Environmental cues under climate change: teasing apart the influence of photoperiod and temperature during the development of dormancy in spruce (Picea glauca)

Background/Question/Methods

Timely responses to environmental cues enable synchronization of life history transitions essential for health and survival of northern boreal tree species. The activity-dormancy transition influences bud dormancy and the inability of the meristem to resume growth, an important adaptation within forest trees. Both photoperiod and temperature have been implicated in the development of dormancy, contributing to the trade-off between active growth and bud set, thereby reducing risk of winter injury. However, while photoperiodic cues remain static under climate change, temperature cues may vary, contributing to possible asynchrony in phenological and associated physiological transitions important to forest health. Understanding the relative contribution of photoperiod and temperature to dormancy acquisition will be important as silvicultural and seed transfer programs are adapted to current and future climates. Using a combination of photoperiod (long/short day) and temperature (warm/cold) treatments under controlled conditions, we examined developmental and physiological traits important to the growth-dormancy transition; including bud formation, cold hardiness and photosynthesis across two provenances of Picea glauca spanning a broad latitude in Alberta, Canada. Depth of dormancy achieved under different daylength and temperature conditions was assessed examining the timing and ability of spruce seedlings to resume growth following transfer from treatment to favorable growth conditions.

Results/Conclusions

External environmental cues were not required to initiate bud formation in Picea glauca, however, environmental cues mediated the trajectory of bud formation and the transition to an endodormant state, integral to the annual growth cycle of conifers. Phenotypic data suggested short days promoted bud development by suppressing second flush, and colder temperatures delayed bud development.  Spruce seedlings exhibited significant trade-offs between development of cold hardiness and photosynthesis in response to environmental cues, with temperature strongly influencing the magnitude of daylength response. This suggests a dual control of physiological traits in response to environmental cues. Dormancy was achieved under multiple conditions, but true endodormancy was only achieved under long day and warm nights. The depth of dormancy, as indicated by the inability to resume growth under favorable conditions, varied across experimental treatments. These experiments point towards a fine-scale sensitivity of spruce to environmental cues, as life history transitions are conditioned on the interactions between photoperiod and temperature. Our data indicates temperature interacts with photoperiod, therefore, incorporating local constant photoperiods into varying climatic niche models will be important for the development of seed transfer guidelines across silvicultural programs under climate change.