes in North Carolina were periodically measured during the past 25 yr. On constructed marshes, the macrophyte community developed quickly, and within 5 to 10 yr, aboveground biomass and MOM were equivalent to or exceeded corresponding values in natural marshes. After 15-25 yr, benthic infauna density and species richness were greater than in the natural marshes. Soil bulk density decreased, and organic C and total N increased over time in constructed marshes, but after 25 yr, soil organic C and N reservoirs were much smaller than in a 2000-yr-old natural marsh. Organic C accumulation was similar in constructed and natural marshes with 12-24% of the net primary production buried annually. Nitrogen accumulation was much higher in constructed marshes (7-12 g · m Ϫ2 · yr Ϫ1 ) than in natural marshes (2-5 g · m Ϫ2 · yr Ϫ1 ), reflecting the open biogeochemical cycles and paucity of N in these young ecosystems. Different ecological attributes develop at different rates, with primary producers achieving equivalence during the first 5 yr, followed by the benthic infauna community 5-10 yr later. Accumulation of soil nutrients to levels similar to those of reference marshes may require more time.

Detrital Decomposition As a Measure of Ecosystem Function in Created Wetlands

“…~c c o m~a n~i n g changes in the rate of detritus processing provide plausible measures of ecosystem function. A number of studies have confirmed that benthic organic matter accumulates, at least initially, in some created wetlands, and that natural wetland substrate has a higher organic content and a lower density than that of young created wetlands (Craft et al 1999and 2003, Zedler and Callaway 1999, Nair et al 2001. Further, there is evidence that the rates of detritus decomposition (Atkinson and Cairns 2001) and carbon mineralization (Craft et al 2003) increase with age in created wetlands.…”

Cordgrass Biomass in Salt Marshes

“…For example, created marshes of S. maritima in the Odiel Marshes (Southwest Iberian Peninsula) develop faster (within 2-4 years with maximum net aerial primary productivity of ca. 600 g DW m -2 yr -1 following Castillo et al, 2008a) than North American marshes of S. alterniflora (Craft et al, 1999(Craft et al, , 2002(Craft et al, , 2003Edwards & Mills, 2005), which seems to be related to warmer winters in Iberian salt marshes. Thus, S. alterniflora productivity decreases with latitude and air temperature along the western Atlantic coast of North America (Kirwan et al, 2009).…”

“…Among cordgrasses with a "guerrilla" growth-form, S. alterniflora in western Atlantic low salt marshes accumulates between 100 and 1100 g DW m -2 . Changes in S. alterniflora biomass between populations are related mainly with its short and tall forms, varying its shoot height between 20 and 140 cm, also with a highly variable shoot density that changes markedly between 100 and 4000 shoot m -2 (Craft et al, 1999(Craft et al, , 2002(Craft et al, , 2003Proffitt et al, 2005;Culbertson et al, 2008;Darby & Turner 2008b;McFarlin et al, 2008;Sala et al, 2008;Tyrrell et al, 2008;Buchsbaum et al, 2009;Gonzalez Trilla et al, 2009;Krull & Craft, 2009;Michel et al, 2009;Holdredge et al, 2010). One year after invading Chinese marshes, S. alterniflora accumulated ca.…”

“…Primary productivity and biomass accumulation are important indicators of success for salt marsh creation and restoration projects (Edwards & Mills, 2005). Although plant biomass accumulation is a key factor in the functioning of Spartina dominated marshes, other ecological attributes, such as species richness and distribution, benthic infauna density or soil nutrient reservoirs, may develop at different rates than cordgrass biomass in restored wetlands (Craft et al, 1999;Onaindia et al, 2001;Craft et al, 2003;. Below-and above-ground biomasses are key functional traits that play very important roles in the ecological behavior of cordgrasses.…”

Sediment remobilisation from decomposing cordgrass (Spartina anglica) patches on a wave‐exposed intertidal flat

“…In other settings, such as on the east coast of the United States, Spartina spp. are being reestablished as part of estuary restoration programmes (e.g., Moy & Levin 1991;Craft et al 1999).…”