Photoperiod, [CO2] and soil moisture interactively affect phenology in trembling aspen: Implications to climate change-induced migration

•Longer photoperiods and low soil moisture delayed autumnal phenology but had opposite effects on growth.•Elevated [CO2] 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 [CO2]. Trees may migrate northward in response to climate change, which would expose them to different environmental conditions, such as [CO2], 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 [CO2] (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 [CO2]. Surprisingly, low moisture reduced cold hardiness of aspen. Elevated [CO2] delayed bud break in the spring and the effects were magnified by the low moisture. The interactive effects of photoperiod, CO2, 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.