Optical properties of leaves and needles for boreal tree species in Europe

Reliable information on the optical properties of leaves and needles is needed for parameterization of radiative transfer models and interpretation of remotely sensed data. The optical properties also convey information about the structure and biochemical constituents of the leaf or needle tissues, and can be linked to the photosynthetic processes of plants. Currently, very little is known about the optical properties of tree species in the European boreal zone. To bridge this gap, we measured directional-hemispherical reflectance and transmittance factors of the three most common tree species in this zone: Scots pine, Norway spruce and Silver birch. The measurements covered a wide spectral range from 350 to 2500 nm with a high spectral resolution of 3-10 nm. To explore the driving factors of the observed leaf-level optical properties, supplementary measurements of structural and biochemical traits of leaves and needles were made. The results showed that the transmittance of pine and spruce needles is clearly lower than reflectance, whereas for birch reflectance and transmittance were similar. In conifers, exposed needles had higher albedo than the shaded needles. Also, the spectra of needles were more dependent on canopy position than the spectra of birch leaves. The relationships between narrowband reflectances and chlorophyll and nitrogen percentage concentrations and specific leaf area were similar for all species, but the strongest correlations were observed for birch.