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Keywords:

  • carbon cycling;
  • carbon isotope;
  • carbon reserves;
  • fine roots;
  • forest;
  • residence time;
  • starch;
  • sugars

Summary

  • Fine roots play a key role in the forest carbon balance, but their carbon dynamics remain largely unknown.
  • We pulse labelled 50 m2 patches of young boreal forest by exposure to 13CO2 in early and late summer. Labelled photosynthates were traced into carbon compounds of < 1 and 1–3 mm diameter roots (fine roots), and into bulk tissue of these and first-order roots (root tips).
  • Root tips were the most strongly labelled size class. Carbon allocation to all size classes was higher in late than in early summer; mean residence times (MRTs) in starch increased from 4 to 11 months. In structural compounds, MRTs were 0.8 yr in tips and 1.8 yr in fine roots. The MRT of carbon in sugars was in the range of days.
  • Functional differences within the fine root population were indicated by carbon allocation patterns and residence times. Pronounced allocation of recent carbon and higher turnover rates in tips are associated with their role in nutrient and water acquisition. In fine roots, longer MRTs but high allocation to sugars and starch reflect their role in structural support and storage. Accounting for heterogeneity in carbon residence times will improve and most probably reduce the estimates of fine root production.