Origin and Age of Carbon in the Cellulose of Mid-Lattitude Tree Rings

Cellulose of tree rings is often assumed to be predominantly formed by direct assimilation of CO₂ by photosynthesis and consequently can be used to reconstruct past atmospheric ¹⁴C concentrations at annual resolution. Yet little is known about the extent and the age of stored carbon from previous years used in addition to the direct assimilation in tree rings. Here, we studied ¹⁴C in earlywood and latewood cellulose of four different species (oak, pine, larch and spruce), which are commonly used for radiocarbon calibration and dating. These trees were still growing during the radiocarbon bomb peak period (1958–1972). We compared cellulose ¹⁴C measured in tree-ring subdivisions with the atmospheric ¹⁴C corresponding to the time of ring formation. We observed that cellulose ¹⁴C carried up to about 50% of the atmospheric ¹⁴C signal from the previous 1–2 years only in the earlywood of oak, whereas in conifers it was up to 20% in the earlywood and in the case of spruce also in the latewood. The bias in using the full ring of trees growing in a temperate oceanic climate to estimate atmospheric ¹⁴C concentration might be minimal considering that earlywood has a low mass contribution and that the variability in atmospheric ¹⁴C over a few years is usually less than 3‰.