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Historical temperature records reveal that the boreal forest has been subjected to a significant lengthening of the thermal growing season since the middle of the last century, and climate models predict that this lengthening will continue in the future. Nevertheless, the potential phenological response of trees to changes in growing season length remains relatively undocumented, particularly for evergreen boreal tree species growing in cold environments. Here, we used the recently defined zero growth (ZG) concept to extract and characterize the metrics of seasonal radial growth dynamics for 12 balsam fir trees subjected to a 12-year soil warming experiment using high resolution radius dendrometer measurements. The ZG concept provides an accurate determination of growth seasonality (onset, cessation, duration, growth rates, and total growth) for these slow-growing trees characterized by significant shrinkage in tree diameter due to dehydration in the winter. Our analysis revealed that, on average, growth onset starts at day 152 ± 7 (±1 SE, 31 May–1 June) and ceases at day 244 ± 27 (31 August–1 September), for a growing season duration of about 3 months (93 ± 26 days) over a 12-year period. Growing season duration is mainly determined by growth cessation, while growth onset varies little between years. A large part (80%) of the total growth occurs in the first 50 days of the growing season. Given the dynamics of growth, early growth cessation (shorter growing season) results in a higher average seasonal growth rate, meaning that longer growing seasons are not necessarily associated with greater tree growth. Soil warming induces earlier growth cessation, but increases the mean tree growth rate by 18.1% and the total annual growth by 9.1%, on average, as compared to the control trees. Our results suggest that a higher soil temperature for warmed trees contributes to providing better growth conditions and higher growth rates in the early growing season, when the soil temperature is low and the soil water content is elevated because of snowmelt. Attaining a critical soil temperature earlier, coupled with lower soil water content, may have contributed to the earlier growth cessation and shorter growing season of warmed trees.

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Despite new knowledge in recent years, our understanding of the phenology of wood formation for various species growing in different environments remains limited. To enhance our knowledge of the tree growth dynamics of boreal tree species, we investigated the average seasonal, monthly, daily, and diel patterns of tree growth and water status from 11 years of observations with the 15 min and 1.5 µm resolved stem radial size variation data of 12 balsam fir (Abies balsamea (L.) Mill.) trees growing in a cold and humid boreal environment. Growth only occurred above an air temperature threshold of 9–10 °C, and the maximal growth rate over the year (23–24 June) was synchronous with the maximal day length (20–21 June) and not with the maximal air temperature, which occurred on average about 2 weeks later (4–5 July). Tree growth was mostly restricted by air temperature and solar radiation under these cold and wet boreal conditions, but our results also highlight a turgor-driven growth mechanism. Diel dynamics reveal that tree growth is minimal during the day when the stem dehydrates, and higher past midnight when the stem is fully rehydrated. This pattern suggests that carbon assimilation through photosynthesis occurs primarily during the day, while energy production and carbon allocation to woody tissues occur primarily at night via cellular respiration. Overall, our results show that the temporal patterns of the growth and water status of balsam fir growing in cold and humid boreal environments are controlled by a set of environmental factors that influence various physiological processes and mechanisms, many of which still need to be documented.