Monitoring Forest Health with Sun-Induced Chlorophyll Fluorescence Observations and 3-D Radiative Transfer Modeling

This study present in situ measurements and model simulations aimed to understand the ability of sun-induced fluorescence (SIF) and other physiological and structural hyperspectral indices as an early indicator of forest decline. Experiments were conducted over an oak forest (Quercus ilex) affected by water stress and Phytophthora infection in the southwest of Spain. The robustness of the SIF quantification through the Fraunhofer Line Depth (FLD) principle with three spectral bands F (FLD3) was assessed using high-resolution hyperspectral imagery. FluorFLIGHT, a modified version of the 3-D radiative transfer model FLIGHT was developed to enable the simulation of canopy radiance and reflectance including fluorescence effects accounting for forest structure. Fluorescence retrievals performed better than structural and physiological indices. Albeit other pigment-related vegetation indices such as CRI550515 and RNIRCRI700 were also strongly related to physiological variables. The 3D modelling approach significantly improved the relationship between Fs and SIF and enabled the quantification of SIF as a function of fractional cover, leaf area index and chlorophyll content, yielding significant relationships between Fs ground-data measurements and fluorescence quantum yield estimated with FluorFLIGHT. The methodology also demonstrated its capabilities for mapping SIF at the crown level to detect early damage assessment in oaks undergoing forest decline.