Agricultural and Forest Meteorology 232 (2017) 422–432 Contents lists available at ScienceDirect Agricultural and Forest Meteorology journal homepage: www.elsevier.com/locate/agrformet Comparison of ground and satellite-based methods for estimating stand-level fPAR in a boreal forest Titta Majasalmi a,∗ , Pauline Stenberg a , Miina Rautiainen b,c a University of Helsinki, Department of Forest Sciences, PO Box 27, FI-00014 Helsinki, Finland b Aalto University, School of Engineering, Department of Real Estate, Planning and Geoinformatics, PO Box 15800, 00076 AALTO, Finland c Aalto University, School of Electrical Engineering, Department of Radio Science and Engineering, PO Box 13000, 00076 AALTO, Finland a r t i c l e i n f o Article history: Received 11 December 2015 Received in revised form 13 September 2016 Accepted 14 September 2016 Keywords: fAPAR LAI Landsat 8 Vegetation index Allometric model Photon recollision probability a b s t r a c t 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 R 2 = 0.34) with the ground-based fPAR. Our study showed that the absolute differ- ence 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. © 2016 Published by Elsevier B.V. 1. Introduction The fraction of absorbed Photosynthetically Active Radiation (fPAR, in the wavelength region of 400–700 nm) quantifies the amount of solar energy that is available for photosynthesis by green vegetation. The diurnal and seasonal courses of fPAR are dynamic and influenced by both vegetation structure and opti- cal properties (e.g. Mõttus et al., 2012). Measuring fPAR in field conditions requires access to a tower to place the upward-looking above-canopy sensor, because fPAR is calculated based on radiation transmitted by the canopy. Thus, the spatial area that can be sam- pled using optical measuring systems is restricted by the limited ∗ Corresponding author. Current address: Norwegian Institute of Bioeconomy Research, PO Box 115, NO-1431 Ås, Norway. E-mail addresses: titta.majasalmi@helsinki.fi, titta.majasalmi@gmail.com (T. Majasalmi), pauline.stenberg@helsinki.fi (P. Stenberg), miina.a.rautiainen@aalto.fi (M. Rautiainen). number of towers. In addition, to obtain representative averages of daily fPAR a large number of sensors is needed, and thus modeling of fPAR often preferred (e.g. Mõttus et al., 2012). fPAR is often modeled based on the Leaf Area Index (LAI), which can be obtained either using optical canopy transmittance mea- surements or allometric biomass models. Canopy transmittance may be measured using different field instruments (e.g. LAI-2000, hemispherical photography) and converted to LAI based on an inversion of the Beer’s law equation (e.g. see review by Bréda (2003)). Allometric biomass models, on the other hand, are based on the pipe model theory, which describes relationships between different tree compartments (e.g. ratio of leaf biomass that can be maintained by the sapwood area of a tree, or ratio between tree diameter-at-breast-height and tree height). Model inputs for allo- metric biomass models are typically taken from forestry databases which contain stand level data (e.g. species, breast-height-diameter and height of the median tree), tree density data (i.e. basal area or http://dx.doi.org/10.1016/j.agrformet.2016.09.007 0168-1923/© 2016 Published by Elsevier B.V.