# Measurement of Diurnal Variation in Needle PRI and Shoot Photosynthesis in a Boreal Forest
## 4 Discussion  
The strongest correlations between the sunlit shoot light use efficiency \(\varepsilon\), photosynthetic assimilation \(P\) and PRI (\(|R|>0.9\), Figure 5a,b) were found during the 2014 campaign when all measurements were made within 24 h. The weakening of the correlation for the multiday 2015  
Figure 5: The correlation between needle PRI and (**a**) the sunlit shoot light use efficiency \(\varepsilon\); (**b**) photosynthetic assimilation \(P\); and (**c**) PPFD. Measurements at high light levels (PPFD \(>\) 1000 \(\upmu\)mol m\({}^{-2}\) s\({}^{-1}\)); \(P\) and \(\varepsilon\) are normalized by their maximum value for each campaign. Figure 6: Correlations between sunlit needle PRI and (**a**) canopy PRI, and (**b**) relative humidity. All measurements made at high light levels (PPFD \(>\) 1000 \(\upmu\)mol m\({}^{-2}\) s\({}^{-1}\)). Canopy PRI data were only available in 2015. Figure 7: (**a**) Chlorophyll (Chl) and (**b**) total carotenoid (Car) concentrations; and (**c**) the pigment pool ratio [Car]/[Chl] for the two needle classes in 2014.  
campaign were possibly caused by the multi-day average curve for \(\varepsilon\) to cope with clouds and temporal matching of the measurements. The sunlit needle PRI was more strongly correlated with \(P\) than with \(\varepsilon\) (Figure 5b): lower PRI indicated a lower \(P\). Furthermore, during the short campaign of 2015, \(P\) varied up to 50% of its daily range during the high PPFD period and, the needle PRI could explain almost all of this variation (\(R^{2}=0.86\)). Together with the lack of correlation between mid-day sunlit needle PRI and PPFD (Figure 5b), this result indicates that daily variations in needle PRI can indeed be used to trace shoot-level photosynthetic activity. Although these correlations only held when PPFD \(>1000\) umol m\({}^{-2}\) s\({}^{-1}\), these high light conditions correspond approximately to between 8 a.m. and 5 p.m. local time and include most remote sensing opportunities. Sunlit needle PRI was only weakly (\(R=0.26\)) correlated with the PRI value measured above the canopy (Figure 6a) despite the apparently similar diurnal cycles of canopy and needle PRI in the afternoon (Figure 4c) and the coincidence of their daily minimum at approximately 3 p.m. Contrastingly, canopy PRI decreases continuously after 7 a.m., while needle PRI has a maximum around 11 a.m. Additionally, shaded needles also contribute to canopy PRI. These factors, however, cannot account for the differences in the ranges of canopy (\(\sim 0.07\)) and sunlit needle (\(\sim 0.02\)) PRI. The direct effects of canopy structure on PRI can be quantified using the harmonic means of needle albedos at the two wavelengths (531 and 570 nm) and the photon recollision probability inside the canopy [34]. Pine needle spectral albedos at 531 and 570 nm in Hyytiala at a similar point in the growing season were measured as 0.222 and 0.216, respectively [35]. Using the within-shoot photon recollision probability of 0.40 [36] and a between-shoot probability of 0.62 (obtained from canopy gap data measured at the site in 2014 using LAI-2000, LI-COR, Lincoln, NE, USA [37]), total within-canopy recollision probability should equal 0.77 [38]. Based on these values, canopy PRI should differ from that of a needle by a factor of 1.2--which was much less than what was measured. The remaining variation in canopy PRI can be attributed by the contribution of understory, the varying contributions of sunlit and shaded needles, and the influence of diffuse clear sky radiation on the overall canopy PRI [21]. The range of the latter could be sufficient to explain all of the variation in canopy PRI considering the range of diffuse and clear sky radiation--from nearly 100% at sunrise until a typical midsummer noon value (diffuse to total PPFD value of approx. 15% as determined by a BF3 sensor (Delta-T Devices, Cambridge, UK) in SMEAR II.