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Rising CO2, global warming, and N deposition create challenging environmental conditions to vegetation. Since elevated CO2 and rising temperature are coupled with each other, it is important to understand their combined effects on plants. We investigated the growth and photosynthetic responses of yellow birch to five levels of nitrogen supply under the current (cCT: current CO2 and temperature) and the predicted future CO2 and temperature conditions (fCT: elevated CO2, current + 4℃ temperature). The results show that fCT and high N supply increased seedling growth but fCT reduced photosynthetic capacity (e.g., maximum rate of Rubisco carboxylation-Vcmax, maximum rate of photosynthetic electron transport-Jmax)) and foliar N concentration. However, the magnitude of the fCT effect declined with increases in N supply. Furthermore, the fCT treatment significantly reduced the Jmax/Vcmax ratio, indicating a possible shift of N allocation from Jmax to Vcmax in the photosynthetic machinery. This result suggests that the photosynthesis of yellow birch may be more limited by electron transport under the predicted future climate condition. Both low N supply and fCT significantly increased photosynthetic N use efficiency (PNUE) and there was a negative relationship between PNUE and photosynthetic capacity. In general, yellow birch grew better under fCT than cCT, particularly above-ground growth. [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]

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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]

The distribution of tree species is expected to shift toward the pole in response to the climate change associated with the elevation of atmospheric CO2 concentration [CO2]. The shift will expose trees to a new photoperiod regime and other environmental conditions. The changes in these factors will likely have interactive effects on the ecophysiological traits of plants. This study investigated how CO2 elevation and change in photoperiod influence the timing of bud development, leaf senescence, and cold hardiness in the fall, and bud break in the spring in boreal white birch (Betula papyrifera Marsh.). Seedlings were exposed to two different [CO2] (AC = 400 μmol mol–1; EC = 1000 μmol mol–1) and four simulated photoperiod regimes in the greenhouse corresponding to each latitude [48 (seed origin), 52, 55, and 58°N] for two growing seasons. We found that EC advanced the initiation of leaf color change (10% leaf color change) in the fall by 23 days, but delayed the completion date of color change (90%). Leaf senescence started earlier in the photoperiods corresponding to 55 and 58°N latitude than those at 48 and 52°N latitudes under EC, but photoperiod did not affect leaf senescence under AC. Additionally, the temperature causing 50% electrolyte leakage (a measure of susceptibility to freezing damage) was more negative under the photoperiod corresponding to 55° (−46°C) and at 58°N (−60°C) under EC than at the lower latitudes (above −40°C). Budburst in the spring occurred earlier under the photoperiods corresponding to the two highest latitudes under EC, but the trend was opposite under AC. The combination of longer photoperiods and elevated [CO2] resulted in earlier budburst in the spring and later completion of leaf senescence in the fall as well as greater cold hardiness, leading to extended growing seasons from both ends. However, the onset of leaf senescence was earlier than in other treatment combinations. Furthermore, the photoperiod effects were quite different under the ambient [CO2]. Our results suggest that it is extremely important to consider the complex interactions of [CO2] and photoperiod in planning latitudinal seed transfers and in predicting the migration of boreal trees 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]

