Polar day syndrome: differences in growth, photosynthetic traits and sink-size patterns between northern and southern Finnish silver birch (Betula pendula Roth) provenances in native and non-native photoperiods.

Continuous light (CL) is available throughout the polar day for plants in the Arctic during the growing season, whereas provenances of the same species experience a very different environment with non-CL (NCL) just a few latitudes to the south. Both provenances need to acclimate to climate warming, yet we lack comprehensive understanding of how their growth, photosynthesis and leaf traits differ. Further, the provenances presumably have morphological and physiological adaptations to their native environments and therefore differ in response to photoperiod. We tested the height growth, leaf longevity, biomass accumulation, biomass allocation and rates of gas exchange of northern (67°N) and southern (61°N) Finnish silver birch (Betula pendula Roth) origins in CL- and NCL-treatments in a 4-month chamber experiment. Irrespective of photoperiod, 67°N had higher area-based photosynthetic rate (Anet), stomatal conductance (gs) and relative height growth rate (RGR), but lower stomatal density and fewer branches and leaves than 61°N. Photoperiod affected height growth cessation, biomass and photosynthetic traits, whereas leaf longevity and many leaf functional traits remained unchanged. In CL, both provenances had lower gs, higher RGR, increased shoot:root ratio and increased sink sizes (more branching, more leaves, increased total plant dry weight) compared with NCL. In NCL, 67°N ceased height growth earlier than in CL, which altered biomass accumulation and distribution patterns. Northern conditions impose challenges for plant growth and physiology. Whether a provenance inhabits and is adapted to an area with or without CL can also affect its response to the changing climate. Northern birches may have adapted to CL and the short growing season with a 'polar day syndrome' of traits, including relatively high gas exchange rates with low leaf biomass and growth traits that are mainly limited by the environment and the earlier growth cessation (to avoid frost damage).

Strategy by latitude? Higher photosynthetic capacity and root mass fraction in northern than southern silver birch (Betula pendula Roth) in uniform growing conditions.

Growth of northern trees is limited by short growing seasons. In multi-year trials, northern trees usually grow less than southern ones but can have higher gas exchange, whereas differences in biomass allocation and its relation to photosynthesis are less known. We characterized silver birch (Betula pendula Roth) provenances from southern (latitude 61°) and northern (latitude 67°) Finland in uniform chamber conditions. In a time-series experiment, we measured traits related to growth, biomass allocation and photosynthesis, and determined gas exchange responses to temperature and light. We found provenance differences in photosynthetic capacity and growth. The northern provenance allocated relatively more to roots, having a higher root mass fraction and lower shoot:root ratio than the southern provenance. On the other hand, the northern provenance had fewer leaves and lower total leaf dry weight (DW) than the southern provenance. The northern provenance attained higher rates of net photosynthesis (Anet) and higher stomatal conductance (gs) in all measured temperatures and higher photosynthesis at the optimum temperature (Aopt) than the southern provenance, but there was no difference in the optimum temperature of photosynthesis (Topt, 18.3 °C for the southern provenance vs 18.9 °C for the northern one). Photosynthetic light response curves showed no between-provenance differences. In a time-series, the northern provenance had higher Anet than the southern provenance, but gs was similar. The northern provenance had higher maximum quantum yield of photosystem II photochemistry (Fv/Fm) than the southern provenance. There were no differences between provenances in height, total plant DW, shoot DW, root DW or shoot mass fraction. Our results suggest that the provenances occupy a common thermal niche, or can at least relatively quickly acclimate to a common growth temperature. Thus, carbon assimilation of these northern trees may not be significantly affected by rising temperatures alone. In an equal photoperiod and optimal conditions, we found different one-season biomass accumulation strategies: southern trees grow with more leaves, while northern trees reach similar total assimilation (total DW, height) with more efficient photosynthetic capacity per leaf area (higher gas exchange, higher Fv/Fm) and relatively more investment in the below-ground fraction of the plant.

Differences in growth and gas exchange between southern and northern provenances of silver birch (Betula pendula Roth) in northern Europe.

Due to its ubiquity across northern latitudes, silver birch (Betula pendula Roth) is an attractive model species for studying geographical trait variation and acclimation capacity. Six birch provenances from 60 to 67°N across Finland were grown in a common garden and studied for provenance and genotype variation. We looked for differences in height growth, photosynthetic gas exchange and chlorophyll content index (CCI) and compared the gas exchange of early and late leaves on short and long shoots, respectively. The provenances stratified into southern and northern groups. Northern provenances attained less height growth increment and had higher stomatal conductance (gs) and lower intrinsic water-use efficiency (WUE, Anet/gs) than southern provenances, whereas net photosynthesis (Anet) or CCI did not show clear grouping. Short shoot leaves had lower gs and higher WUE than long shoot leaves in all provenances, but there was no difference in Anet between shoot types. The separation of the provenances into two groups according to their physiological responses might reflect the evolutionary history of B. pendula. Latitudinal differences in gas exchange and water use traits can have plausible consequences for global carbon and water fluxes in a warming climate.