Remote sensing methods for mapping the onset and progression of spartina alterniflora marsh dieback in coastalLouisiana

摘要

We investigated the use of remote sensing in mapping the occurrence and monitoring the progression of amarsh dieback phenomenon observed in coastal Louisiana. Within our investigation, we demonstrated that the early andlater stages of dieback progression were generally associated with green and red edge leaf reflectance changes and trends,while the later stages of marsh impact were represented by trends in blue and red leaf reflectance. Near infrared (nir) leafreflectance trends were inconclusive while relative changes in predicted leaf carotene and chlorophyll were indicative ofstressed plants. The nir/red ratio followed blue and red reflectance trends while the nir/green ratio mimicked the greenand red edge reflectance trends indicating impact onset and progression as well as indicating later stages of impact. Thenir/green also provided a convenient method to determine the relative time since dieback onset and offered an operationalsolution that would be amenable for current operational satellite remote sensing,Following our successful linkage of leaf optical changes to marsh dieback onset and progression, we applied our newlydeveloped methods to changes in site-specific canopy reflectance spectra. Canopy reflectance spectra (~20 m groundresolution gained from a helicopter platform) were collected from marsh sites occupied during the leaf spectral analysesand sites exhibiting visual signs of dieback. Two scalesof analyses were employed. One represented wholewhole-spectra hyperspectral sensors such as the NASA EO1Hyperion and the other broadband spectral sensorssuch as the NASA EO1 Advanced Land Imager and theLandsat Enhanced Thematic Mapper (limited to about400 nm to 1000 nm). Spectral indicators generated inthe whole-spectra analysis were used to determine thepercent dead and healthy marsh composition at eachsite. The compositions were used classified into groupsof marsh sites exhibiting similar levels of dieback.Separately, blue, green, red, red-edge, and nearinfrared (nir) spectral bands extracted from the fieldspectra and nir/green and nir/red band transforms wererelated to marsh dieback and progression indicatorscalculated from classifications of the 35-mm slidescollected with the canopy reflectance recordings. Boththe whole spectra and broadband spectral indicatorscould distinguish differences in (1) healthy marsh, (2)live marsh impacted by dieback, and (3) dead marsh.Broadband indicators provided some determination ofdieback progression within the impacted live marshwhile whole spectra hyperspectral indicators offeredincreased discrimination of dieback onset andprogression.