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A Statistical Forest Reflectance Model

The variability of forest reflectance among hemiboreal forests can be described with a fewbasis functions. Five basis functions describe almost 98% of variability of directional reflectancespectra in the optical spectral domain (400–1700 nm) in forest stands at the top of a canopy in nadir.A statistical forest reflectance model (SFRM) was developed, the input parameters of which are theforest parameters measured in the course of regular forest inventory. Nadir spectral reflectance ofa forest stand is expressed in the SFRM as a linear combination of basis functions, the weights ofwhich are linear combinations of the 15 stand parameters in the forest inventory database. Multiplecorrelations of the weights on the forest inventory parameters are determined separately for pine,spruce, and broadleaf forests. The basis functions are found from low altitude airborne measurementsover managed forests in southeastern Estonia, where a forest management database is available. Themodel was validated against more than 3000 spectral signatures of forest stands from Sentinel-2Multispectral Imager (MSI) measurements over a test site in southeastern Estonia. In most cases, themodel predicts the forest reflectance spectrum at nadir with a relative error about 20–40%. The errorsof reflectance values are less than 0.02 in most cases. The sole exception is the reflectance of broadleafstands, which in near infrared bands of Sentinel-2 MSI is overestimated by 0.02–0.05.