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 conditio n of salt marsh ecosystems. This paper describes simultaneous uses of held sampling and remote sensing approaches to understand salt marsh e cosystem functions and species distributions and discusses the implica tions for salt marsh monitoring using remote sensing. Three sites alon g the Petaluma River near the entrance into San Pablo Bay, California, which represented a range of soil salinity, water content, and nutrie nts, were studied. Standing biomass was directly assessed by field sam pling and indirectly estimated through canopy reflectance. The sites w ere dominated by almost monotypic stands of Salicornia virginica, Spar tina foliosa, and Scirpus robustus. For Salicornia, we found a positiv e relationship between salinity and biomass up to a threshold of 42 g/ kg, after which biomass declined monotonically with increasing salinit y. No Scirpus or Spartina were found at soil salinities >20 g/kg. Alth ough significantly different levels of nitrate and ammonium nitrogen w ere found in the interstitial water and soils at these sites, no stron g relationships were found between biomass and nitrate nitrogen. Soil ammonium nitrogen, in contrast, was positively related to biomass. Soi l redox and salinity increased with elevation and distance from the sh oreline, while soil moisture and H2S decreased. Canopy biomass was est imable using remotely sensed spectral vegetation indices at 58-80% acc uracy depending on. species. Simple Vegetation Index (VI) and Atmosphe rically Resistant Vegetation Index (ARVI) measured by handheld field s pectrometers 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 veg etation 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 abundan ce. Narrow band reflectance features measured with a handheld spectrom eter can be used to predict canopy plant water content (R-2 = 63%). In terpolated estimates of water content from field-measured canopy refle ctance were shown to relate to variation in salinity and soil moisture . Canopy water content was estimated from Airborne Advanced Visible In frared Imaging Spectrometer data, which showed similar spatial pattern s at the site. Results indicate that both biomass production and canop y water content can be accurately determined from remotely sensed spec tral measures. Species-specific differences in these characteristics m ay be used for monitoring species distribution and abundance from airb orne or satellite images.