# Measurement of Diurnal Variation in Needle PRI and Shoot Photosynthesis in a Boreal Forest
## 4 Discussion
To untangle the structural effects on canopy PRI, data on the spectral composition of incident radiation and the fraction of shaded foliage in the field of view of the sensor would be required. Hence, we could not corroborate a clear link between canopy and needle PRI as has been reported earlier [7; 39], as more research is required to quantify this connection. The sunlit needle PRI did not follow illumination (i.e., PPFD) for the measured Scots pine trees (Figure 4), a finding which contradicts earlier findings in other species [40; 41]. According to our results, the diurnal cycle of needle PRI was a complicated one. Both the sunlit and shaded needles had the same PRI values in the morning (Figure 3). This is not surprising considering they have similar carotenoid to chlorophyll content ratios (Figure 7), which is the primary driver of needle PRI [17] and a similar canopy location. It should be noted that, contrary to other studies, the shaded needles measured by us were sampled from the topmost canopy layer with a high fraction of the visible sky. They differed from sunlit needles only by receiving less direct sunlight before and at noon. Needle PRI increased slightly before noon until 11 a.m., after which the PRI of sunlit needles decreased to the daily minimum value at approx. 3 p.m. Correlation with shoot photosynthesis (Figure 5) corroborates that the PRI decrease is indeed caused by biochemical changes leading to photosynthetic downregulation. By 8 p.m., the PRI values of both sunlit and shaded shoots had recovered to their morning values. Unfortunately, clear days suitable for leaf-level diurnal measurements are hard to achieve in many regions. Leaf-level PRI measurements are laborious and need to be carried out manually. It could be argued that the limited number of successful measurement campaigns limits the generality of the daily needle PRI curves presented here. The measurements have also limited spatial coverage due to the difficulty in accessing multiple fully grown tree crowns. However, a lack of a needle PRIrecession before noon has been confirmed for the same area from carefully-corrected airborne imaging spectroscopy data [22]. We found a strong and significant correlation between sunlit needle PRI and relative humidity (Figure 6b) in 2014, but in 2015 the correlation was not significant (\(p>0.1\)). Under high light conditions, relative humidity and sunlit shoot downregulation were significantly correlated (\(0.1<R^{2}<0.5\), \(p<0.01\), data not shown). Unfortunately, the spatial and temporal limitations in sampling mentioned above make it impossible to determine the mechanistic causes of the PRI variations. The strong correlation may indicate a co-variation with some environmental parameter (e.g., solar irradiance) and need not indicate a functional dependence. To integrate the temporally-sparse PRI measurements provided by remote sensing satellites to daily and seasonal photosynthesis, it is critical to fully understand the processes involved in creating the observed temporal changes. The uncoupling of needle PRI and PPFD depend on, for example, how photoassimilates are used in relation to absorbed radiative energy, and the role of sinks in the relationship between photosynthetic assimilation and water transport. Determining the normal diurnal processes would clarify the causes of afternoon downregulation, and enable detection of deviations from the normal behaviour. The diurnal cycle of PRI is likely to be different for different species growing in different climatic conditions. An alternative would be to study this phenomenon under controlled conditions (following [PERSON] et al. [19]), but this is difficult for fully-grown trees and would inevitably exclude some environmental factors. Our study supports the finding that PRI is correlated with midday LUE and photosynthesis in mid-summer conditions under high light conditions similar to those on cloudless days during remote sensing satellite overpasses.