Longer photoperiods negate the CO2 stimulation of photosynthesis in Betula papyrifera Marsh: Implications to climate change-induced migration.

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.

CO2 Elevation and Photoperiods North of Seed Origin Change Autumn and Spring Phenology as Well as Cold Hardiness in Boreal White Birch.

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.

Photoperiod and CO2 elevation influence morphological and physiological responses to drought in trembling aspen: implications for climate change-induced migration.

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 (Vcmax) and the maximum rate of electron transport for RuBP regeneration (Jmax) 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 Vcmax and Jmax in response to LSM varied with photoperiod (Vcmax: 36% at 52°N, 22% at 55°N, 47% at 58°N; Jmax: 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.