7In the near future, climate warming is expected to produce more severe and frequent periods of drought 8 with consequent water stresses for boreal species. In this paper, we present a high resolution analysis of 9 chronologies of stem radius variations in black spruce under rain exclusion. Prolonged rain exclusions were applied 10 for three consecutive summers to trees on four sites along a latitudinal gradient. The stem radius variations of 11 control and treated trees were monitored year-round at an hourly resolution with automatic point dendrometers. The 12 seasonal patterns of shrinking and swelling were analyzed using a sequential analysis technique and the daily 13 patterns of contraction and expansion were extracted. Overall, the treated trees followed their daily cycles of 14 contraction and expansion during the rain exclusions and no cumulative difference in stem expansion was observed 15 over the three years. Trees subjected to rain exclusion showed larger stem contractions in summer on three out of 16 four sites and larger winter contractions were observed on the northern sites. This study shows that a repeated 26SR; Data analysis and interpretation, critical manuscript revision.

Climate warming is modifying the movement of air masses over Northern latitudes, producing warming and cooling events across the boreal regions. These new conditions changes may mismatch plant phenology from weather conditions, and affect the growing period of trees. Understanding the processes of local adaptation in bud phenology can help to predict the response of plants to these rapid and unexpected environmental changes. Our study monitored bud burst and bud set weekly during four growing seasons in black spruce [Picea mariana (Mill.) B.S.P.] saplings planted in a common garden and originating from five provenances representing the whole latitudinal distribution of the closed boreal forest in Quebec, Canada. We compared the variance in bud phenology among populations and years, and analysed the relationships with temperatures at the origin sites. Bud burst and bud set occurred in mid‐May and mid‐July, respectively, with a large variability among provenances and between the study years. A delayed bud phenology was observed in the provenances from warmer sites, with bud burst and bud set being 1.1 and 1.4 days later for every additional degree in mean annual temperature at the origin site respectively. Populations with earlier bud bursts also showed earlier bud sets, thus the growing season was similar among provenances. The heritability of bud set was higher than that of bud burst, with estimates of 0.26 and 0.21 respectively. On average, variance in bud phenology among provenances reached 5.3%, which was higher than that within provenances (2.6%). The factor year explained 37.7%–69.7% of the variance in bud phenology. Synthesis. The findings demonstrate the evolutionary response to climate variations in the boreal biome. Endogenous and environmental components have different effects on the phases of bud burst and bud set. This complex interaction among drivers of bud phenology may prevent reliable predictions of changes in the growing season under changing climate. An earlier growth reactivation due to higher spring temperatures could result in higher frost risks to the developing buds. Despite a longer period for photosynthesis expected for the evergreen black spruce under warmer conditions, the period spent for bud development could remain unchanged.

Global warming is affecting tree growth and forest productivity, especially in the Northern boreal ecosystems. Wood quality, which is largely determined by anatomical traits of wood, is vital for the forest industry and global carbon sequestration. Cambium activity, wood density, fiber length and microfibril angle are the anatomical traits that determine wood quality, depending on market demands. Within the global warming scenario, a comprehensive understanding of these traits is still lacking and urgently required for both the forest industries and ecological researches. In this review, we identify that large proportions of mature wood, high wood density, longer fiber or tracheid length and low microfibril angles are the anatomical traits closely related with high wood quality. Higher temperatures could trigger onset and ending of cambial cell division, thus affecting wood quality by modulating duration of the growing season. Climate warming could also affect wood quality by impacting earlywood and latewood formation, as well as changing wood density, fiber length and microfibril angle depending on different species and growing conditions. In addition, this review indicates that the anatomical traits involved in wood quality are diverse and depend on the intended use. Improving our knowledge about the underlying mechanisms of how the wood anatomical traits respond to a changing environment with extreme climate events is thus still a crucial topic in the forest sciences. Selection of species and provenances best adapted to climate warming will be necessary to improve quality without sacrificing volume. Studies on wood traits and their relation to climate should therefore focus on a multitude of aspects including the physiology and genetics of boreal tree species.

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