Monitoring Pacific Coast Salt Marshes Using Remote Sensing

The rapid decline in the extent and health of coastal salt marshes has created a need for nondestructive methods for evaluating the condition of salt marsh ecosystems. This paper describes simultaneous uses of field sampling and remote sensing approaches to understand salt marsh ecosystem functions and species distributions and discusses the implications for salt marsh monitoring using remote sensing. Three sites along the Petaluma River near the entrance into San Pablo Bay, California, which represented a range of soil salinity, water content, and nutrients, were studied. Standing biomass was directly assessed by field sampling and indirectly estimated through canopy reflectance. The sites were dominated by almost monotypic stands of Salicornia virginica, Spartina foliosa, and Scirpus robustus. For Salicornia, we found a positive relationship between salinity and biomass up to a threshold of 42 g/kg, after which biomass declined monotonically with increasing salinity. No Scirpus or Spartina were found at soil salinities >20 g/kg. Although significantly different levels of nitrate and ammonium nitrogen were found in the interstitial water and soils at these sites, no strong relationships were found between biomass and nitrate nitrogen. Soil ammonium nitrogen, in contrast, was positively related to biomass. Soil redox and salinity increased with elevation and distance from the shoreline, while soil moisture and H2S decreased. Canopy biomass was estimable using remotely sensed spectral vegetation indices at 58–80% accuracy depending on species. Simple Vegetation Index (VI) and Atmospherically Resistant Vegetation Index (ARVI) measured by handheld field spectrometers were the best estimators of green biomass for high cover of Salicornia. Soil Adjusted Vegetation Index (SAVI) and Soil Adjusted and Atmospherically Resistant Vegetation Index (SARVI) gave the best estimates for Spartina while the Global Environment Monitoring Index (GEMI) was the best estimate for Scirpus. The relationships between vegetation indices and biomass were developed from field spectra. The VI was used to estimate spatial patterns of biomass across the salt marsh from Landsat satellite Thematic Mapper (TM) data. The TM image showed spatial patterns corresponding with species zones and biomass abundance. Narrow band reflectance features measured with a handheld spectrometer can be used to predict canopy plant water content (R2 = 63%). Interpolated estimates of water content from field-measured canopy reflectance were shown to relate to variation in salinity and soil moisture. Canopy water content was estimated from Airborne Advanced Visible Infrared Imaging Spectrometer data, which showed similar spatial patterns at the site. Results indicate that both biomass production and canopy water content can be accurately determined from remotely sensed spectral measures. Species-specific differences in these characteristics may be used for monitoring species distribution and abundance from airborne or satellite images.