LAUSR

Past research suggests that climate change will cause the climate envelopes of various tree species to shift 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 [CO2] (AC 400 vs. 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 (HSM) vs. low (LSM, -2 MP) for two growing seasons in greenhouses. Both morphological and physiological responses were observed. The LSM 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 Vcmax and 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.