The onset of xylogenesis is not related to distance from the crown in Smith fir trees from the southeastern Tibetan Plateau

Young and mature trees are usually characterized by asynchronous growth resumption of xylem. Here, we test the hypothesis that the later onset of xylem growth in older trees is related to the longer distance of the stem from the developing buds, which represents the main source of hormones triggering vascular tissue differentiation. We compared the onset of xylogenesis at different heights along the stems of young and mature Smith fir (Abies georgei var. smithii (Viguie & Gaussen) W. C. Cheng & L. K. Fu)) trees in the Sygera Mountains, southeastern Tibetan Plateau. Xylem formation was monitored weekly in 2012 on anatomical sections of wood microcores. The onset of xylogenesis differed between young and mature trees, with most phases occurring 2 weeks later in mature trees. No effect of the sampling height was observed on the growth resumption. Our results suggest that the later resumption of xylogenesis in older conifer trees is not related to their longer distance from the crown.