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Past research suggests climate change will cause the climate envelopes of various tree species to shift to higher latitudes and can lead to a northward migration of trees. However, the success and scope of the migration are likely affected by factors that are not contained in the climate envelope, such as photoperiod and interactive effects of multiple environmental factors, and these effects are currently not well understood. In this study, we investigated the interactive effects of CO2 concentrations ([CO2]), photoperiod and soil moisture on the morphological and physiological traits of Populus tremuloides Michx. We grew seedlings under two levels of [CO2] (ambient [CO2] (AC) 400 vs elevated [CO2] (EC) 1000 μmol mol−1), four photoperiod regimes (growing season photoperiods at 48 (seed origin), 52, 55 and 58°N latitude) and two soil moisture regimes (high soil moisture (HSM) vs low soil moisture (LSM), −2 MPa) for two growing seasons in greenhouses. Both morphological and physiological responses were observed. Low soil moisture reduced leaf size, total leaf area and height growth by 33, 46 and 12%, respectively, and increased root/shoot ratio by 20%. The smaller leaf area and increased root/shoot ratio allowed the seedlings in LSM to maintain higher the maximum rate of Rubisco carboxylation (V cmax) and the maximum rate of electron transport for RuBP regeneration (J max) than control seedlings (55 and 83% higher in July, 52 and 70% in August, respectively). Photoperiod and [CO2] modified responses to LSM and LSM altered responses to photoperiod and [CO2], e.g. the August photosynthetic rate was 44% higher in LSM than in HSM under EC but no such a difference existed under AC. The increase in V cmax and J max in response to LSM varied with photoperiod (V cmax: 36% at 52°N, 22% at 55°N, 47% at 58°N; J max: 29% at 52°N, 21% at 55°N, 45% at 58°N). Stomatal conductance and its reduction in response to LSM declined with increasing photoperiod, which can have significant implications for soil moisture effect on northward migration. This study highlights the need to consider the complex interactions of [CO2], photoperiod and soil moisture when planning assisted migration or predicting the natural migration of boreal forests in the future. [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]

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]