Synchronisms and correlations of spring phenology between apical and lateral meristems in two boreal conifers

Phenological synchronisms between apical and lateral meristems could clarify some aspects related to the physiological relationshipsamong the different organs of trees. This study correlated the phenological phases of bud development and xylem differentiationduring spring 2010–14 in balsam fir (Abies balsamea Mill.) and black spruce [(Picea mariana Mill. (BSP)] of theMonts-Valin National Park (Quebec, Canada) by testing the hypothesis that bud development occurs after the reactivation ofxylem growth. From May to September, we conducted weekly monitoring of xylem differentiation using microcores and buddevelopment with direct observations on terminal branches. Synchronism between the beginning of bud development and xylemdifferentiation was found in both species with significant correlations between the phases of bud and xylem phenology. Degreedaysum was more appropriate in assessing the date of bud growth resumption, while thermal thresholds were more suitable forcambium phenology. Our results provide new knowledge on the dynamics of spring phenology and novel information on thesynchronisms between two meristems in coniferous trees. The study demonstrates the importance of precisely defining thephases of bud development in order to correctly analyse the relationships with xylem phenology.