Increased inter-annual variability in budburst dates towards the northern range edge of black spruce

Species spreading across large geographical ranges show adaptations to local conditions. Thermal gradients can produce clines in phenology among populations. The spatial heterogeneity of temperature fluctuations under the current warming may alter phenological patterns due to the specific responses of the vegetation to thermal conditions. In this study, we calculated the growing degree days (GDD) required for budburst during 2015-2021 in five provenances originating from the latitudinal distribution of black spruce (Picea mariana (Mill.) B.S.P.) and growing in a common garden in Simoncouche, Quebec, Canada. The thermal model was combined with tem-perature to reconstruct the spatiotemporal variations in budburst during 2000-2021 across the boreal forest in Quebec, Canada. In the common garden, budburst started from late April to late May and finished from late May to late June, occurring 7-10 days earlier in the coldest provenances. The GDD for the phase 'open bud' varied from 185 +/- 42 to 230 +/- 40 for the coldest and warmest provenance, respectively. Across the study area (48-54 degrees N), 'open bud' was estimated from mid-May to late June, and the 'exposed shoot' occurred from mid-June to mid -July. Budburst gradually occurred later and showed an increased inter-annual variability towards the northern range edge, following the reduction and increasing variability in spring temperature. For each degree of latitude, the delay in the phases of open bud and exposed shoot decreased from 6 to 1.5 days and 4.6 to 0.7 days, respectively, showing a non-linear pattern. Our results confirmed the hypothesis that the temporal variability in budburst increases towards the species range edge. The ongoing climate change can speed up heat accumulation and advance budburst in black spruce. Northern populations could exhibit the greatest phenological changes due to the limiting thermal conditions and faster warming predicted for these regions. Such an advancement of spring phenology would expose black spruce to a higher risk of late frost.