Effect of shade on simultaneous estimation of non-photosynthetic and photosynthetic vegetation cover using the NDVI-NSSI normalized difference triangular space

ABSTRACT

The non-photosynthetic vegetation – soil separation index (NSSI) and normalized difference vegetation index (NDVI) can be used to simultaneously estimate the fractional cover of non-photosynthetic vegetation (fNPV), photosynthetic vegetation (fPV), and bare soil (fBS) in vegetation ecosystems. However, these estimates suffer from problems due to shading by dense vegetation or topography. Based on field-measured data and on hyperspectral data acquired by unmanned aerial vehicles, we analyse how shading affects the morphology of NDVI-NSSI and enhanced vegetation index – NSSI (EVI-NSSI). We also investigate the simultaneous estimation of fNPV, fPV, and fBS. The results show that the NDVI-NSSI normalized difference feature space mitigates the impact of shade. Shade causes the NDVI-NSSI and EVI-NSSI to shift rightward parallel to the BS axis and leftward parallel to the PV axis, respectively. Although such shifts may hinder the determination of NPV, PV, and BS endmembers, they do not affect the spectral separation of NPV. Overall, with the NDVI-NSSI and EVI-NSSI methods, the estimation error for fNPV under shaded conditions is approximately 5% greater than that under illuminated conditions. Shaded green vegetation more strongly affects the estimation error of fPV and fBS in EVI-NSSI than in NDVI-NSSI, whereas fBS and fPV depend more strongly on shade than fNPV.

KEYWORDS:

• Shade
• non-photosynthetic vegetation
• separation
• coverage
• triangular space method

Disclosure statement

No potential conflict of interest was reported by the author(s).

CRediT authorship contribution statement

Cuicui Zhu and Jia Tian designed the experiment, and performed the overall data analysis and drafted the manuscript. Jia Tian and Qingjiu Tian proposed the theory of this study and provided comments and suggestions for the manuscript and edited the manuscript. Shanhua Wang checked the manuscript and provided provide suggestions for the manuscript writing. Xiaoqiong Wang and Qianjing Li provided comments and suggestions for the manuscript. All authors read and approved the final manuscript.

Additional information

Funding

This work was supported by the National Key R&D Program of china [2023YFF1303903], the Open Fund of Key Laboratory of Urban Land Resources Monitoring and Simulation, Ministry of Natural Resources [KF-2022-07-018], the China Postdoctoral Science Foundation Project [2021M701653], the National Natural Science Foundation of China [42101321], and Open Fund of State Key Laboratory of Urban and Regional Ecology [Grant No. SKLURE2023-2-6]. We sincerely thank AZUP International for aid with UAV-Borne Hyperspectral data collection.