Recent Trends in Satellite Vegetation Index Observations Indicate Decreasing Vegetation Biomass in the Southeastern Saline Everglades Wetlands
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We analyzed trends in time series of the normalized difference vegetation index (NDVI) from multitemporal satellite imagery for 2001–2010 over the southeastern Everglades where major changes in vegetation structure and type have been associated with sea-level rise and reduced freshwater flow since the 1940s. Non-parametric trend analysis using the Theil-Sen slope revealed that 84.4 % of statistically significant trends in NDVI were negative, mainly concentrated in scrub mangrove, sawgrass (Cladium jamaicense) and spike rush (Eleocharis cellulosa) communities within 5 km of the shoreline. Observed trends were consistent with trends in sawgrass biomass measurements made from 1999 to 2010 in three Long-term Ecological Research (LTER) sites within our study area. A map of significant trends overlaid on a RapidEye high-resolution satellite image showed large patches of negative trends parallel to the shoreline in and around the “white zone,” which corresponds to a low-productivity band that has moved inland over the past 70 years. Significantly positive trends were observed mainly in the halophytic prairie community where highly salt tolerant species are typically found. Taken as a whole, the results suggest that increased saline intrusion associated with sea-level rise continues to reduce the photosynthetic biomass within freshwater and oligohaline marsh communities of the southeastern Everglades.
KeywordsMODIS NDVI Non-parametric trend analysis Sawgrass marsh Mangroves Above-ground biomass production
The authors wish to thank Raymond Turner of Center for Southeast Tropical Advanced Remote Sensing (CSTARS) for providing the RapidEye image. We are grateful to the many scientists of the FCE LTER who have made their field data publicly available. Support for this research was provided by NASA WaterSCAPES (Science of Coupled Aquatic Processes in Ecosystems from Space) Grant NNX08BA43A.
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