Biophysical remote sensing of salt marshes in South-East United States
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Study of salt marsh biophysical properties is imperative to understand its response to environmental change. We developed protocols for mapping biophysical properties of salt marshes such as Green Leaf Area Index (GLAI), Canopy Chlorophyll (CHLc), Vegetation Fraction (VF), and aboveground Green Biomass (GBM) using moderate resolution satellite images and in-situ data for the salt marshes in south-eastern United States. The time-series products derived using the biophysical models have been able to capture the spatio-temporal effects of the environmental events affecting the salt marshes of the region. We also tested the performance of different smoothing functions to derive noise-free phenology for Louisiana (LA) and Georgia (GA) salt marshes from the time-series GBM composites, and selected the best smoothing function to derive and analyze phenological parameters for salt marsh habitats. Long-term trend analysis of phenological parameters indicate positive changes in the base GBM values, and mostly negative changes in the GBM amplitude and small seasonal integral, which indicate overall progressive decline in the rates of photosynthesis and biomass allocation in the salt marsh ecosystem. This observed decline in photosynthesis and biomass allocation may be attributed to elevated atmospheric carbon dioxide (CO2) levels and sea level rise. Finally we attempted to map Gross Primary Productivity (GPP) for a salt marsh habitat in the Gulf Coast, using the GBM composites and in-situ GPP estimates from eddy covariance CO2 flux towers. The time-series composites and phenological charts developed using the biophysical GPP model was able to capture the effect of different environmental events such as dieback and hurricane landfall. The results illustrate the relative efficiency of MODIS in analyzing salt marsh biophysical properties. This is the first study to employ MODIS images to study the long-term trends in biophysical characteristics of salt marshes in south-east United States. The methods described in this study as well as the biophysical products derived using the methods has the potential to improve our ability to predict their productivity and carbon sequestration potential. These techniques could also be used to assess the success of previous and ongoing salt marsh restoration projects, and evaluate the productivity of marshes under threat from both natural and anthropogenic drivers.