Decoding the seasonal clock of sugar maple: Timings and sequence of its phenological events

The seasonal sequence of physiological processes is essential to understand the strategies of carbon allocation and the dynamics of sap production in maple. We monitored the phenological events in sugar maple (Acer saccharum Marsh.) from 2018 to 2022 in Simoncouche (QC), Canada. We measured sap exudation, xylem water transport and radial growth using rain gauges and dendrometers installed on four adult trees. Leaf phenology was estimated with MODIS-derived Normalized Difference Vegetation Index (NDVI) in maple stands near the study site. The sugar season started in late March and ended in early May, at the onset of tension-driven water transport. The sugar season corresponded with the period of snowmelt and increase in soil water content. Budbreak and complete leaf expansion were observed one and three weeks after the ending of sap exudation, respectively, when the snow had disappeared and soil temperature started rising. Radial growth occurred two weeks after leaf expansion and ended in early August. The cessation of xylem water transport caused leaf fall in early November. Phenology advanced up to three weeks during the warm spring 2021. The synchronism between the sugar season and tension-driven water transport confirms the physiological limit for sap exudation, suggesting the importance of optimizing yield within the time window of rehydration after winter dormancy. In the context of ongoing climate warming, producers could benefit from paying more attention to the environmental and physiological signals triggering the sap season rather than relying solely on historical calendars of sap production.