# Measurement of Diurnal Variation in Needle PRI and Shoot Photosynthesis in a Boreal Forest
## 3 Results  
The needle PRI values showed considerable variation during the measurement campaigns (Figure 2). The measurement campaigns were characterized by mostly broken skies typical of the region with measurements concentrated on the days with smaller variations incident PPFD. PRI varied between 0.010 and 0.045 in 2014, and between \(-0.011\) and 0.025 in 2015. Needle PRI values in the pooled PRI data showed diurnal variations (Figure 3): the sunlit needle PRI values before noon are statistically significantly (\(p<0.05\)) different from the afternoon minima in both campaigns. The 90% confidence interval for the mean needle PRI was 0.0060 in 2014, and 0.0078 in the 2015 campaign. The confidence interval was considerably smaller than the measured PRI ranges of 0.026 and 0.038 in 2014 and 2015, respectively. Before noon, the PRI of sunlit and shaded needles were not statistically significantly different. In the afternoon, PRI values of shaded needles exhibited different trends in the two campaigns. In 2014, shaded needle PRI decreased simultaneously with sunlit needle PRI within the 90% confidence margin (Figure 3a); in 2015, shaded needle PRI remained constant, or increased slightly, while the values for sunlit needles decreased to recover in the evening (Figure 3b). The daily PRI curve for sunlit needles loosely followed the photosynthetic assimilation \(P\) and the light use efficiency \(\varepsilon\) of sunlit shoots in 2014 (Figure 3a). Figure 3: Diurnal variation in shaded (empty symbols) and sunlit (filled symbols) needle PRI (pooled data, left axes); and the photosynthetic assimilation \(P\) (dashed line) and the light use efficiency \(\varepsilon\) of sunlit shoots (dotted line, right axes). \(P\) and \(\varepsilon\) are normalized by their maximum daily values; (**a**) 2014; and (**b**) 2015. Error bars indicate 90% confidence limit for mean needle PRI. The canopy LUE \(\varepsilon_{can}\) daily curve, as measured by SMEAR II (Figure 4), differed from the daily PRI curve and the sunlit shoot \(\varepsilon\). Canopy LUE followed the PPFD time course, lacked a morning plateau, and was almost symmetrical around noon (Figure 4). The canopy PRI, PRI\({}_{can}\) (Figure 4b, available only in 2015), on the other hand, had a minimum of approximately 0.005 at around 3 p.m., concurrently with the minimum in sunlit needle PRI. However, the morning increase of PRI\({}_{can}\) (to the value of 0.070) took place much earlier than that of sunlit needles, before 7 a.m.  
We found no systematic correlations between sunlit needle PRI and sunlit shoot photosynthesis on the daily scale (data not shown). However, when including only mid-day measurements using the threshold PPFD \(>\) 1000 umol m\({}^{-2}\) s\({}^{-1}\), we found significant correlations between sunlit needle PRI and the shoot LUE, \(\varepsilon\) (\(p\) = 0.006 and \(p\) = 0.047 for 2014 and 2015, respectively, Figure 5a); in 2014, when the data were obtained within 24 h, \(R^{2}\) = 0.83. Using the same PPFD threshold, we found stronger correlations between sunlit needle PRI and the shoot photosynthetic assimilation \(P\) (\(p\) = 0.008 and \(p\) = 0.001 for 2014 and 2015, respectively, Figure 5b). We found no statistically significant correlation between needle PRI and shoot PPFD, Figure 5c): the regressions had different slopes and were statistically insignificant (\(p\) > 0.25). At the canopy level (i.e., SMEAR II data), sunlit needle PRI and photosynthesis data were not correlated (data not shown) regardless of light level. However, at high light levels (PPFD \(>\) 1000 umol m\({}^{-2}\) s\({}^{-1}\)), canopy and sunlit needle PRI were correlated (Figure 6a), although the correlation was not significant (\(p\) = 0.2). We also found systematic correlations (the same direction for all campaigns) between needle PRI and ambient air relative humidity (Figure 6b), although the correlations were not always significant (\(p\) = 0.02 and \(p\) = 0.12 for 2014 and 2015, respectively).