Modeling directional forest reflectance with the hybrid type forest reflectance model FRT

摘要

The directional multispectral hybrid type forest radiative transfer model FRT by Kuusk and Nilson (2000) was compared using directional reflectance factor of high angular resolution measured over threemature hemiboreal forest stands in Estonia. The model systematically underestimated backward scattering even in the case of the best available input data, especially in coniferous stands. The measured BRF distribution could be reproduced if the spherical leaf angle distributionwas replaced by erectophile in order to bettermodel the angular dependence of gap fraction, and the Ross-Nilson area scattering phase functionwas replaced by significantly more asymmetric Henyey-Greenstein phase function. The limitedamount of available data does not allowdeciding howuniversal are the applied asymmetry parameter values. The asymmetry of the phase function may depend on the species composition, stand age, and site type.