An algorithm to derive the fraction of photosynthetically active radiation absorbed by photosynthetic elements of the canopy (FAPAR(ps)) from eddy covariance flux tower data

Abstract

The fraction of absorbed photosynthetically active radiation (FAPAR) is a key vegetation biophysical variable in most production efficiency models (PEMs). Operational FAPAR products derived from satellite data do not distinguish between the fraction of photosynthetically active radiation (PAR) absorbed by nonphotosynthetic and photosynthetic components of vegetation canopy, which would result in errors in representation of the exact absorbed PAR utilized in photosynthesis. The possibility of deriving only the fraction of PAR absorbed by photosynthetic elements of the canopy (i.e. FAPAR(ps) ) was investigated. The approach adopted involved inversion of net ecosystem exchange data from eddy covariance measurements to calculate FAPAR(ps) . The derived FAPAR(ps) was then related to three vegetation indices (i.e. Normalized Difference Vegetation Index (NDVI), Medium Resolution Imaging Spectrometer (MERIS) Terrestrial Chlorophyll Index (MTCI) and Enhanced Vegetation Index (EVI)) in an attempt to determine their potential as surrogates for FAPAR(ps) . Finally, the FAPAR(ps) was evaluated against two operational satellite data-derived FAPAR products (i.e. MODIS and CYCLOPES products). The maximum FAPAR(ps) from the inversion approach ranged between 0.6 and 0.8. The inversion approach also predicted site-specific Q₁₀-modelled daytime respiration successfully (R² > 0.8). The vegetation indices were positively correlated (R² = 0.67-0.88) to the FAPAR(ps). Finally, the two operational FAPAR products overestimated the FAPAR(ps). This was attributed to the two products deriving FAPAR for the whole canopy rather than for only photosynthetic elements in the canopy.