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Tree migration to higher latitudes may occur in response to future changes in climate, exposing the trees to higher concentrations of carbon dioxide ([CO2]), new photoperiods, different levels of soil moisture, and other new conditions. These new conditions can influence the physiology, survival, and growth of trees. This study examined the interactive effects of [CO2], photoperiod, and soil moisture on the morphology and resistance to xylem cavitation in trembling aspen (Populus tremuloides Michx.). One-year-old seedlings, in greenhouses, were exposed to two [CO2] (ambient [CO2] 400 μmol·mol−1 or an elevated [CO2] 1000 μmol·mol−1), four photoperiod regimes corresponding to latitudes 48°N (seed origin), 52°N, 55°N, and 58°N, and two levels of soil moisture (60%–75% and 13%–20% of field capacity) for one growing season. Seedling growth, leaf size, specific leaf area, biomass allocation, and xylem resistance to cavitation (water potentials for 20%, 50%, and 80% loss of hydraulic conductivity) were assessed. The seedlings under the longest photoperiod regime (58°N latitude) had greatest height and biomass but smallest specific leaf area. Under the elevated [CO2], however, the longest photoperiod regime significantly reduced xylem resistance to drought-induced cavitation compared with the photoperiod corresponding to 48°N. These results suggest that when migrating to higher latitudes, trembling aspen may grow faster but could become less resistant to drought and more prone to hydraulic failure during a drought spell. [ABSTRACT FROM AUTHOR]

Trees may migrate northward in response to climate change and become exposed to new photoperiod and soil moisture regimes. This study assessed the impacts of photoperiod and its interaction with soil moisture and carbon dioxide concentration ([CO2]) on the hydraulic conductivity in jack pine ( Pinus banksiana Lamb.) and its vulnerability to xylem embolism. Seedlings were exposed to 400 vs. 950 μmol·mol−1 [CO2], 60%-70% vs. 30%-40% (of field capacity) soil moisture, and photoperiods of seed origin and 5° and 10° north of seed origin in greenhouses. Cavitation vulnerability curves were measured for determining the xylem pressure at which 50% hydraulic conductivity was lost (ΨPLC50). It was found that elevated [CO2] significantly increased hydraulic conductivity, whereas low soil moisture decreased it. Under elevated [CO2], the xylem became progressively more vulnerable to embolism with changes in photoperiod regime from the seed origin to 10° north of the seed origin, as indicated by the progressively less negative ΨPLC50. However, no such a trend was detected under the ambient [CO2]. The results suggest that the species may become less resistant to drought as the atmospheric [CO2] increases, hindering the northward migration or seed transfers. Even within its current natural distribution range, trees near its northern boundary of the range may be more vulnerable to embolism as the atmospheric [CO2] increases even without any change in moisture conditions. [ABSTRACT FROM AUTHOR]

CO2–light interactions can influence the competition among boreal plants, but are poorly understood. We investigated the effect of such interactions on the growth and biomass of 1-year-old black spruce (Sb) (Picea mariana (Mill.) BSP) and white spruce (Sw) (Picea glauca (Moench) Voss) grown with CO2 concentrations ([CO2]) of 360 and 720 µmol·mol–1 under 30%, 50%, and 100% light, in greenhouses. There were significant two-way and three-way interactions. Root collar diameter (RCD) of Sw decreased with decreasing light, while in Sb, there was no significant difference in RCD for plants grown under 50% or 30% light. Height was greater for plants grown under 100% light than if the plants were shaded. Elevated [CO2] increased RCD by 33% and enhanced stem volume by 67%, 98%, and 84% under 100%, 50%, and 30% light, respectively. The CO2 enhancement of total biomass was relatively higher under lower light, and greater for Sb than Sw. Elevated [CO2] decreased specific leaf area under 50% light only. Root mass was generally higher under 100% light than when shaded. Elevated [CO2] increased the root mass of Sb under 100% light, but decreased it under 30% light. Elevated [CO2] decreased the shoot/root ratio under 100% light, but increased it under 30% light. Our data suggest that raising [CO2] will likely increase species competitiveness under low light conditions, and that the increase will be greater in species that are relatively shade tolerant. [ABSTRACT FROM AUTHOR]

Compared with the effects of spring frosts on opening buds or newly flushed tissues, winter freezing damage to conifers, owing to temperature fluctuations prior to budbreak, is rare and less known. In this study, changes in cold hardiness (measured based on electrolyte leakage and needle damage) and spring budbreak were assessed to examine the responses of four boreal conifer species - black spruce ( Picea mariana (Mill.) B.S.P.), white spruce ( Picea glauca) (Moench) Voss), jack pine ( Pinus banksiana Lamb.), and lodgepole pine ( Pinus contorta Dougl. ex. Loud.) - to different durations of experimental warming (16 °C day to -2 °C night with a 10 h photoperiod, except for night temperatures during November warming (+2 °C)). Seedlings showed increased responses to warming from November to March, while the capacity to regain the cold hardiness lost to warming decreased during the same period. This suggests an increasing vulnerability of conifers to temperature fluctuations and freezing damage with the progress of chilling and dormancy release from fall to spring. Both lodgepole pine and jack pine initiated spring growth earlier and had greater responses to experimental warming in bud phenology than black spruce and white spruce, suggesting a greater potential risk of frost/freezing damage to pine trees in the spring. [ABSTRACT FROM AUTHOR]