¹⁴C‐Age of Carbon Used to Grow Fine Roots Reflects Tree Carbon Status

The time elapsed between carbon fixation into nonstructural carbohydrates (NSC) and their use to grow tree structural tissues can be estimated by 14 C ages. Reported 14 C‐ages indicate that NSC used to grow root tissues (growth NSC) can vary from < 1 year to decades. To understand the controls of this variability, we compared 14 C‐ages of leaf, branch, and root tissues from two conifers ( Larix decidua , Pinus mugo ) in a control valley site and an alpine treeline ecotone where low temperatures restrict tree growth. Our results of increasing respiration rate and NSC concentration with ecotone elevation suggest an excess of C assimilation over growth and an increase in fresh NSC supply. Greater flow of fresh NSC through needles and branches could explain their young growth NSC (< 2 years). A smaller inflow of fresh NSC into roots could explain older growth NSC ages, which increased from 2 to 10 years from the valley to the bottom of the ecotone, and then declined to 6 years at the ecotone top. Rather than species differences that were small, environmental conditions over years appear to be the primary driver of C allocation dynamics, which are reflected in the 14 C‐ages of fine roots.