Remote sensing-based structural and functional assessments of Phragmites australis diebacks in the Mississippi River Delta

Abstract

• Extensive wetland changes can be caused by abiotic and biotic stressors. • Changes in wetland structure and condition can significantly impact vital navigation, flood risk, and ecosystem health services. • Remote sensing and hot spot analyses were used to measure episodic disturbance impacts on wetland condition. • The results show remotely collected data can provide enhanced spatial and temporal measures of wetland plant extent and condition as a function of landscape level dynamics. The Lower Mississippi River Delta (MRD) is dominated by Phragmites australis which provides a stabilizing force, protecting marsh communities from erosion and storm-related impacts. The MRD has experienced recent die-offs of Phragmites stands, which have coincided with a number of abiotic (hurricane and water-level) and biotic ( Phragmites scale) stressors. During this event, previously healthy stands have died or experienced stunted growth, resulting in conversion to replacement species or to open water. This study utilized remote sensing methods to (1) evaluate changes in Phragmites health and distribution in the MRD through time; (2) assess changes in plant cover and floristic quality before, during, and after the dieback event; (3) evaluate changes in landscape patterns (i.e., percentage of landscape, patch density, total edge, and aggregation index); and (4) evaluate changes in channel length and width as a function of the dieback event. Normalized Difference Vegetation Index data showed an active delta landscape with below typical plant biomass/health in periods of hurricane and dieback impacts, with recovery to health values more typical for the MRD after the dieback event. Vegetation species level assessments showed the MRD landscape experienced decreasing coverage and floristic quality during the disturbance periods, and low and nominally increasing vegetation cover and quality through the dieback and recovery periods. Landscape metrics showed similar trends, where wetland areas that experienced event-related stress during the hurricane and dieback periods, showed some level of rebound during the recovery period. And finally, bank-line change analyses showed a significant increase in the rate of shortening and widening of the Mississippi River Passes since the die-back event began. This study serves as a methodological basis for deriving vegetation trend assessments and integrating those results with landscape metrics to prioritize areas of interest; all of which are essential for effective management and mitigation of aquatic nuisance vegetation.