NDVI and Fluorescence Indicators of Seasonal and Structural Changes in a Tropical Forest Succession
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ORIGINAL ARTICLE
NDVI and Fluorescence Indicators of Seasonal and Structural Changes in a Tropical Forest Succession Syed M. Irteza1 · Janet E. Nichol1,2 · Wenzhong Shi1,3 · Sawaid Abbas1 Received: 6 August 2020 / Accepted: 28 August 2020 © King Abdulaziz University and Springer Nature Switzerland AG 2020
Abstract In this research, the Sun-induced Chlorophyll Fluorescence (SIF) signal was derived from field-based spectrometers and Hyperion satellite imagery of Hong Kong’s Country Parks by applying the Fraunhofer Line Difference (FLD) method. Due to high levels of atmospheric aerosols and water vapour in the study area, a modified FLD method (FLD-M) was tested for its ability to deliver improved SIF estimates by cancelling out the effects of atmospheric scattering. The SIF results were compared with the Normalised Difference Vegetation Index (NDVI), and analysed according to seasonality, five successional age groups of forest and monoculture exotic plantations. Results indicate that SIF methods are more sensitive to seasonal phenology and diurnal fluctuations than the NDVI, indicating its greater sensitivity to photosynthetic activity. The SIF responds earlier and stronger, to senescence in winter and green-up in summer. Field spectrometer data showed NDVI to be unresponsive to time of day, whereas fluorescence responds to changes in sunlight intensity from 10:15 a.m. to 02:00 p.m. Both seasonal and diurnal results indicate that SIF is better than NDVI in representing the subtle changes in vegetation conditions. Although both NDVI and SIF distinguish between the four woody structural stages of vegetation, only SIF can distinguish between forest and exotic plantations, the difference being greater in the dry season. The study provides an improved operational remote sensing methodology for investigating the health status of tropical secondary forest, where atmospheric turbidity is high. Keywords FLD · Hong Kong · Hyperspectral · NDVI · SIF · Tropical forest
1 Introduction Sun-induced Chlorophyll Fluorescence (SIF) is an indirect measure of photochemical processes and carbon sequestration. However, it remains unclear how SlF and photosynthesis are linked diurnally and across the growing season, as well as how plant productivity is related to the SIF signal. * Janet E. Nichol [email protected] * Sawaid Abbas [email protected] 1
Department of Land Surveying and Geo‑Informatics, The Hong Kong Polytechnic University, Hung Hom, Hong Kong, China
2
Department of Geography, School of Global Studies, University of Sussex, Sussex, UK
3
Department of Land Surveying and Geo‑Informatics, Smart Cities Research Institute, The Hong Kong Polytechnic University, Hung Hom, Hong Kong, China
Retrieval of the fluorescence signal by remote sensors can potentially provide a unique understanding of the photosynthesis cycle of vegetation, since SIF is a direct indicator of canopy photosynthesis (Mohammed et al. 2019). As different species have different fluorescence emissions (Mohammed et al. 2019), this can be useful
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