Tracking mangrove light use efficiency using normalized difference red edge index

Abstract

Strong temporal and spatial heterogeneity of mangrove carbon fluxes makes it difficult to accurately assess mangrove carbon budgets at both site and regional scales. The light use efficiency (LUE) model provides a promising remote sensing approach, however, the lack of robust spectral metrics for tracking mangrove LUE hinders the integration of carbon flux and remote sensing measurements. To close this gap, here we examined the potential of six relevant spectral metrics, including red edge position, red edge reflectance, red valley reflectance, green peak reflectance, normalized difference vegetation index, and normalized difference red edge index (NDRE), for tracking canopy LUE based on two-year (2021–2022) simultaneous measurements of tower-based hyperspectral and eddy covariance (EC) data in a subtropical mangrove of southeastern China. The results indicated that mangrove LUE had strong daily and seasonal variations with the value down-regulated by increasing vapor pressure deficit (VPD) and air temperature. The canopy spectral reflectance curve changed seasonally showing elevated reflectance with increasing VPD over the entire visible/red-edge bands. Among the spectral metrics, NDRE was found to be the only statistically significant correlated to LUE at both daily and monthly scales, showing a positive and linear NDRE-LUE linkage. To the best of our knowledge, this is the first study to explore the links between mangrove LUE and red edge-related spectral metrics across temporal scales using simultaneous hyperspectral and EC measurements. The NDRE-LUE linkage confirmed here provides a basis for establishing robust remote sensing approaches to map mangrove LUE and carbon fluxes with ready-available satellite data.

Keywords

• Field spectrometer
• Eddy covariance
• Sentinel-2
• VPD
• Zhangjiang estuary