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

Summary The fraction of absorbed photosynthetically active radiation ( FAPAR ) is a key vegetation biophysical variable in most production efficiency models ( PEM s). 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 ) T errestrial C hlorophyll I ndex ( MTCI ) and E nhanced V egetation I ndex ( 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 Q 10 ‐modelled daytime respiration successfully ( R 2  > 0.8). The vegetation indices were positively correlated ( R 2  = 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.