Carbon allocation between tree root growth and root respiration in boreal pine forest.

Soil respiration, i.e. respiration by mycorrhizal roots and by heterotrophic organisms decomposing above- and below-ground litters, is a major component in ecosystem carbon (C) balances. For decades, the paradigm has been that the biomass of fine roots of trees turns over several times a year, which together with large inputs of above-ground litter leaves little room for the contribution from root respiration. Here, we combine the results of a recent tree girdling experiment with the C budget of the classic Swedish Coniferous Forest (SWECON) project, in which root growth and turnover were estimated to be high. We observe that such a high rate of root turnover requires an unlikely high C use efficiency for root growth, and is not consistent with the 1:1 relation between root: heterotrophic respiration obtained in the girdling experiment. Our analysis suggests that 75% of the C allocated to roots is respired, while 25% is used for growth, and hence that root growth and turnover were grossly overestimated in the SWECON study.