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Background and aims: The main objectives of this study were to determine how the carbon age of fine root cellulose varies between stands, tree species, root diameter and soil depth. In addition, we also compared the carbon age of fine roots from soil cores of this study with reported values from the roots of the same diameter classes of ingrowth cores on the same sites. Methods: We used natural abundance of C to estimate root carbon age in four boreal Norway spruce and Scots pine stands in Finland and Estonia. Results: Age of fine root carbon was older in 1.5-2 mm diameter fine roots than in fine roots with <0.5 mm diameter, and tended to be older in mineral soil than in organic soil. Fine root carbon was older in the less fertile Finnish spruce stands (11-12 years) than in the more fertile Estonian stand (3 and 8 years), implying that roots may live longer in less fertile soil. We further observed that on one of our sites carbon in live fine roots with the 1.5-2 mm diameter was of similar C age (7-12 years) than in the ingrowth core roots despite the reported root age in the ingrowth cores - being not older than 2 years. Conclusions: From this result, we conclude that new live roots may in some cases use old carbon reserves for their cellulose formation. Future research should be oriented towards improving our understanding of possible internal redistribution and uptake of C in trees. [ABSTRACT FROM AUTHOR]