Measured and modelled albedos in Finnish boreal forest stands of different species, structure and understory

Abstract

Total shortwave and photosynthetically active radiation (PAR) albedos of eight boreal forest stands of different species (pine, spruce and birch) and structure were measured, and the results were compared to albedos simulated with an existing forest albedo model. The measured shortwave and PAR albedos were rather insensitive to leaf area index (LAI), the largest differences in albedo occurring between tree species. A thinning operation in one of the pine stands caused a short term increase in the PAR albedo but did not affect the total shortwave albedo, probably due to a low understory albedo in the near-infrared (NIR). In the coniferous stands, the white-sky albedos simulated using the PARAS albedo model ranged from 0.071 to 0.101 and were on average about 10% lower than the measured daily coniferous forest albedos in overcast conditions (0.075–0.120). However, the modelled PAR albedos (0.009–0.025) were clearly lower (by 49%) than the measured ones (0.026–0.037) in the coniferous stands. In the single birch stand, on the other hand, the modelled and measured PAR albedos were similar, but the modelled shortwave albedo was notably higher (0.237) than the measured one (0.167) in overcast conditions. The modelled albedos showed a much weaker trend along the diurnal course of solar zenith angle (SZA) than did the measured ones. A sensitivity analysis was conducted in order to find out possible reasons for the differences between the modelled and measured albedos.