Spatial variations in salinity stress across a coastal landscape using vegetation indices derived from hyperspectral imagery

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Chlorophyll fluorescence and landscape-level reflectance imagery were used to evaluate spatial variations in stress in Myrica cerifera and Iva frutescens during a severe drought and compared to an extremely wet year. Measurements of relative water content and the water band index (WBI970) indicated that the water stress did not vary across the island. In contrast, there were significant differences in tissue chlorides across sites for both species. Using the physiological reflectance index (PRI), we were able to detect salinity stress across the landscape. For M. cerifera, PRI did not differ between wet and dry years, while for I. frutescens, there were differences in PRI during the 2 years, possibly related to flooding during the wet year. There was a positive relationship between PRI and ΔF/F' for M. cerifera (r2 = 0.79) and I. frutescens (r2 = 0.72). The normalized difference vegetation index (NDVI), the chlorophyll index (CI), and WBI970 were higher during the wet summer for M. cerifera, but varied little across the island. CI and WBI970 were higher during 2004 for I. frutescens, while there were no differences in NDVI during the 2 years. PRI was not significantly related to NDVI, suggesting that the indices are spatially independent. These results suggest that PRI may be used for early identification of salt stress that may lead to changes in plant distributions at the landscape level, as a result of rising sea level. Comparsions between the two species indicate that variations in PRI and other indices may be species specific.

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