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In response to global warming, trees are expected to shift their distribution ranges to higher latitudes. The range shift will expose them to novel environmental conditions, such as new photoperiod regimes. These factors can interact with rising atmospheric CO2 ([CO2]) to affect trees' physiology and growth. This study simulated future environmental conditions to investigate photosynthetic responses to changes in photoperiod regimes (seed origin [48°N], 52, 55, and 58°N) and [CO2] (ambient 400 vs. elevated 1000 μmol mol−1) in white birch (Betula papyrifera Marsh.) seedlings. Our results show that elevated [CO2] stimulated leaf photosynthesis (Pn) at the two lower latitudes (48 and 52°N). However, this stimulation by elevated [CO2] was lost in the two higher latitudes (55 and 58°N). Elevated [CO2] led to the downregulation of maximum Rubisco activity (Vcmax) for the two higher latitudes, and maximum electron transport rate (Jmax) and triose phosphate utilization (TPU) at 58°N, while it enhanced Jmax and TPU for the two lower latitudes. Increased instantaneous water‐use efficiency (IWUE) for the two lower latitudes was primarily attributed to the CO2 stimulation of Pn while the higher IWUE under the photoperiod regimes of 55 and 58°N latitudes was explained by reduced water loss. Photoperiod effects varied with [CO2]: Pn increased at the photoperiod regimes of 55 and 58°N in ambient [CO2] while it tended to decline under these photoperiods in elevated [CO2]. Our study suggests that the photosynthesis of white birch will likely respond negatively to northward migration or seed transfer in response to climate change. [ABSTRACT FROM AUTHOR]

White birch (Betula papyrifera Marsh.) seedlings were grown under two carbon dioxide concentrations (ambient: 360 μmol mol−1 and elevated: 720 μmol mol−1), three soil temperatures (5, 15 and 25 °C initially, increased to 7, 17 and 27 °C, respectively, 1 month later) and three moisture regimes (low: 30–40%; intermediate: 45–55% and high: 60–70% field water capacity) in greenhouses. In situ gas exchange and chlorophyll fluorescence were measured after 2 months of treatments. Net photosynthetic rate (An) of seedlings grown under the intermediate and high moisture regimes increased from low to intermediate Tsoil and then decreased to high Tsoil. There were no significant differences between the low and high Tsoil, with the exception that An was significantly higher under high than low Tsoil at the high moisture regime. No significant Tsoil effect on An was observed at the low moisture regime. The intermediate Tsoil increased stomatal conductance (gs) only at intermediate and high but not at low moisture regime, whereas there were no significant differences between the low and high Tsoil treatments. Furthermore, the difference in gs between the intermediate and high Tsoil at high moisture regime was not statistically significant. The low moisture regime significantly reduced the internal to ambient CO2 concentration ratio at all Tsoil. There were no significant individual or interactive effects of treatment on maximum carboxylation rate of Rubisco, light-saturated electron transport rate, triose phosphate utilization or potential photochemical efficiency of photosystem II. The results of this study suggest that soil moisture condition should be taken into account when predicting the responses of white birch to soil warming. [ABSTRACT FROM PUBLISHER]

