Explicitly accounting for needle sugar pool size crucial for predicting intra-seasonal dynamics of needle carbohydrates δ O and δ C.

We explore needle sugar isotopic compositions (δ O and δ C) in boreal Scots pine (Pinus sylvestris) over two growing seasons. A leaf-level dynamic model driven by environmental conditions and based on current understanding of isotope fractionation processes was built to predict δ O and δ C of two hierarchical needle carbohydrate pools, accounting for the needle sugar pool size and the presence of an invariant pinitol pool. Model results agreed well with observed needle water δ O, δ O and δ C of needle water-soluble carbohydrates (sugars + pinitol), and needle sugar δ C (R  = 0.95, 0.84, 0.60, 0.73, respectively). Relative humidity (RH) and intercellular to ambient CO concentration ratio (C /C ) were the dominant drivers of δ O and δ C variability, respectively. However, the variability of needle sugar δ O and δ C was reduced on diel and intra-seasonal timescales, compared to predictions based on instantaneous RH and C /C , due to the large needle sugar pool, which caused the signal formation period to vary seasonally from 2 d to more than 5 d. Furthermore, accounting for a temperature-sensitive biochemical O-fractionation factor and mesophyll resistance in C-discrimination were critical. Interpreting leaf-level isotopic signals requires understanding on time integration caused by mixing in the needle sugar pool.