Does the age of fine root carbon indicate the age of fine roots in boreal forests?

To test the reliability of the radiocarbon method for determining root age, we analyzed fine roots (originating from the years 1985–1993) from ingrowth cores with known maximum root age (1–6 years old). For this purpose, three Scots pine (Pinus sylvestris L.) stands were selected from boreal forests in Finland. We analyzed root C age by the radiocarbon method and compared it with the abovementioned known maximum fine root age. In general, ages determined by the two methods (root C age and ingrowth core root maximum age) were in agreement with each other for roots of small diameter (\0.5 mm). By contrast, in most of the samples of fine roots of larger diameter (1.5–2 mm), the C age of root samples of 1987–1989 exceeded the ingrowth core root maximum age by 1–10 years. This shows that these roots had received a large amount of older stored carbon from unknown sources in addition to atmospheric CO2 directly from photosynthesis. We conclude that the C signature of fine roots, especially those of larger diameter, may not always be indicative of root age, and that further studies are needed concerning the extent of possible root uptake of older carbon and its residence time in roots.