Remote sensing of canopy light use efficiency using the photochemical reflectance index - Model and sensitivity analysis

A growing number of studies have shown that reflectance changes at 531 nm, associated with the xanthophyll cycle and the related thylakoid energisatio n are widespread among plant species. The photochemical reflectance index ( PRI), derived from narrow band reflectance at 531 and 570 nm has been relat ed with some success to photosynthetic light use efficiency (LUE). Such a r elationship would enable the estimation of stand photosynthesis from remote ly sensed data. However, canopy PRI is an integral of the component leaf re sponse weighted by the strength of the signal from each leaf to the sensor. This analysis investigates the extent to which canopy structure, view, and illumination angles are likely to influence the measured canopy PRI. A one -dimensional ray tracing radiative transfer model was used to estimate ligh t distribution within a canopy and the dynamic response of individual folia r elements, based oil a published relationship between PRI and LUE and a si mple photosynthetic light response function. The model estimated the LUE of the canopy, based oil both incident and absorbed light, and reflectance of the canopy at the desired wavelengths and hence the canopy PR1. A range or solar zenith, leaf area index (LAI), leaf angle distributions (LAD), and s oil types were used to determine the likely influence on measured canopy PR I. The results show a positive correlation between PRI and LUE variation at canopy scale. However, the index shows a greater variation of view angle t han most vegetation indices. The index is strongly influenced by varying so il background for LAI < 3. At large viewing angles (> 30 degrees) the index is also sensitive to LAD. Correction for Rayleigh scattering is necessary to relate the index to ground measured PRI. Results Show that the PRI value is most sensitive to changes in LAI. Utilisation of the relationship to pr edict or improve estimates of canopy LUE based oil either absorbed or incid ent light will require an independent estimate of LAI change between dates/ locations of in situ measurements and of remote sensing observations. (C) 2 001 Elsevier Science Inc. All rights reserved.