One-year-old jack pine (Pinus banksiana Lamb.) and current-year white birch (Betula papyrifera Marsh.) seedlings were grown in ambient (360 ppm) or twice ambient (720 ppm) atmospheric CO2 concentration ([CO2]) and at three soil temperatures (Tsoil = 7, 17 and 27 °C initially, increased to 10, 20 and 30 °C two months later, respectively) in a greenhouse for 4 months. In situ foliar gas exchange, in vivo carboxylation characteristics and chlorophyll fluorescence were measured after 2.5 and 4 months of treatment. Low Tsoil suppressed net photosynthetic rate (Pn), stomatal conductance (gs) and transpiration rate (E) in jack pine in both CO2 treatments and gs and E in white birch in ambient [CO2], but enhanced instantaneous water-use efficiency (IWUE) in both species after 2.5 months of treatment. Treatment effects on gs and E remained significant throughout the 4-month study. Low Tsoil reduced maximal carboxylation rate (Vcmax) and PAR-saturated electron transport rate (Jmax) in jack pine in elevated [CO2] after 2.5 months of treatment, but not after 4 months of treatment. Low Tsoil increased actual photochemical efficiency of photosystem II (PSII) in the light (ΔF/Fm′) in jack pine, but decreased ΔF/Fm′ in white birch after 4 months of treatment. In response to low Tsoil, photosynthetic linear electron transport to carboxylation (Jc) decreased in jack pine after 2.5 months and in white birch after 4 months of treatment. Low Tsoil increased the ratio of the photosynthetic linear electron transport to oxygenation (Jo) to the total photosynthetic linear electron transport rate through PSII (Jo/JT) in both species after 2.5 months of treatment, but the effects became statistically insignificant in white birch after 4 months of treatment. High Tsoil decreased foliar N concentration in white birch. Elevated [CO2] increased Pn, IWUE and Jc but decreased Jo/JT in both species at both measurement times except Jc in white birch after 2.5 months of treatment. Elevated [CO2] also decreased gs and E in white birch at high Tsoil, Vcmax in both species and triose phosphate utilization in white birch at low Tsoil after 4 months of treatment, and ΔF/Fm′ in white birch after 2.5 months of treatment. Elevated [CO2] also increased foliar N concentration in both species. Low Tsoil caused no permanent damage to PSII in either species, but jack pine responded and acclimated to low Tsoil more quickly than white birch. Photosynthetic down-regulation and a decrease in photosynthetic electron transport to photorespiration occurred in both species in response to elevated [CO2]. [ABSTRACT FROM AUTHOR]

• 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]

• Photoperiod at a latitude 4° north of seed origin increased growth by 15% and biomass by 18% in white birch. • Photoperiods at even higher latitudes origin did not improve growth or biomass further. • Biomass allocation to roots decreased with increasing photoperiod from 48 to 58°N latitude. • CO 2 elevation did not affect the above responses. Global warming induced northward migration will expose trees to longer photoperiod regimes during the growing season. The phenotypic ability of trees to take advantage of the longer photoperiods and elevated atmospheric CO 2 concentration ([CO 2 ]) will likely be a critical factor for determining their success and performance at the new locations. We investigated how growth, biomass, and biomass allocations respond to the interactive effects of photoperiod regimes and [CO 2 ] in white birch (Betula papyrifera Marsh.). Seedlings were exposed to ambient (400 μmol mol−1) or elevated concentration (1000 μmol mol−1) [CO 2 ], and four photoperiod regimes corresponding to 48 (seed origin), 52, 55, and 58°N latitude for two growing seasons. Our results show that growth, biomass, and biomass allocation were affected by photoperiod regime but not by [CO 2 ]. Seedling growth and biomass were stimulated by the three photoperiod regimes north of the seed origin. Plants under the photoperiod regime of 52°N were 15% higher in growth and 18% higher in biomass than under the photoperiod regime of the seed origin (48°N). However, increases in photoperiod regimes beyond that of 4° north of the seed origin did not lead to an additional increase in growth and biomass. The differences in biomass components among the three longer photoperiods were statistically insignificant, but the leaf biomass and stem biomass were higher under the longer photoperiods relative to the seed origin. While the differences between two adjacent photoperiods were not always statistically significant during the two growing seasons, biomass allocated to roots showed a general decreasing trend with increases in photoperiod from the seed origin to 58°N latitude. Our results suggest that despite the limited plasticity in growth and biomass displayed in much higher latitudes, white birch will likely benefit from the longer photoperiod regimes during the growing season associated with migration or seed transfer to higher latitudes. [ABSTRACT FROM AUTHOR]