14C-age of carbon used to grow fine roots reflects tree carbon status
O_LI"Bomb" 14C ages in trees indicate the time elapsed between carbon fixation into nonstructural carbon (NSC) and its use for metabolism and growth. It remains unknown why newly grown aboveground tissues have a narrow range of young 14C-ages, while fine root ages can span decades. C_LIO_LIWe measured 14C in tissues of two coniferous species along an alpine treeline ecotone and used a mixing model to estimate the fraction of NSC that is metabolically active. We expected that greater growth limitation higher in the ecotone would supply more freshly fixed NSC to respiration, active NSC, carbohydrates, and roots, resulting in younger 14C-ages of NSC used to grow tissues. C_LIO_LIResults confirmed an increase in fresh NSC supply with elevation. Needle and branch growth was supported by young NSC (< 2 yr) with little elevation influence, while root growth was supported by older NSC that declined from 10 to 6 yr with elevation. C_LIO_LIMassive allocation of fresh NSC to needles and branches could explain their young 14C-ages. Variable fine root 14C-ages reflect tree C status, with older ages when aboveground growth limits the contribution of new C belowground, while young ages represent greater delivery of fresh NSC to roots. C_LI