Spiral ascent does not homogenize xylem transport: evidence from stable isotope tracing in Scots pine
Ruth-Kristina Magh, Reimo Lutter, Nils Henriksson, Hyungwoo Lim, Tomas Lundmark, John D MarshallAbstract
The functional role of spiral grain remains debated. It is unclear whether spiral grain promotes tangential mixing of xylem resources or whether transport remains sectorial, as predicted by the pipe model of tree architecture. To address this question, we traced the natural uptake and transport of isotopically labelled water (2H) and dissolved nitrogen (as nitrate: K15NO3) in mature Scots pine (Pinus sylvestris L.) growing in a boreal forest.
A combined label solution was applied to a 1 m2 soil patch, and at the end of the season, five trees exhibiting clear label uptake were felled, and stem discs were collected along the stem axis. The most recent growth ring was analysed for incorporation of the 2H label, while 15N incorporation was assessed in current-year needles.
We show that water taken up from the soil remains largely confined to discrete flow paths following the root trajectory, with minimal tangential spreading during transport through the stem. The labelled signal follows spiral grain but does not substantially mix across it, indicating that spiral grain does not function as an effective mechanism for lateral redistribution within the stem. Significant widening of the labelled sector occurs only upon reaching the crown, suggesting that increased connectivity at the branching level, rather than stem-level mixing, drives redistribution. In contrast, concurrently absorbed nitrogen is rapidly and mostly uniformly distributed across the canopy. This decoupling between water and nutrient pathways demonstrates that xylem transport is primarily source-driven and structurally constrained, whereas nutrient allocation operates at the whole-canopy level.
By providing field-based evidence of tracer transport following natural root uptake, this study challenges the assumption that spiral grain enhances tangential mixing within the stem and highlights the need to account for sectorial transport pathways in ecohydrological studies.