• Freeze-thaw events reduce stem hydraulic conductivity and delay spring phenology in white birch. • Longer photoperiods increase hydraulic conductivity. • Longer photoperiods advance spring phenology under elevated CO 2 but delay it under ambient CO 2. Past studies indicate that narrower conduits such as those in diffuse-porous species are less vulnerable to freeze-thaw (FT) induced embolism and also facilitate the refilling of embolized xylem conduits early in the spring, resulting in an earlier bud break. In this study, we investigated if a novel environmental condition associated with climate change-induced northward migration will affect the vulnerability to FT-induced embolism and spring phenology in white birch. Seedlings were grown under ambient (400 μmol mol−1) or elevated CO 2 concentration (1000 μmol mol−1), and four photoperiod regimes corresponding to 48 (seed origin), 52, 55, and 58 °N latitude. We found that the longest photoperiod (corresponding to 58 °N latitude) significantly increased the maximum specific hydraulic conductivity of the stem. CO 2 concentration ([CO 2 ]) and photoperiod had no significant impact on the vulnerability to FT-induced embolism. The treatment of 5 freeze-thaw cycles (+5 to −20 °C) led to an 11 % loss of hydraulic conductivity in dormant seedlings that had been stored at −4 °C for 3 months while the effect of such a treatment in the fall was much smaller. This result suggests that freeze-thaw events in late winter or spring can impair the hydraulic conductivity of the xylem which in turn may negatively affect the physiology of the trees. Indeed, the FT treatment in this study delayed budburst and leaf expansion in the spring. It is interesting to note that photoperiods had the opposite effect on budburst under different [CO 2 ]: longer photoperiods led to earlier budburst in the spring under elevated [CO 2 ], but delayed budburst under ambient [CO 2 ]. The synergistic effect of longer photoperiods and CO 2 elevation suggests that the growing season for white birch may be longer than what we predict from either factor alone at a migration site in the future when [CO 2 ] will be much higher. [ABSTRACT FROM AUTHOR]

Elevated CO2 and warmer temperature occur simultaneously under the current climate change. However, their combined effects on the photosynthetic traits in boreal trees are not well understood. This study investigated the morphological and photosynthetic responses of yellow birch (Betula alleghaniensis Britt.) to a combined treatment of CO2 and temperature (ambient, ACT (400 μmol mol−1 CO2 and current temperature) vs elevated, ECT (750 μmol mol−1 CO2 and current +4 °C temperature)). It was found that ECT significantly reduced leaf-area based photosynthetic rate (A n ), maximum Rubisco carboxylation rate (V c max), photosynthetic electron transport rate (J max), leaf nitrogen concentration, respiration and mesophyll conductance. There were two interesting findings: first, the primary mechanism of photosynthetic limitation shifted from Ribulose-1,5-bisphosphate (RuBP) carboxylation (related to V c max) to RuBP regeneration (related to J max) in response to ECT, leading to decreased transition point (C i-t and A n-t ) from RuBP carboxylation to regeneration; second, the increase in total leaf area in response to ECT more than compensated for the downregulation of leaf-area based photosynthesis, leading to greater biomass in ECT than in ACT. We proposed a new protocol for evaluating photosynthetic limitations by comparing the relative relationship between the transition point (C i-t and A n-t ) and the photosynthetic rate at growth CO2 (C i-g and A n-g ). Furthermore, we found that J max (RuBP regeneration) was the primary limitation to A n under ECT. [ABSTRACT FROM AUTHOR]

Predicting photosynthetic acclimation to elevated CO2 and warming is difficult because they have opposite effects. We investigated physiological and morphological responses in white birch (Betula papyrifera Marshall) to a combination of CO2 and temperature (ACT — 400 µmol·mol–1 CO2, current temperature; ECT — 750 µmol·mol–1 CO2, current + 4 °C temperature). ECT reduced photosynthesis, maximum Rubisco carboxylation (Vcmax), maximum electron transport rate (Jmax), photorespiration, daytime respiration, leaf N, and stomatal and mesophyll conductance, but increased biomass, height, total leaf area, electron partitioning to carboxylation and oxygenation ratio, and CO2 compensation point. The photosynthetic acclimation is consistent with the optimal carbon gain theory (carbon gain drives the coordination of carboxylation, electron transport, and respiration). While the photosynthetic acclimation was similar to acclimation to elevated CO2, ECT reduced Jmax/Vcmax, which is consistent with the response to warming but opposite to the response to elevated CO2, suggesting that thermal acclimation may be the primary mechanism of photosynthetic acclimation to ECT and ECT probably altered N allocation between machinery for carboxylation and that for ribulose-1,5-bisphosphate regeneration. The increase in total leaf area by ECT more than offset the negative effect of photosynthetic downregulation on carbon sequestration, resulting in faster growth and greater biomass under ECT. [ABSTRACT FROM AUTHOR]