The fractional absorption of photosynthetically active radiation (fPAR) is frequently a key variable in models describing terrestrial ecosystem–atmosphere interactions, carbon uptake, growth and biogeochemistry. We present a novel approach to the estimation of the fraction of incident photosynthetically active radiation absorbed by the photosynthetic components of a plant canopy (fChl). The method uses micrometeorological measurements of CO2 flux and incident radiation to estimate light response parameters from which canopy structure is deduced. Data from two Ameriflux sites in Oklahoma, a tallgrass prairie site and a wheat site, are used to derive 7-day moving average estimates of fChl during three years (1997–1999). The inverse estimates are compared to long-term field measurements of PAR absorption. Good correlations are obtained when the field-measured fPAR is scaled by an estimate of the green fraction of total leaf area, although the inverse technique tends to be lower in value than the field measurements.
The inverse estimates of fChl using CO2 flux measurements are different from measurements of fPAR that might be made by other, more direct, techniques. However, because the inverse estimates are based on observed canopy CO2 uptake, they might be considered more biologically relevant than direct measurements that are affected by non-physiologically active components of the canopy. With the increasing number of eddy covariance sites around the world the technique provides the opportunity to examine seasonal and inter-annual variation in canopy structure and light harvesting capacity at individual sites. Furthermore, the inverse fChl provide a new source of data for development and testing of fPAR retrieval using remote sensing. New remote sensing algorithms, or adjustments to existing algorithms, might thus become better conditioned to ‘biologically significant’ light absorption than currently possible.