Species with transcontinental distribution or spread over wide geographical regions develop populations with growth traits genetically adapted to the local climate. The aim of this study was to investigate the ecotypic sensitivity of bud break, a strong adaptive trait, to a changing environment. Six phenological phases of bud break were monitored daily on black spruce [Picea mariana (Mill.) BSP] seedlings submitted to different temperatures (12, 16 and 20 °C) and photoperiods (14, 18 and 22 h). Six provenances were tested in growth chambers, produced from seeds collected along the whole latitudinal range of the closed boreal forest in Quebec, Canada. Bud break lasted 13.3 days on average and occurred earlier in seedlings from colder sites. The annual temperature of the sites suitably tracked the clinal variation among ecotypes, providing a clear biological explanation for the environmental signal driving the adaptive divergence of populations to the local climate. Increasing temperature induced an earlier bud break according to a non-linear pattern with greater advancements observed between 12 and 16 °C. Photoperiod was significant, but sensitivity analysis indicated that its effect on bud break was marginal with respect to temperature. No interaction of provenance × treatment was observed, demonstrating an ecotypic convergence of the responses to both factors. Changes in the growing conditions could substantially modify the synchronization between bud phenology and climate, thus exposing the developing meristems of black spruce to frost damage. However, similar advancements of bud break could be expected in the different ecotypes subjected to warmer temperatures or longer day lengths.