A photoperiod treatment of continuous light (CL) and non-continuous light (NCL) was applied in (III). The photoperiod affected many aspects of growth, height growth cessation, biomass and GE, but did not affect most leaf traits. Both provenances had higher RGR, shoot:root ratio, WUE and larger sink-sizes but lower RMF and gs in CL than in NCL. 67°KI had higher and 61°PU had lower Amass in CL than in NCL. When 67°KI was subjected to NCL, it ceased its height growth earlier than in CL (III). Therefore, the photoperiod especially affected the biomass accumulation and allocation patterns of 67°KI. The LMA of 61°PU was higher in CL than in NCL, but other treatment effects in leaf traits were not found (III). These included leaf longevity, which was hypothesized to be lower in CL than in NCL and in 67°KI than in 61°PU, but no treatment or provenance differences were found.
In (I), the GE of different leaf types was compared. Short shoot (early) leaves had lower gs and higher WUE than long shoot (late) leaves (I), and short shoot leaf gs increased throughout the growing season
(Supplementary Material of I). The results agree with other studies on birch where lower gs in early leaves have been observed (Miyazawa and
Kikuzawa 2004, Hoshika et al. 2013). However, these studies also observed lower Anet in early leaves, whereas in (I) leaf type did not have a significant effect on Anet. Leaf type differences can be important as the flushing of these leaves is timed differently – early spring leaves are more likely to flush during high soil moisture. However, provenance differences and the acclimation plasticity of early and late leaves require further study.
3.2 Differences in the growth performance and biomass
earlier compared to more southern ones (Viherä-Aarnio et al. 2006), as was also seen in the field (Supplementary Fig. S6c of I). 67°KI stopped height growth earlier than 61°PU also in the NCL treatment (III), but when the photoperiod was relaxed (article II and CL in III) obviously earlier height growth cessation did not take place. Earlier height growth cessation happens because northern birches have a shorter critical night length requirement (Viherä-Aarnio et al. 2006, Luquez et al. 2008), that is, growth cessation is under strong photoperiodic control. Accordingly, Viherä-Aarnio et al. (2013) observed that when silver birch origins are transferred to Finnish mid-latitude CGs the height growth and volume yield is best if the latitudinal transfer distance is zero or slightly (~2°) to the north. Similarly, in a British trial with young silver birch seed-origins, best growth was seen in provenances that were transferred ~2° to the north (Lee et al. 2015).
Tedla et al. (2019) saw an increase in white birch (Betula papyrifera Marsh.) height growth and biomass when the photoperiod was shifted northward a maximum of 4°. Consequently, translocations larger than a few degrees to either north or south have been warned to diminish gains and increase risk to survival, although the occasional study has found that even much larger (southward) transfers carry little adverse effects (Rousi et al. 2012).
Recent evidence has pointed out that transfer effects may also heavily depend on the nutrient status of the receiving soil (Possen et al. 2021). At the CG in (I), both a south- and northward translocation was done;
northern provenances were translocated ~3 – 5° to the south and southern provenances ~0 – 2° to the north. Local provenances and those
translocated to the CG a maximum of 2° to the north had better growth than the ones translocated 3° or more to the south (I).
Some plants were discarded during the experiments. In (I), most plants experienced a long-distance translocation to the CG, yet only two were discarded as they did not grow. Similarly in (II), only two plants were discarded, which can be due to coincidence, e.g., failed rooting. In (III), six plants were discarded (five 67°KI plants from the NCL-treatment and one 61°PU plant from the CL-treatment). Discarded plants obviously could not be measured, but having to discard several 67°KI plants from the NCL- treatment can itself be considered a result, indicating relatively high
mortality or failed growth of northern plants in non-continuous light. It should be noted that the experiments in (II) and (III) lasted for three to four months, which is a slightly unrealistically long growing period for Finland.
When kept in the same conditions, the total accumulated biomass was not different between the provenances (II, III). Instead, the provenances differed in biomass allocation to the relative mass fractions. In the field, northern provenances produced less leaves and less total leaf area than southern ones (Supplementary Figs. S6a,b of I) and in the chambers, 67°KI produced only a few branches and a small number of leaves (II, III) but had higher RMF (II). At the same time, absolute root mass did not differ
between provenances (II, III). These were seen irrespective of photoperiod, and may indicate adaptations to northern conditions. 67°KI also had a lower shoot:root ratio than 61°PU (II), although it was higher in CL for both provenances (III). Provenance differences in biomass allocation were more obvious in (II) than in (III), and this is likely more of a data problem rather than a reflection of reality, caused by missing data of 67°KI in NCL
(discarded plants). High allocation to roots may be adaptive in the north (Reich et al. 2014, Zadworny et al. 2016) and would direct resources away from above-ground biomass. A similar case is seen in conifers, where northern ecotypes allocate more to roots than southern ones (Oleksyn et al. 1992, Johnsen and Seiler 1996) and high-altitude Norway spruce (Picea abies L.) populations have lower relative biomass-allocation to needles and stems and higher allocation to roots than low-altitude conifers (Oleksyn et al. 1998). A potential explanation for these observations is that roots may continue to grow after height growth cessation as long as temperatures permit it (Blume‐Werry et al. 2016, Radville et al. 2016). As root phenology may thus be unlinked from above-ground phenology, and as northern provenances cease height growth earlier than southern ones, northern provenances may in this way increase their final relative root mass fraction. It has even been proposed that increased nutrient acquisition (higher N) could thus explain high GE rates in northern populations (Zadworny et al. 2016). Additionally, a tree with a small number of branches could be less susceptible to snow-load damage, but branch damaging in northern settings has never been quantified for the exact
same provenances used in this thesis. For the same reason, it would be of interest to investigate branch angles. It is known that southern silver birches translocated to the far north of Finland lose their tops and become bushy more likely than more northern silver birches, suggesting that the number of branches and branch angles are indeed important (Raulo 1976, 1979).
