Quantifying root lateral distribution and turnover using pine trees with a distinct stable carbon isotope signature

Summary

• 1
  In order to help assess spatial competition for below-ground resources, we quantified the effects of fertilization on root biomass quantity and lateral root distribution of mid-rotation Pinus taeda trees. Open-top chambers exposed trees to ambient or ambient plus 200 µmol mol⁻¹ atmospheric CO₂ for 31 months.
• 2
  Tank CO₂ was depleted in atmospheric ¹³C; foliage of elevated CO₂ trees had δ¹³C of −42·9, compared with −29·1 for ambient CO₂ trees.
• 3
  Roots 1 m from the base of elevated CO₂-grown trees had more negative δ¹³C relative to control trees, and this difference was detected, on average, up to 5·8, 3·7 and 3·7 m away from the trees for 0–2, 2–5 and >5 mm root-size classes, respectively. Non-fertilized tree roots extended as far as fertilized trees despite the fact that their above-ground biomass was less than half that of fertilized trees.
• 4
  These results are informative with respect to root sampling intensity and protocol, and the distances required between experimental manipulations to evaluate below-ground processes of independent treatments.
• 5
  Fine-root turnover has usually been estimated to range from weeks to 3 years, representing a major avenue of carbon flux. Using a mixing model we calculated that 0–2 mm roots had a mean residence time of 4·5 years indicating relatively slow fine-root turnover, a result that has major implications in modelling C cycling.