Hyperspectral Reflectance Response of Freshwater Macrophytes to Salinity in a Brackish Subtropical Marsh

Abstract

Coastal freshwater wetlands are threatened by increased salinity due to relative sea level rise and reduced freshwater inputs. Remote radiometric measurement of freshwater marsh canopies to detect small shifts in water column salinity would be useful for assessing salinity encroachment. We measured leaf hyperspectral (300-1100 nm) reflectance of freshwater macrophytes (cattail, Typha latifolia and sea oxeye, Borrichia frutescens) in a field study in a subtropical brackish (2.5-4.5 parts per thousand salinity, per thousand) marsh to determine salinity effects on visible and near-infrared spectral band reflectance and to identify reflectance indices sensitive to small (1 per thousand) changes in wetland salinity. For sea oxeye, floating-position water band index [fWBI = R(900)/minimum(R(930) - R(980)), where R(lambda) = reflectance at band lambda], normalized difference vegetation index [NDVI = (R(774) - R(681))/(R(774) + R(681))], and a proposed wetland salinity reflectance ratio (WSRR = R(990)/R(933)) were sensitive to salinity with R2 of 40, 35, and 65%, respectively (p < 0.01). For cattail, NDVI and photochemical reflectance index [PRI = (R(531) - R(570))/(R(570) + R(531))] were sensitive to salinity with R2 of 29 and 33%, respectively (p <or= 0.01). Higher salinity significantly reduced mean reflectance of sea oxeye in 328- to 527-nm and 600- to 700-nm wavebands (p < 0.05), which corresponded to chlorophyll bands. Reflectance of cattail was not significantly affected by the highest salinity, although the spectral band most affected was 670 nm (p < 0.10), which is a chlorophyll a band. Our findings indicate that hyperspectral radiometry can detect the response of emergent freshwater plants to changes in wetland salinity, which would help with monitoring salinity effects on coastal wetlands.

Spectral indices for the detection of salinity effects in melon plants

“…An increment in the soil salinity generally tends to induce a decrease in leaf reflectance in the near infrared spectral region (Leone et al, 2007;Peñuelas et al, 1997b), but an increase in near infrared reflectance with salinity was observed in this study. Previous studies reported the same pattern found in the near infrared reflectance when salt-tolerant species were irrigated with moderately saline water (Poss et al, 2006, Tilley et al, 2007. Similarly, Zhang et al (2011) observed a rise in near infrared reflectance for salt-tolerant species growing on moderately saline soils in a wetland environment.…”

Multiple Stable States and Catastrophic Shifts in Coastal Wetlands: Progress, Challenges, and Opportunities in Validating Theory Using Remote Sensing and Other Methods

“…In addition to automated analysis methods, vegetation classes have long been digitized by hand [79][80][81]. Remote sensing imagery has been used to map coastal topography and flooding [56,63,[82][83][84][85][86] and to map physiological responses of vegetation to salinity or pollution [87][88][89][90]. Some studies have used remote sensing to map coastal or vegetation change over time at two, or a few, time points [91][92][93][94][95][96][97], including analyses of effects of invasive species [59,98].…”

Soil Salinity Mapping and Monitoring in Arid and Semi-Arid Regions Using Remote Sensing Technology: A Review

“…Normally, unhealthy vegetation has a lower photosynthetic activity, causing increased visible reflectance and the reduced near-infrared reflectance (NIR) from the vegetation [24]. This pattern has been found in various plants subjected to salinity stress [25]. Therefore, based on this finding, several vegetation indices (VIs) such as Normalized Differential Vegetation Index (NDVI) and Soil Adjusted Vegetation Index (SAVI) have been used as indirect indicators assess and map soil salinity.…”