Abstract
We explore needle sugar isotopic compositions (δ18 O and δ13 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 δ18 O and δ13 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 δ18 O, δ18 O and δ13 C of needle water-soluble carbohydrates (sugars + pinitol), and needle sugar δ13 C (R2 = 0.95, 0.84, 0.60, 0.73, respectively). Relative humidity (RH) and intercellular to ambient CO2 concentration ratio (Ci /Ca ) were the dominant drivers of δ18 O and δ13 C variability, respectively. However, the variability of needle sugar δ18 O and δ13 C was reduced on diel and intra-seasonal timescales, compared to predictions based on instantaneous RH and Ci /Ca , 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 18 O-fractionation factor and mesophyll resistance in 13 C-discrimination were critical. Interpreting leaf-level isotopic signals requires understanding on time integration caused by mixing in the needle sugar pool.