Temporal photosynthetic carbon isotope signatures revealed in a tree ring through 13CO2 pulse‐labelling

Using a combined method of pulse‐labelling trees and analysing detailed distribution of 13C tracer within tree rings, we studied how photo‐assimilates incorporated on a given day are then distributed in a tree ring. A branch of a 4‐year‐old Cryptomeria japonica D.Don tree growing in Tsukuba, Japan was pulse‐labelled with non‐radioactive 13CO2 on two occasions: 29 May 2001 and 18 September 2001. Two discs were cut from the stem on 4 March 2002, one immediately under and the other 0.5 m below the branch and put through high‐resolution δ13C analysis. δ13C peaks were observed in both the earlywood and latewood of the concerned tree ring, corresponding to each pulse‐labelling date. The earlywood peaks was broader than the latewood peaks, possibly reflecting seasonal variation of the width of wood developing zone. Half‐widths of the peaks were measured and used as indicators for the potential time resolution of tree‐ring isotope analysis. The half‐widths of the peaks indicated a time resolution no finer than 8.7–28 and 33–42 d in the early and latewood, respectively. Holocellulose extraction yielded only a slight change to the shape of the δ13C peaks. 13C tracer pulse‐labelled in May and September reached tangentially different locations in the lower disc, suggesting a seasonal change in the pathway of carbohydrates. Local consumption of spring assimilates and long‐distance downward transport of autumn assimilates were also suggested.