Management of Smooth Cordgrass (Spartina alterniflora) Stubble in Freshwater Production Ponds

Smooth cordgrass (Spartina alterniflora) is commonly used in wetland management projects throughout the United States. It is a perennial grass native to intertidal saline marshes along the Atlantic and Gulf of Mexico coasts (Godfrey and Wooten 1979) that decreases coastal erosion by reducing wave energy and accumulating suspended sediments (Redfield 1972, Nepf 1999). Reducing coastal erosion is essential in Louisiana, because it has the highest rate of coastal erosion in the continental United States (Couvillion et al. 2011). To reduce coastal erosion and protect coastal habitats, communities, and industries, numerous wetland management projects are completed each year. Projects range from reintroduction of native plants in deteriorated coastal areas to wetland construction in deteriorated areas or areas that have entirely eroded to open water.

Vegetative propagation of smooth cordgrass is common in Louisiana. Most federal and state restoration projects in Louisiana require 'Vermilion' smooth cordgrass variety because it is known to survive and perform well based upon scientific evaluations and successful use in restoration projects (Fine and Thomassie 2000). Requiring nursery grown Vermilion also eliminates the need to harvest smooth cordgrass material from natural marsh environments, which can cause additional erosion due to reduced stem densities and physical disruptions caused by foot and boat traffic and removal of plant and soil material (DeLaune et al. 1994, Nepf 1999, Kirwan et al. 2008, Bouma et al. 2009). Efficient and economical methods to produce smooth cordgrass in freshwater plant production nurseries are essential to provide large numbers of plants for wetland management projects. Management of previous years' plant stubble is one practice that could greatly increase the efficiency of smooth cordgrass plant production nurseries. Stubble removed by burning or mowing is known to increase plant and seed yields in perennial crops (Thompson and Clark 1989, Young et al. 1998, Cuomo et al. 1999, Meints et al. 2001, Bueno et al. 2007) and could also increase stem yields of aquatic plants, such as smooth cordgrass. The objective of this study was to determine smooth cordgrass yields in freshwater production ponds when plant stubble was burned or mowed.

In 2006 we transplanted smooth cordgrass plants to two 50 m × 30 m freshwater ponds with 0.6 m levees in Baton Rouge, LA. Ponds were flooded with 0.2 m of water and periodically drained to simulate tidal conditions. On January 23, 2009 and February 19, 2010, one half of each pond (50 m × 14 m) was burned and one fourth of each pond (50 m × 7 m) was mechanically mowed to remove previous years' stubble; the remaining one fourth of the pond was undisturbed to serve as the untreated control. A one meter wide pathway between each treatment area was mechanically maintained as a non-vegetated walkway within each pond. In 2010, stubble removal was delayed one month because of significant precipitation; between December 2009 and January 2010 more than 44 cm of precipitation occurred, while less than 24 cm precipitation occurred between December 2008 and January 2009.

Number of stems and tillers in a 625 cm² quadrat and stem diameter of five plants were measured every four weeks for nine months. Stems were defined as growth greater than 10 cm tall while tillers were defined as new growth less than 10 cm tall. Stem diameter was measured 5 cm above the soil surface with digital calipers. A total of 51 quadrats, 17 quadrats per treatment, were measured. Thirteen quadrats per treatment were measured in one pond because location of plants transplanted in 2006 could be determined. Four quadrats per treatment were measured in the second pond because location of original plants could not be determined. Fresh weight of aboveground biomass was measured nine months after stubble removal by harvesting stems from 625 cm² quadrats and immediately measuring weight. Number of stems and tillers and stem diameter data were analyzed with Analyses of Variance as repeated measures (PROC MIXED, SAS Institute version 9.1.3). Biomass data were analyzed with Analysis of Variance as completely random design (PROC MIXED, SAS Institute version 9.1.3). Stubble removal treatment was considered a fixed effect while year and...