Interaction of age and elevation on xylogenesis in Juniperus przewalskii in a cold and arid region

Exogenous factors (such as temperature and water availability) as well as endogenous factors (such as tree age) have effects on cambial phenology and intra-annual tree growt dynamics. However, how these factors interactively influence this physiological process remains unclear. Here, we compared biweekly cambial activity, timing and duration of xylogenesis in adult (< 250 yr) and old (> 400 yr) trees of Juniperus przewalskii Kom. along an altitudinal gradient on the northeastern Tibetan Plateau. We found that the onset of cell production was 10–11 days earlier in adult than in old trees, both at high and low elevations. However, the end of cell production was mainly correlated with water availability varying with elevation, regardless of age. In addition to earlier growth onset, adult trees were characterized by a longer growing season and higher growth rates, resulting in a higher number of xylem cells. We found that at relatively wet high elevations, tree age exerts a more dominant control on cell production, whereas at relatively dry low elevations, the effect of tree age on tree growth became weak or negligible. Our study demonstrates that age interacts with environmental constraints to determine intra-annual growth dynamics in this cold and arid region.