## Materials and Methods
### Spectral Data  
The dual-field-of-view optical system which used a pair of Ocean Optics USB2000+ spectrometers operated in dual field of view mode, sampling at a 30 min time step using custom written automated software, and fully described in Reference [55] was installed on the SMEAR II eddy covariance (EC) flux tower during in early April 2011. It operated continuously between April 23 and September 30 (days of year 113 to 273), acquiring simultaneous incoming solar irradiance and canopy-reflected radiance every 15 min.  
The canopy-pointing optical fiber, with a FOV of 24.8\({}^{\circ}\) (yielding an off nadir instantaneous FOV of approximately 400 m\({}^{2}\) on the canopy) was attached to the tower at 7 m above the tree tops, with azimuth and viewing angles of 280\({}^{\circ}\) and 70\({}^{\circ}\) (relative to nadir), respectively. This configuration allowed us to measure about 400 m\({}^{2}\) of mostly sunlit pine crowns when the sun was at 35\({}^{\circ}\) degrees or more above the horizon (see Figure 5 in Reference [55]) minimizing the impact of ground cover dynamics on APAR (e.g., snow vs green ground vegetation) and facilitating the investigation of the impact of canopy LUE dynamics on SiF. The system cosine-corrected optical fiber (CC-3, Ocean Optics Inc., Dunedin, FL, USA) was attached to the tip of a vertical pole at the top of the tower, sitting at about 1 m above the canopy-pointing fiber. This resulted in an almost completely unobstructed view of the sky; only a small part of the nearby SMEAR II tall tower was included in the fiber IFOV.  
Intensity counts recorded by the spectrometers were converted to radiation fluxes, namely downwelling irradiance and upwelling radiance, using the procedure described in Reference [55]. Calibration coefficients used to calculate fluxes were obtained from an on-site calibration of the optical system prior to the start of the study period. Calculated radiation fluxes were also used to derive a spectral reflectance spectrum for each periodic acquisition by the optical system.  
Diurnal variations in cloud cover contribute to making physiological interpretation of SiF and spectral reflectance data more difficult. Apart from the obvious direct effects of clouds decreasing the overall PPFD and subsequently SiF signal, clouds also increase the ratio of diffuse-to-direct radiation, which will change the radiation field within the canopy and the relative contribution of different canopy components to the measured SiF signal. To minimize potential cloud effects on the spectral data, we visually inspected time series of 30 min photosynthetically active photon flux density (PPFD), which was measured for the whole sampling period by a quantum sensor (Li-19 QSZ, LI-COR Inc., Lincoln, NE, USA) on the nearby SMEAR II radiation tower. We split the dataset into clear and cloudy observations through a visual inspection of the shape of PPFD diurnal patterns: We considered as clear spectral data points only those ones that occurred on a Gaussian response in PPFD over the diurnal course [56]. Finally, to minimize the impact of structural effects on spectral data, we calculated solar zenith angle (SZA) values for each spectral acquisition using the site geographic coordinates and date-time of measurement; these were used as filtering conditions in regression analyses.