Linda Benedict, Nyman, John A. | 11/28/2011 11:46:54 PM
Managing and restoring coastal wetlands requires knowledge of wetland conditions and the factors creating the desired conditions. Flooding stress, nutrient starvation and salinity stress are the most likely suspects in wetlands that have higher loss rates or appear less productive than typical wetlands. Diagnosing the cause depends primarily on information inferred from plant species growing in the wetlands. The dominant species and the species richness can be used by many wetland managers to infer levels of salinity stress and flooding stress, but not nutrient limitation. Even with salinity stress and flooding stress, experts may disagree about which is more important at a particular site. Coastal wetland managers and restoration planners could benefit from new tools to help them objectively diagnose factors limiting plant growth.
LSU AgCenter scientists have been working to modify a technique that farmers use to diagnose their crops so that marsh managers and restoration planners can similarly diagnose coastal wetlands. Farmers and home gardeners can clip leaves from their fields and lawns, send the leaf tissue to the LSU AgCenter’s Soil Testing and Plant Analysis Laboratory, and a few weeks later receive a diagnosis of the factors most likely to be limiting plant production. Plant analysis diagnoses nutrient deficiency, toxicity and imbalance of major and micro-nutrient elements of more than 90 crops and lawn plants. Numerous scientists have studied these plants under known conditions of nutrient deficiency or toxicity and then identified a chemical consequence of those conditions in the leaf tissue. Until now, such information did not exist for coastal wetland grasses.
The research focused on Spartina patens, also known as saltmeadow cordgrass, which occurs from fresh to saline marshes but dominates intermediate and brackish marshes. Work began with experimental projects growing saltmeadow cordgrass under a range of known flooding, nutrient and salinity conditions in the greenhouse and the field. The health of those plants and the chemical makeup of their leaves were used to develop chemical signatures of flooding stress, nutrient starvation and salinity stress. Fifteen elements and various combinations of ratios of those elements were considered when trying to identify chemical consequences of flooding stress, nutrient deficiency and salinity stress. Identifying a signature of flooding stress was simpler because flooding stress apparently is unaffected by salinity stress and nutrient starvation. Identifying signatures of salinity stress and nutrient starvation was more complex, however, because those factors interact in such a way that growth is limited by salinity even when nutrient content is high.
Work continued with field studies designed to challenge the chemical signatures developed from the experimental work. Study sites included Four League Bay, which is southeast of Morgan City, over to Calcasieu Lake, which is south of Lake Charles. Public lands involved were the Cameron Prairie National Wildlife Refuge, Rockefeller Refuge and Marsh Island Refuge. Private landowners also cooperated including Miami Corporation. Most of the field study sites were within a mile or two of a sensor operated by the Louisiana Office of Coastal Protection and Restoration, which records water level and salinity hourly. More importantly, salinity and nutrient levels were measured in water collected from the study plots where saltmeadow cordgrass health was measured. The field challenges revealed that the new technique accurately identified salinity stress and nutrient limitation. The flooding stress indicator appeared to work but could not be tested rigorously because the water level recorder data did not accurately reflect flooding and drainage conditions at the sampling sites. Thus, while marsh managers and restoration planners can confidently use the new tool to diagnose nutrient starvation and flooding stress, the flooding stress indicator is not yet ready for use. Additional field work is planned that will incorporate adjacent study plots and water level recorders.
Andy Nyman, Associate Professor, School of Renewable Natural Resources, LSU AgCenter, Baton Rouge, La.
(This article was published in the fall 2011 issue of Louisiana Agriculture magazine.)