Both provenances accumulated more biomass in CL than in NCL. This is not surprising, as the longer photoperiod represents a larger pool of resources. The earlier growth cessation of 67°KI in NCL likely explains how the photoperiod appeared to affect especially its relative mass fractions, but both provenances had a lower relative allocation to roots (lower RMF) in CL than in NCL (III). Total biomass accumulation was also negatively correlated with RMF and positively with the shoot:root ratio
(Supplementary Material of II). This shift in allocation may be an important observation for the future, if southern provenances migrate to areas of CL and decrease their RMF. However, in such a scenario there is likely to exist a multitude of effects affecting allocation, such as new temperatures, soil conditions and herbivory.
In (I) and (III), northern provenances consistently had higher relative height growth rates than southern ones and RGR was calculated in various different ways, increasing confidence in the observation. In (III), growth rate was examined with RGR and with a linear regression through the most active growth during 32 – 95 DAP (Supplementary Fig. S2b of III). As a potential pitfall, 32 DAP was chosen as the start point of the linear
regression, even though for 67°KI it could be argued that active growth in the NCL-treatment began later at 46 DAP. This could intuitively be thought to affect the slope, but if the regression is run from 46 – 95 DAP, the slope does not change prominently or become significant (not shown). Thirdly, in the CL-treatment of (III), the linear regression line of 67°KI crossed the regression line of 61°PU (Supplementary Fig. S2b of III), and a similar phenomenon was observed in (II) when a model was regressed through the entire duration of growth (Fig. 2c in II). This is further evidence that 67°KI can show a good height growth rate as long as the photoperiod is long enough not to cause growth cessation. Additionally, while absolute
growth rate (AGR) did not show a significant provenance difference (Supplementary Fig. S2a of III), it did again demonstrate the same
phenomenon where the curve of 67°KI outpaced 61°PU during the phase of best growth.
It should be noted that high genotypic variability most certainly
operated behind all growth-related results. In the CL-treatment of (III), the northern genotype Ki27 expressed a slender phenotype with good height growth but very little branching and in parallel relatively little leafing (Supplementary Fig. S1 of III). At the same time, the other northern genotype Ki7 grew to a more moderate height but with more branching and leaves (genotype differences not statistically tested), albeit branches remained short in length (not measured, but obvious to the eye). This is in contrast to the southern genotypes in the CL-treatment, which were more balanced in phenotype; although Pu17, Pu18, Pu25 and Pu30 did show genotypic variation especially in the number of leaves and branches, they differed less among each other in height compared to the northern ones, and all had lengthy branches (as seen in Fig. 10 and Supplementary Fig. S1 of III).
Daily, seasonal and latitudinal variations in photoperiod, light intensity or spectral quality were not incorporated into the setups in (II) and (III). The experiments lasted for three to four months, the length of an entire
growing season, and the intensity and quality of light undergo large
changes during this time span in nature. During the spring in the north, the day-length first rapidly increases (increasing faster further north) before reaching a plateau at the summer solstice (with CL in the Arctic), and then decreases towards winter with the rate of change again depending on latitude. Light intensity and quality are modified by changes in sun elevation angle, affecting phenology and several growth and leaf traits (Pecot et al. 2005, Tsegay et al. 2005, Sellaro et al. 2010, Kalaitzoglou et al.
2019, Brelsford et al. 2019, Chiang et al. 2019, Kotilainen et al. 2020). Thus, it is conceivable that tree growth (and photosynthetic leaf traits) could become differently entrained in a more realistic setup. For example, a simple experiment comparing plant height growth in both a realistic and a constant photoperiod could give insight into whether plant development
followed a different trajectory in these conditions. Another possibility would be to only first give the plants a realistic, changing variation in day- length, then a constant photoperiod, to examine whether this would be enough to entrain the plants.
Figure 10. Example comparison of the same genotypes in (a) CL and (b) NCL chosen to highlight the provenance differences in response to
photoperiod in (III). The northern genotype (Ki27) shown here exhibits the typical response of low biomass and short final height in NCL, whereas the southern genotype (Pu30) does not show a clear phenotypic response to day-length. Photographs taken near the end of the experiment.
Photographs of all plants are shown in Supplementary Fig. S1 of III.
3.3 Photosynthetic gas exchange and chlorophyll fluorescence