The new product fAPARchl is better than fAPARcanopy to describe terrestrial ecosystem photosynthesis (GPP)

Existing global climate models have been unable to accurately describe the intensity of photosynthetic activity or to discriminate this functionality among terrestrial vegetation canopies/ecosystems. Many satellite-based production efficiency models (PEMs), land-atmosphere interaction models and biogeochemical models (e.g., SiB, CLM and CASA) have used the concept of the fraction of photosynthetically active radiation (PAR) absorbed for vegetation photosynthesis (fAPARPSN) in their modeling work. These models typically use fAPAR for the whole canopy (fAPARcanopy) (usually denoted as FPAR or fAPAR) to represent fAPARPSN. However, this widely used FPAR parameter has proved to be physiologically insufficient to describe or retrieve terrestrial ecosystem photosynthesis. A much better alternative is to utilize the fraction of PAR absorbed by chlorophyll throughout a canopy/ecosystem (i.e., fAPARchl) to replace FPAR in these calculations. In this study, we present examples of fAPARchl, leaf fAPARNPV (the non-photosynthetic canopy fraction, without chlorophyll) and fAPARcanopy at 30 m spatial resolution for deciduous forests, evergreen forests and crops, obtained from Earth Observing One (EO-1) Hyperion satellite imagery. The differences obtained between fAPARchl and fAPARcanopy are significant for all of these vegetation types across the whole growing season. For instance, for the evergreen forests, fAPARchl changes seasonally, whereas the seasonal trend for fAPARcanopy is flat. Consequently, these differences translate into significant differences in estimates of fAPARPSN. We suggest modeling scientists should compare simulation outputs using fAPARcanopy versus fAPARchl, to check whether the differences are significant.