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 analyzed the relationships with the 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. Results demonstrate the plastic response of bud burst to changing temperatures and suggest the effects of endogenous factors on bud set. The earlier growth reactivation due to global warming occurring under higher frost risks in spring are expected to produce damage to the developing buds. Meanwhile, the ability of bud phenology to match the inter-annual variability in weather could help to cope with the changing environmental conditions expected in the future.

Seed collection and common garden

Seeds were collected in June 2012, from 8-11 mother trees randomly selected in each site (Table 1). According to the availability and accessibility of the canopy,  a minimum of 10 cones were collected per trees from each stand. Seeds from each tree were kept separated to characterize the variability among half-sib families within populations. After seed extraction, the seedlings were grown under controlled conditions until reaching a suitable size to be planted in the field. In July 2014, a total of 422 seedlings were planted in a 0.5 ha forest gap in SIM, the southernmost site (Table 1).  Seedlings were planted according to a random design at a distance of 2 m × 2 m. Two rows of non-experimental black spruce seedlings were planted on each side of the plantation to avoid edge effects. 

Phenology

Bud phenology was recorded weekly between May and October in 2015, 2017-2019. The apical buds were used to discriminate the different phenological phases of bud burst and bud set (Dhont et al., 2010). Six bud burst phases were defined (1) open bud, with a pale spot at the tip; (2) elongated bud, with lengthening brown scales; (3) swollen bud, with smooth and pale-coloured scales; (4) translucent bud, with needles visible through the scales; (5) split bud, with needles still clustered; and (6) exposed shoot, with needles spreading outwards. Five phases of bud set were defined: (1) white bud; (2) beige bud, with beige scales; (3) brownish bud, with a substantial increase in bud size; (4) brown bud, with needles spreading outwards; and (5) spread needles, with the needles completely spread outwards.