Tracking seasonal changes of leaf and canopy light use efficiency in a Phlomis fruticosa Mediterranean ecosystem using field measurements and multi-angular satellite hyperspectral imagery

摘要

Numerous normalized difference spectral indices (NDSIs) derived from leaf measurements and CHRIS/ PROBA hyperspectral and multi-angular satellite images were examined for their capacity to track seasonal variations of leaf (ε_(leaf)) and canopy (ε_(can)) light use efficiency of a Mediterranean phryganic ecosystem. A series of seasonal field ecophysiological measurements, i.e. leaf area index (LAI), leaf photosynthesis and leaf reflectance, were conducted on the Phlomis fruticosa shrubs at the days of CHRIS acquisitions over the study site. Leaf scale analysis confirmed background theory on the relationship of the photochemical reflectance index (PRI) with ε_(leaf) and provided a detailed view of the wavelengths that can be used in PRI formulation for the specific species. In canopy scale analysis, PRI and some alternative formulations of this index based on CHRIS bands, presented the most significant relationships with ε_(can), indicating that this index preserves its efficiency in satellite observations for the specific ecosystem. Additionally, spectral indices related to chlorophyll and water content were found to present good relationships with ε_(can). Taking into account the functional relationship between ε_(can) and chlorophyll content, a combination of the xanthophyll de-epoxidation band (531 nm) with 701 nm CHRIS band in a NDSI is suggested as an alternative to the original PRI formulation that could improve seasonal ε_(can) estimations. The satellite observation geometry effects on the determination of ε_(can) were not very intense for the studied ecosystem. However, the most effective viewing direction was proved to be the backward scattering, while zenith observations were the least efficient for the specific ecosystem, most probably due to increased background effects. Even though the sensitivity of the original PRI formulation to ε_(can) was reduced in forward scattering viewing directions, when 531 nm xanthophyll de-epoxidation band was replaced with higher wavelength bands (540-550 nm), a strong PRI-ε_(can) relationship reappeared. These findings indicate possible shift of xanthophyll de-epoxidation signal according to viewing direction.