Mapping properties of vegetation in a tidal salt marsh from multi-spectral satellite imagery using the SCOPE model

ABSTRACT

Quantitative remote sensing of vegetation biophysical and biochemical parameters through the inversion of physical models is of great significance for monitoring vegetative growth, ecosystem function, and the global carbon and nitrogen cycles. However, mapping and comparing vegetation parameters in a tidal saltmarsh based on multi-spectral satellite imagery has been rarely documented. In this study, a coupled model (Soil-Canopy-Observation of Photochemistry and Energy fluxes, SCOPE) was used to quantitatively invert the spatial patterns of key parameters such as the leaf area index (LAI), leaf chlorophyll content (C_ab), and fraction of absorbed photosynthetically active radiation (fPAR) of salt marsh vegetation from Land Remote-Sensing Satellite 8 (Landsat-8), Sentinel-2A, and RapidEye multispectral satellite imagery. The results show that RapidEye outperformed Landsat-8 and Sentinel-2A in the retrieval of LAI and C_ab, with higher coefficients of determination (R² = 0.539 and 0.488, respectively) and lower normalized root mean square errors (NRMSE = 0.32 and 0.28, respectively). Without the red-edge band, the R² between the modelled and observed LAI and C_ab for RapidEye decreased by 27.83% and 5.33%, respectively, while the NRMSE increased by 3.13% and 46.43%, respectively, highlighting the important role of the red-edge band in vegetation parameter retrieval. Tidal flooding resulted in poor retrieval results from Sentinel-2A. However, the accuracy of C_ab inversion was significantly improved by removing the sampling sites near tidal trenches or in low-lying areas, indicating that tidal inundation exerted a notable effect on the retrieval of saltmarsh vegetation parameters when using multi-spectral images. In addition, the spatial pattern of the simulated fPAR from RapidEye is consistent with that of the gross primary productivity. This study provides an alternative method for estimating ecosystem functions in a tidal saltmarsh wetland, as well as for large-scale and high-precision monitoring of wetland restoration that is beneficial for the ecological protection of coastal zones.

Acknowledgements

We acknowledge the grants from the National Key R&D Program of China (#2017YFC0506000), the National Natural Science Foundation of China (#41801253), and the project  “Coping with deltas in transition” within the Programme of Strategic Scientific Alliances between China and The Netherlands (PSA), financed by the Chinese Ministry of Science and Technology (MOST) (#2016YFE0133700) and Royal Netherlands Academy of Arts and Sciences (KNAW). We also acknowledge Dr. Christiaan van der Tol (Twente University) for providing us the SCOPE model code. We thank LetPub (www.letpub.com) for its linguistic assistance during the preparation of this manuscript.

Disclosure statement

No potential conflict of interest was reported by the authors.