Contrasting effects of warming and drought on autumn phenology of photosynthesis and growth in white spruce.

Evergreen conifers rely on photoperiod and temperature cues to regulate autumn phenology. Climate change delays autumn cooling, whereas photoperiod remains unaffected, and this mismatch might have consequences for the timing of autumn phenology. Climatic stresses, such as drought and heat during summer, might also impact the timing of phenological events, including photosynthetic downregulation and growth cessation. We investigated the single and combined impacts of summer drought and autumn warming on photosynthetic downregulation and growth cessation in a northern and southern family of white spruce (Picea glauca). In a temperature free-air controlled experiment combined with rainout structures, we exposed white spruce seedlings to ambient temperature and rainfall, summer drought, autumn warming, or a combination of summer drought followed by autumn warming. Warming delayed the downregulation of photosynthesis in both families compared to the control seedlings, with the southern family showing increased photosynthetic activity compared to the northern family. Despite extended photosynthetic activity, secondary growth cessation was not delayed by warming and did not vary between families. Warming affected latewood xylem development, an important component of secondary growth, by increasing and decreasing lumen area in the northern family and southern family, respectively, indicating the northern family may have been more cold-limited under past selective pressures. Summer drought had minimal impacts on photosynthetic downregulation and growth cessation in either family. We conclude that warming-induced extension of photosynthetic activity in autumn may not translate into increased growth and carbon sequestration due to strong photoperiodic and genetic constraints on growth cessation in white spruce.