Relationship between forest density and albedo in the boreal zone

Abstract

The relationship between albedo and forest areas is complex. Little is known about the driving factors of albedo in the boreal zone. Using a radiative transfer model and an extensive forest inventory database, we simulated albedo of forest stands composed of the most abundant tree species of Fennoscandia – Scots pine, Norway spruce and Silver birch. The physically-based radiative transfer model allowed us to uncouple the driving factors of the forest albedo. We analyzed separately how biomass, canopy cover, and species composition influence the shortwave albedo of a boreal forest. The albedos differed significantly between species and increased with solar zenith angle. The lowest values were observed for spruce stands, followed by pine stands and the highest values were observed for birch stands. Diurnal courses of albedo were tightly related to forest density as quantified by biomass or canopy cover. The albedos generally decreased with increasing stand biomass and canopy cover whereas the sensitivity to solar angle increased as the stands became denser. The sharpest decrease in albedo was observed at low biomass values, after which the albedo remained relative stable. The strength of the relationships was weaker for larger solar zenith angles.