AbstractPremise of The StudyMost dendroecological studies focus on dominant trees, but little is known about the growing season of trees belonging to different size classes and their sensitivity to biotic factors. The objective of this study was to compare the dynamics of xylem formation between dominant and suppressed trees of Abies fabri of similar age growing in the Gongga Mountains, southeastern Tibetan Plateau, and to identify the association between xylem growth and climate.MethodsThe timing and duration of xylogenesis in histological sections were investigated weekly during the 2013–2015 growing seasons.Key ResultsOur investigation found that timing and duration of xylogenesis varied with canopy position and its associated tree size. Xylogenesis started 6–14 days earlier, and ended 5–11 days later in dominant trees than in suppressed trees, resulting in a significantly longer growing season. Dominant trees also exhibited higher temperature sensitivity of tracheid production rate than suppressed trees.ConclusionsThe observed differences in xylogenesis among trees suggested that competition affects tree growth by reducing the growing period in suppressed trees. Representative climate–growth relationships should involve trees of all size classes when evaluating the effects of the environment on forest dynamics.