Late frost as a major driver of ecotypic differentiation in bud break of sugar maple

Benjamin Marquis, Xiali Guo and Roberto Silvestro, Sciences Fondamentales, Université du Québec à Chicoutimi, Chicoutimi, QC, Canada, Xiali Guo, South China Botanical Garden, Chinese Academy of Sciences, Guangzhou, China, Ping Ren, College of Life Sciences, Anhui Normal University, Wuhu, China, Qiao Zeng, Key Lab of Guangdong for Utilization of Remote Sensing and Geographical Information System, Guangdong Open Laboratory of Geospatial Information Technology and Application, Guangzhou Institute of Geography, Guangzhou, China, Siddhartha Khare, Department of Biology | Quebec Center for Biodiversity Science, McGill University, Montréal, QC, Canada, Jian-Guo Huang, South China Botanical Garden, Chinese Academy of Sciences, Beijing, China, Eryuan Liang, Institute of Tibetan Plateau Research, Chinese Academy of Sciences, Beijing, China, Jean-Daniel Sylvain, Direcitone de la recherche forestière, Ministère de la Faune et des Parcs du Québec, Québec, QC, Canada, Sylvain Delagrange, Dept des Sciences Naturelles - Institut des Sciences de la Forêt tempérée (ISFORT), UQO, Ripon, QC, Canada, Patricia Raymond, Direction de la recherche forestière, Ministère des Ressources naturelles du Québec, Québec, QC, Canada, Sergio Rossi, Département des Sciences Fondamentales, Université du Québec à Chicoutimi, Chicoutimi, Canada

Background/Question/Methods
In temperate and boreal climates, dormancy results from the evolutionary pressure imposed by the cold temperatures during winter. We expect that growth reactivation in trees has evolved according to the local climate with the aim of limiting damage of late frosts to the new developing tissues. Accordingly, budbreak of different populations should reflect the differences in the timings of late frost at the provenance origin. To test our hypothesis, experiments on 29 sugar maple provenances sampled across eastern Canada were conducted in the field and under controlled conditions. Fieldwork consisted in budbreak observations conducted twice a week over 524 one-year-olds seedlings that were planted in two common gardens established in two contrasting climate regimes: Chicoutimi (48.38 °N, 71.04 °W, mean annual temperature 3.7 °C) and Ripon (45.78 °N, 75.09 °W, mean annual temperature 6.4 °C). Results from the field experiment were compared to those obtained from the warming experiment conducted in growth chamber using two thermal conditions (14/10 and 18/14 °C day/night temperature). In total, 5229 and 18750 bud observations were collected from the field and growth chamber analysis respectively.
Results/Conclusions
Results from the field and the warming experiments showed that budbreak began and ended earlier for provenances originating from colder sites. Budbreak also began and ended earlier by 5 and 7 days for trees experimentally warmed and for trees planted in the warmer site respectively compared to trees under controlled conditions and for trees planted at the colder site. Moreover, the minimum air temperature of April at provenance origin explained the intra-species variations in budbreak timing observed in the field. Since the timing of late frosts, which ranged from day of year 90-120 at the provenance origin corresponds to the beginning and ending dates of April, minimum air temperature in April is a good proxy of late frost timing. Overall, we showed (1) that budbreak in sugar maple populations is locally modulated to avoid damage from late frosts, (2) that the future warmer climate should advance budbreak and (3) that trees from colder sites should be more exposed to future late frost than trees from warmer sites. These important ecotypic differences in budbreak timing between sugar maple provenances will help to determine which provenances should be select for an assisted migration program aiming to maintain ecosystem functions under climate change.