Comparison of ground and satellite-based methods for estimating stand-level fPAR in a boreal forest

- Allometric estimates of LAI and Beer's law are not sufficient to estimate fPAR.
- Canopy gaps have an important role in determining canopy fPAR in a boreal forest.
- Satellite-based fPAR products underestimate daily mean fPAR in northern latitudes.

The fraction of absorbed Photosynthetically Active Radiation (fPAR, 400-700 nm) can be retrieved from satellite measurements and is used to estimate and monitor ecosystem productivity and vegetation phenology. Although fPAR depends primarily on the Leaf Area Index (LAI), it is also influenced by the incoming radiation field and thus varies diurnally and seasonally. The overall aim of this study was to compare different ground and satellite-based fPAR estimates. The field data consisted of nearly 1000 plots located in the southern boreal forest zone in Finland. For all plots, data on canopy transmittance and forest inventory variables were available. We compared two different approaches to estimate fPAR and studied which satellite-based vegetation indices are best correlated with the instantaneous ground reference fPAR. Finally, a canopy fPAR model was used to assess some underlying assumptions of the satellite-based fPAR products: the similarity between instantaneous (at the time of satellite overpass) and daily-integrated fPAR. Results showed that allometric estimates of LAI and Beer's law are not sufficient to estimate fPAR in a boreal forest, because canopy openness strongly influences the relationship between canopy LAI and fPAR. We found that vegetation indices calculated from Landsat were only moderately correlated (maximum R2 = 0.34) with the ground-based fPAR. Our study showed that the absolute difference between fPAR at the moment of the satellite overpass and daily integrated fPAR exceeds the 0.05 fPAR-units limit set by Global Climate Observing Systems −network in northern latitudes during the peak growing season. Thus, at high latitudes covered by boreal forests, the satellite-based fPAR systematically underestimates the daily integrated black sky fPAR.