Leaf and needle spectra were measured for three samples per tree. A sample refers to a leaf or a set of up to approx. 20 needles (depending on needle dimensions). Leaves and needles were randomly picked and detached from the tree. Only healthy green leaves and needles were measured. For conifers, all needle age classes were mixed randomly. Measurements of spectral directional-hemispherical reflectance (DHRF) and transmittance (DHTF) factors were performed with ASD FieldSpec 3 non-imaging spectrometer (serial number 16007) attached to an ASD RTS-3ZC integrating sphere with a halogen light source. Special sample holders (‘needle carriers’, see Fig. 1 in [5]) were used to attach the needle samples to the integrating sphere. The needle carriers were used also when measuring leaves, to ensure a comparable measurement. The needles were placed in the carrier side-by-side, so that between-needle distance equalled 0.5–1 × needle width. Thus, the number of measured needles depended on needle dimensions. Spruce needles were placed in two rows because they were shorter than the diameter of the sample port in the integrating sphere. The measurement protocol followed the protocol described in [5], except that there the needle carriers were 0.3 mm thick and here 0.8 mm. The protocol for both reflectance and transmittance included measurement of white reference, and measurements of both sides of the sample. The sides were adaxial (‘upper’) and abaxial (‘lower’). Spruce needles were symmetric and no adaxial and abaxial sides could be distinguished. However, both sides of the sample were measured also for spruce needles to ensure equal number of observations for each species. The measurement of reflectance included also a measurement of stray light. All spectral measurements were performed in digital numbers and converted to DHRF and DHTF in the data processing. Integration time in the measurements was 1.09 s for each individual spectrum, and 10 individual spectra were averaged into one measurement. To determinate the gap fractions in the needle samples, we scanned the carriers with needles in them, using Epson Perfection V550 digital film scanner (8-bit grayscale images in 800 dpi resolution).

For one sample tree per species, bark reflectance spectra were measured utilizing the same protocol as described above. Three to five bark samples per tree were peeled off from the tree stem from different heights, and placed in the needle carrier for the measurements. The bark outer surface was then measured for DHRF. Note that one of the bark sample trees (an oak) was taken outside of the set of 18 trees measured in the goniometer, and thus the total number of trees in our dataset is 19 (Table 1). No other measurements were conducted for this one oak tree.

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