## 2 Materials and Methods
### Measurements of NDVI and Fluorescence  
Optical parameters are estimated from spectral measurements using autonomous spectrometers that are mounted on top of the towers above the forest canopies. Measurements at each station employ a pair of JAZ spectrometers (Ocean Optics, Dunedin, FL, USA) that operate simultaneously to collect irradiance and reflected radiance data [29]. Operational setup, calibration procedure, and setup for measurements at the stations are described in [PERSON] et al. [28]. Instruments have a resolution (FWHM) of 1 nm, and they scan at steps of approximately 0.3 nm in the visible and near-infrared area. Spectrometers are housed in two insulated boxes (Figure 1) that ensure thermal stability during the measurements. Key features of pre-season preparations of the measurements are calibration of the up-welling and down-welling radiation flux using a Spectralon panel and radiometric calibration of each measuring unit using a HL-2000 calibration lamp (Ocean Optics, USA). Following these procedures, we are able to capture spectra accurately with radiometric units. These steps were the key prerequisites for estimations of fluorescence from measured data. Measurements of the spectra in the field started at 07:00 and ended at 19:00 every day at 5 min intervals. Protocols, calibrations, and measurements were identical for both sites. The fiber optic components of the spectrometers were mounted on the tops of the meteorological towers at a distance approximately 4 m to the south side to avoid artificial shading of the measured area (Figure 1). Field of view of the measurements is 21\({}^{\circ}\) for field-stop sensor, and optical fiber measuring solar irradiance spectrum is equipped with a CC-3 cosine corrector (Ocean Optics). Reflectance at Bily Kriz (spruce forest) was measured at a height of 20 m above the canopy, which corresponded to a sampling area of approximately 80 m\({}^{2}\) and covered a broad area around seven spruce trees. At the Stitna site, the setup resulted in data acquisition from one tree crown. The optical parameters were calculated as half-hourly averages. NDVI is estimated from reflectance at 720 and 780 nm, following work by [PERSON] et al. [30] as:  
\[\mathrm{NDVI}=\frac{\mathrm{R780-R720}}{\mathrm{R780+R720}} \tag{1}\]  
Estimations of SIF in the O\({}_{2}\)A and O\({}_{2}\)B atmospheric absorption bands were based on measuring the response in a single waveband with the maximum absorption by oxygen (Figure 2). The 3-FLD approach was applied to the spectra to estimate fluorescence. The equation was applied to two telluric oxygen absorption bands, O\({}_{2}\)B (687 nm) and O\({}_{2}\)A (760.4 nm). Estimation of fluorescence from radiance spectra used the following calculations:  
\[\mathrm{SIF}=\frac{\mathrm{L}(\lambda\mathrm{in})-\frac{\mathrm{E}(\lambda \mathrm{in})-\mathrm{\{wleft}+\mathrm{L}(\lambda\mathrm{left})+\mathrm{wright} -\mathrm{L}(\lambda\mathrm{right})\}}{\mathrm{wleft}+\mathrm{E}(\lambda \mathrm{left})+\mathrm{wright}-\mathrm{E}(\lambda\mathrm{right})}}{1-\frac{ \mathrm{E}(\lambda\mathrm{in})}{\mathrm{wleft}+\mathrm{E}(\lambda\mathrm{left })+\mathrm{wright}-\mathrm{E}(\lambda\mathrm{right})}} \tag{2}\]  
where  
\[\mathrm{wleft}=\frac{\lambda\mathrm{in}-\lambda\mathrm{left}}{\lambda \mathrm{right}-\lambda\mathrm{left}} \tag{3}\]  
and  
\[\mathrm{wright}=\frac{\lambda\mathrm{right}-\lambda\mathrm{in}}{\lambda \mathrm{right}-\lambda\mathrm{left}} \tag{4}\]  
L(\(\lambda_{\mathrm{left}}\)) and L(\(\lambda_{\mathrm{right}}\)) in these equations represent the radiances in the left and right shoulders of the oxygen absorption band (Figure 2), respectively, and E(\(\lambda_{\mathrm{left}}\)) and E(\(\lambda_{\mathrm{right}}\)) similarly represent the solar irradiances. \(\mathrm{wleft}\) and \(\mathrm{wright}\) are the weights of the two bands outside the oxygen absorption band. Wavelengths of 756.2 nm (\(\lambda_{\mathrm{left}}\)), 760.4 nm (\(\lambda_{\mathrm{in}}\)), and 775.3 nm (\(\lambda_{\mathrm{right}}\)) were selected to estimate SIF in the O\({}_{2}\)A band.