Ecotypic differentiation of black spruce populations: temperature triggers bud burst but not bud set

Phenology is one of the major mechanisms of adaptation to the environment. Species spreading over a wide geographical range can evolve ecotypes that are able to grow and reproduce under particular local conditions.We compared the thermal conditions during bud phenology in black spruce [Picea mariana (Mill.) BSP] populations in order to assess differences among ecotypes. The phases of bud burst and bud set were monitored weekly during 2015, 2017 and 2018 in saplings growing in a common garden, and originating from a latitudinal range across the whole closed boreal forest of Quebec, Canada. Provenances from the colder sites exhibited both earlier bud burst and bud set, with differences of 8 and 11 days, respectively, between the northern and southern provenances. Bud burst occurred under colder temperatures in provenances from the northern sites. The phase of open bud occurred at 4°C in the northernmost provenance, compared to 8°C in the southernmost one. Bud set occurred in summer, when temperatures still exceeded 20°C, and no difference was observed between provenances. Black spruce populations exhibit a clear clinal differentiation in ecotypes showing temperature-adapted bud burst of the apical meristem. The need to complete formation of the winter bud and hardening before autumn leads bud set to being independent of the air temperature. Warmer conditions can affect the timings of spring phenology by anticipating bud burst in black spruce, although no direct effect may be expected for bud set.