From phenology to forest management: Ecotypes selection can avoid early or late frosts, but not both

Forest managers use artificial regeneration to influence tree species composition and productivity. The selection of plant material could assume a leading role in forest planning, mainly when aiming to increase the adaptation of stands within a context of climate change. In this study, we investigated the timing of bud burst and bud set in five black spruce [Picea mariana (Mill.) B.S.P.] ecotypes growing in a common garden and originating from a latitudinal range in the boreal forest of Québec, Canada. Bud phenology was monitored weekly during the 2015, 2017 and 2018 growing seasons. On average, the bud burst process lasted 23 days, occurring 1.2 days later for each degree Celsius of increase in mean annual temperature at the provenance site. Bud set duration was 55 days and occurred 1.8 days later for each degree Celsius of increase in mean annual temperature at the provenance site. We demonstrated that both bud burst and bud set occurred earlier in individuals from colder sites, which resulted in similar lengths of the growing season among provenances. This clinal variation in the timings of growth resumption and cessation confirms the ecotypic differentiation of black spruce populations and reflects a long-lasting adaptation to the local temperatures in the sites of origin. The findings of this study demonstrate that ecotype selection in black spruce can aim to avoid damage from either early or late frosts, but not both.