Spectral analysis of an intertidal saltmarsh in the Yangtze Estuary

Abstract

To consider different vegetation types, coverage, soil types, soil moisture, and gradient elevation of a tidal flat so as to determine the effect of the tide, three typical saltmarsh vegetation types in the Yangtze Estuary were chosen as research objects. Vegetation transects and samples(1 m×1 m) were selected for the above factors, and the growth environment of the saltmarsh was simulated. An ASD Fieldspec HandHeld 2 was used to measure the spectrum of the saltmarsh vegetation canopy; then the spectral reflectance and the first derivative (350-950 nm) were calculated, and correlations between coverage/soil/tidal level and spectral reflectance were analyzed. Results showed that vegetation coverage and the underlying surface greatly affected spectral reflectance of the saltmarsh. For the same soil conditions, spectral reflectance of different saltmarsh vegetation coverages in the visible light band was negatively correlated (r≈-1.00) with coverage and in the near infrared positively correlated (r≈0.99). For both low coverages and high coverages, spectral reflectance of different soil types in the visible light band was positively correlated (r≈0.97) but in the near infrared was less correlated. The saltmarsh and tidal level were significantly correlated (r≈0.97). Also spectral reflectance of the saltmarsh at high tide was higher than at low tide, and the reflectance increased as the tide rose. When the tide level rose to a certain height, spectral reflectance of the saltmarsh was lower than without a tide, and this generally decreased as the tide level fell. The effect of vetegation coverage, underlying(soil types and soil moisture) should not be ignored when monitoring saltmarsh vegetation.