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Predicting the future distribution of plants requires a mechanistic understanding of how plants cope with their new environmental conditions under a changed climate. We examined the effects of elevated carbon concentration ([CO2]) and elevated Tsoil on the physiological responses to light of a shade-tolerant woody species, mountain maple (Acer spicatum L.). Seedlings were exposed to ambient and elevated [CO2] (392μmolmol−1 vs. 784μmolmol−1), current and elevated soil temperatures (Tsoil 17 vs. 22°C) at high and low (100% vs. 30%) light conditions for one growing season. It was found that elevated Tsoil stimulated net photosynthesis (A) by 18% in the low-light treatment but tended to reduce A in the high-light treatment. Elevated [CO2] increased A by 100% under the elevated Tsoil and by 48% at the current Tsoil. The effect of elevated Tsoil also varied with [CO2]: it increased A by 13% under the elevated [CO2] but reduced A by 17% under the ambient [CO2]. We observed a significant increase in the instantaneous water-use efficiency of photosynthesis (IWUE) in response to the high-light treatment under the elevated [CO2] but not under the ambient [CO2]. Our data indicate that the elevated [CO2] markedly increased nitrogen (N) allocation to the photochemical apparatus of A in seedlings grown under the high-light condition. These results suggest that the predicted increases in atmospheric [CO2] will likely enhance the growth of mountain maple in canopy gaps in the forest, while the predicted elevated Tsoil will negate the benefit of high-light conditions. [ABSTRACT FROM AUTHOR]

• Longer photoperiods and low soil moisture delayed autumnal phenology but had opposite effects on growth. • Elevated [CO 2 ] delayed spring bud flush and the effect was magnified by low soil moisture. • Cold hardiness showed a decreasing trend with increasing photoperiod, and the effect was reduced by elevated [CO 2 ]. Trees may migrate northward in response to climate change, which would expose them to different environmental conditions, such as [CO 2 ], photoperiod, soil moisture. Changes in these environmental conditions will likely influence tree phenology and growth. We grew Populus tremuloides Michx. from seeds for two growing seasons under two [CO 2 ] (400 vs 1000 μmol mol−1), four photoperiod regimes (corresponding to 48 °N (seed origin), 52 °N, 55 °N, and 58 °N latitude), two soil moistures (60 %–75 % vs 13 %–20 %) and examined phenological responses. We found that longer photoperiods extended growing season by delaying autumnal phenology and increased growth. Low moisture also delayed autumnal phenology but reduced growth, and the effects were reduced by elevated [CO 2 ]. Surprisingly, low moisture reduced cold hardiness of aspen. Elevated [CO 2 ] delayed bud break in the spring and the effects were magnified by the low moisture. The interactive effects of photoperiod, CO 2, and soil moisture on phenology, cold hardiness, growth, and growing season indicate the complex nature of northward migration that has been predicated under the scenario of climate change. Our results suggest that these interactions should be considered when planning latitudinal seed transfers and predicting the migration of boreal trees in response to climate change. [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]