Relationships between soil properties and Spartina alterniflora dieback patchiness in an Atlantic salt marsh

Background/Question/Methods

Spartina alterniflora (smooth cordgrass) is a keystone plant species in many salt marshes and is known for its role in trapping sediments and providing the basal resources for complex food webs and high secondary productivity. High plant productivity may contribute to high rates of carbon storage in salt marshes. Large scale dieback of S. alterniflora in the past decade has plagued salt marshes along the major coastlines of the United States. There are many hypothesized drivers of S. alterniflora dieback including changes in herbivory and trophic structures, altered soil redox states, drought and sea level rise. One unanswered question is why does dieback tend to affect distinct areas of salt marshes? We hypothesized that differences in fundamental soil physical and chemical states would lead to patches of S. alterniflora dieback interspersed amongst healthy plant assemblages. We surveyed physical and chemical soil conditions in dieback affected areas and unaffected areas in three marshes on Sapelo Island, Georgia in October 2011. We also re-surveyed dieback locations documented in 2007. Measurements of soil cores from healthy and dieback locations included: pH, salinity, bulk density, percent loss on ignition, soil water content and porosity. 

Results/Conclusions

We found strong differences in the physical states of S. alterniflora dieback soils compared to adjacent healthy areas. However, we did not find strong differences in the chemical states between the study areas. Soil water content, porosity and percent loss on ignition were all significantly lower in dieback areas, whereas bulk density was significantly higher in areas affected by dieback.  Almost half of the dieback sites documented in 2007 were recurrent diebacks in 2011 indicating that spatially discrete dieback is a fundamental geographic feature of the studied marshes. While we cannot establish cause and effect of S. alterniflora dieback, we believe that the observed soil states represent a legacy of dieback with complex interactions between plants, soils and water movement. Our finding that dieback soils store significantly lower amounts of organic matter (by proxy from % LOI) has strong implications for the carbon balance of these highly productive ecosystems. Furthermore, because soil carbon is believed to contribute to the long-term stability of coastal marshes, decreased carbon storage could represent a trajectory towards or the establishment of a new stable state.